Sign up for our monthly newsletter so you never miss the latest from InDepth!
By Reilly Fogarty
See Previous InDepth Story: How To Calculate the Risk Of Pulmonary Oxygen Toxicity by Ran Arieli
These days you would have to be a lunatic (J-valve fanatics, I’m looking at you) to dive without a pressure gauge. Without a gauge, you can’t know how much gas you have left: whether you’re riding the edge of your reserve supply or have enough gas to finish your dive and then some. Planning a dive without understanding oxygen toxicity risk calculations is just as dangerous, but a little more complicated to work through.
Unfortunately for new divers, there just aren’t enough synonyms for “glossing over” to illustrate the number of ways we avoid delving into the complicated science of pulmonary oxygen toxicity management via misdirection, well-meaning anecdotes, or junk science. Like many research theories, what we know about the physiology of the condition is still evolving. Simplified perspectives based on antiquated calculations have a way of initiating a cyclical pattern of compounding mistakes, as one diver explains their best understanding to another, with each diver adding some element of inaccuracy. Despite the challenges, it’s our job as divers to understand how and why we make the decisions that affect our safety.
No single article can hope to cover the scope of a topic like this, but our aim is to provide you with the necessary tools to begin to understand the physiology of pulmonary oxygen toxicity. Our modern understanding is closely intertwined with the history of research into the subject, so while our hope is to educate, we also want to avoid causing harm through misinformation or to give the false impression of complete understanding. Risk mitigation falls to you, as does comprehending the material before you attempt to apply it in your own diving.
The broad strokes of this research can be best understood through the evolution of the unit pulmonary toxicity dose (UPTD) model, the derived equation for repetitive exposure (REPEX) model, and modern approaches developed by Barbara Shykoff, PhD, and Ran Arieli, PhD. Much like the decompression models we use, these are largely (in some cases wholly) unproven models based on compounding theories. Interestingly, the oldest and most problematic of these methods remains the most-used and least frequently questioned by divers. Here’s what you need to know about pulmonary oxygen toxicity.
The Unit Pulmonary Toxic Dose (UPTD) Model
The UPTD model was first developed in 1970 by researchers at the University of Pennsylvania. The theory is based on the loss of vital capacity (VC – the maximum volume of gas that can be voluntarily moved into or out of the lungs) that the researchers believed correlated with the symptoms of pulmonary oxygen toxicity.
The researchers used a small sample group of young men to create a dataset on oxygen exposure symptoms. These participants were subjected to a range of oxygen exposures with PO2 ranging from 1.0 to 3.0 ATA over a variety of exposure durations from 3.5 to 24 hours. To compare exposures across PO2 and exposure time differences, a common unit was created—the UPTD. This unit was defined as the equivalent to one minute of exposure to 100% oxygen at one bar. UPTD could then be calculated as follows and exposures could be compared:
By converting exposures to a common unit, the researchers believed that dives could be compared across depth and gas choices, and repetitive dives could be compared by the sum of their exposure values.
Shortly after the initial work was published, the cumulative pulmonary toxic dose (CPTD) model was created to allow for continuous oxygen exposures above a PO2 of 0.5 ATA to be calculated and expressed in UPTD. As a result of this work, a rudimentary understanding of the onset and presentation of pulmonary oxygen toxicity symptoms began to evolve. It’s important to note, however, that the original UPTD model did not differentiate between dives performed one hour or 23 hours apart—there was no calculation for complete or partial recovery.
UPTD is now often shortened to oxygen toxicity units (OTUs) and the calculations are still widely used even 45 years after their development. It remains unclear whether the model is accurate or the results of UPTD calculations merely overlap adequately with safe limits under common diving conditions, but experts are increasingly finding fault with the model.
Notably, Barbara Shykoff, PhD and US Navy Experimental Diving Unit (NEDU) researcher, has cited several past works from Duke University, Clark, Lambertsen, and others in critique of UPTD applications in the real world. Among her complaints are several significant deviations from the model’s predictions among dives performed at PO2 other than 2 ATA, differences in underlying lung injury and physiology, and disparity between VC changes at mild and extreme PO2 exposures. Research from Clark and Lambertsen has also indicated that the “unit dose” concept may not be applicable in a linear fashion above a PO2 of 1.5 ATA, but requires calculation as a “function of time squared for higher PO2s”.
The Derived Equation for Repetitive Exposure (REPEX) Model
If UPTD was a best-fit model designed for a small data set, the NOAA REPEX model took that applied theory a step further. Developed by R.W. “Bill” Hamilton, PhD, in the early 1980s, the NOAA-derived equation for repetitive exposure (REPEX) model was designed to facilitate recovery and multi-day diving calculations. The model was notably never validated in divers but was built on the concepts of the original UPTD and CPTD model and other supporting studies.
Hamilton’s team set theoretical limits for various exposures, with 850 OTU being an allowable single day exposure but gradually reducing the average daily OTU as missions became longer. He did note that these exposure limit curves would have to be modified with operational experience or desired conservatism, but did not verify the curves with real-world trials. The most often-voiced concern about this model is that it is based on a linear extrapolation of prior data, data that may itself not be accurate. This leaves it with a number of significant flaws and potentially erroneous results, particularly outside the relatively narrow range of exposures for which the UPTD system was designed.
Supporters of the REPEX system might dispute those assertions. It is fair to point out that the NOAA oxygen exposure limit tables have been used extensively for several decades with a great deal of success. This success could be due to accuracy of the model within the applied parameters common in recreational diving. It could also be the result of significant conservatism caused by a large gap between the calculated limits and the real-world exposure limits that incur pulmonary toxicity risk.
Shykoff’s Calculator for Estimating the Risk of Pulmonary Oxygen Toxicity
Shykoff’s work is one of the first notable evolutions in the way we proposed methods to calculate pulmonary oxygen toxicity risk in decades. In a paper published in 2015, she outlines a method by which divers can calculate risk over the course of repeated dives. The method is centered around what Shykoff calls a “incidence-time model” that stipulates a linear relationship in a mid-range exposure value. This makes it possible to not only calculate oxygen toxicity for duration, but also extrapolate equivalent exposure time between exposures at different PO2s. This is coupled with a recovery model and centered around the idea that any dive with a PO2 of 1.3 to 1.4 bar begins some level of pulmonary injury. Shykoff admits that the probability of noticeable injury in small exposures is quite small, but she believes there is a proportional risk between this and intermediate duration exposures. This differs from the UPTD and REPEX models in that it significantly limits its extrapolation of linear relationships only to the intermediate exposures, notable because exposures beyond a small midrange have indicated significant variance from the norm in prior models.
Also important is the verification of Shykoff’s model. While its design relies on a relatively small dataset of 1350 in-water dives, the fact that it has been verified in divers at all separates it from its predecessors. These dives occurred with a PO2 of between 1.3 and 1.4 bar, and were used to create and verify the time-incidence model. An additional dataset using 620 dives was used to validate the recovery model used in Shykoff’s risk calculator.
Interestingly, these data showed that significant recovery did not occur until nearly five hours after a dive, with no measurable recovery apparent after two separate three-hour dives that occurred with a two-hour surface interval between them. Whether this is a function of actual recovery mechanisms or a limitation of the model remains unclear.
In terms of practical application, Shykoff’s model makes a few interesting points. Most notably, she suggests completing shorter dives before longer ones, and a resting dive before one with exercise. This is both to avoid a possibly over-conservative calculation of risk due to the rate of recovery being calculated with the duration of exposure in the denominator, and because recovery after a dive that involves exercise occurs at nearly half the rate as one done at rest. It’s also important to note that zero incidence does not correlate with zero duration. Shykoff cites a 95% confidence limit for this model at 0.8 – 4.9%, meaning that—purely by the numbers—approximately 3% of divers in the model may exhibit signs or symptoms of pulmonary oxygen toxicity at the beginning of the dive with no exposure to elevated PO2. This point must be followed with the note that this is directly comparable to the overall incidence of symptoms found in a dataset of 239 shallow open-water dives with a PO2 of 0.3 bar (resulting in about 6% symptom incidence) and can be “interpreted as the effect of breathing underwater.”
Another key differentiation from prior models is the recovery rate calculation. While prior models used a first-order kinetic model, Shykoff used a sigmoidal shape recovery pattern based on real-world data. This means that recovery rates vary between their initial rate, a maximum rate, and a final slower rate in a non-linear fashion. More complex calculations alone don’t imply superiority, but Shykoff does have some real-word data to back up her model. Experimentally, it appears that injuries from one exposure may begin to diminish even as damage from a following dive occurs, leading to occasional improvement of symptoms in subjects who began a dive with symptoms (note that this is an experimental observation and NOT a recommendation or suggestion of causation).
This model also does not associate severity of oxygen toxicity with exposure duration. It also does not differentiate between severity of exercise, other effects of mild hyperoxia, or PO2 significantly deviating from 1.3 bar or beyond a single exposure of up to eight hours. The model is based on a small dataset, and its results should be treated as almost entirely theoretical in their current form.
Arieli’s Toxicity Index Derived from the Power Equation
Ran Arieli, PhD is the former head of Hyperbaric Physiology Research at the Israel Naval Medical Institute, and takes many of the same issues with the UPTD and REPEX models that Shykoff and their colleagues do. In response, he’s developed a method that focuses on a power-law approach to pulmonary oxygen toxicity risk calculations. This model focuses on the polynomial expression of reactive oxygen and nitrogen species that can be correlated with pulmonary injury, and the assumption that the development of these species is related to the highest power of the PO2 exposure. Combining this with other oxygen toxicity symptoms (decrease in lung capacity, ventilatory drive, or thickness of alveolar wall, for example) modeled by their exposure time, the rate of hydrogen peroxide (a reactive oxygen and nitrogen species precursor) can correlate with the square of time and—Arieli believes—can provide an ability to predict oxygen toxicity risk.
More on how Arieli derived his power equation can be found in a previous InDepth article, “How To Calculate the Risk Of Pulmonary Oxygen Toxicity,” but at its core his model was designed to address a demonstrable difference in pulmonary pathologies found at high and low PO2 exposures. Modeling different risk factors for each resulted in a model that Arieli believes more accurately represents pulmonary oxygen toxicity risk.
This model has the added benefit of being directly verifiable through the measurement of reactive oxygen and nitrogen species, or their precursors. The caveat to this is a correlation between these species, and the expected pulmonary pathology must be assumed, although these relationships are relatively well accepted. Like Shykoff’s model, Arieli’s proposed model has fairly specific parameters. Its initial publication specifically focuses on interpolating risk for exposures done under sedentary to light intensity activity (1 to 4.4 MET) in subjects either dry or immersed in 33°C water. The result is a model which Arieli believes can accurately be used to estimate pulmonary oxygen toxicity risk in divers but which must be carefully applied within the parameters of its original intent.
Applying The Models to Real World Dives
The SS Brandenberg
It’s hard to bring concepts like these into focus without crunching some numbers, so working through an example of a long CCR dive can help illustrate the differences. Take a dive like Massimo Bondone’s exploration of the SS Brandenburg for example. A British steamship torpedoed on February 10th, 1941, the Brandenburg now sits in nearly 199 m/650 ft off the coast of Tuscany, Italy. The Italian explorer and his team identified and dived the wreck in 2017. Their efforts represent the fourth deepest shipwreck exploration dive conducted by tech divers.
Using a closed-circuit rebreather Bondone logged 15 minutes of bottom time and a 490 minute total run time for his dive. This dive is extreme, and difficult to do without significantly exceeding CNS toxicity limits under almost all guidelines, but useful for exploring the limits of pulmonary toxicity which take greater total exposure time to exceed. Assuming a constant PO2 of 1.0 bar, here’s how each of the models works out:
Total toxicity in UPTD can be calculated as follows:
UPTD = t * [(PO2 – 0.5)/(1-0.5)]1/1.2
UPTD = (490 minutes) * [1.0 – 0.5)/(1-0.5)]1/1.2
UPTD = 490
This exceeds the latest US Navy Diving Manual limit for a daily exposure (450 UPTD), although a single extreme exposure limit of 1425 UPTD is designated for medical applications in the operational theater. Bondone’s dive was extreme, but UPTD guidelines put it just beyond the allowable limits for single-day pulmonary oxygen toxicity exposure.
That same dive by REPEX guidelines results in the same unit value – 490 OTU – but has significantly shorter maximum exposure durations.
By these guidelines, a 490 minute exposure to a PO2 of 1.0 bar is more than two hours over the limit for single exposures. There is no way to quantify the increase in risk once the limit is exceeded; the guidelines were designed to be binary. It is interesting to note how much shorter than the UPTD guidelines the REPEX exposure limits are.
The Shykoff method is significantly more involved to calculate by hand, but is available to easily calculate via an online tool Shykoff has published. One issue with calculating this example dive, however, is that the model has been designed only for a PO2 range of 1.3 to 1.4 bar – the lower PO2 ranges commonly used for dives like this fall outside the intended range of the model. Calculating the dive with a higher PO2 but an identical dive time (something that might occur with more conservative gradient factors, for example) gives the following:
Unlike the older models, this calculation gives a probability of symptoms, rather than just a binary judgement on whether the dive fits within acceptable parameters or not. That probability does not imply accuracy in and of itself, but if the mode proves accurate it could provide new possibilities for dive planning in extreme exposures.
Calculating toxicity risk by the power equation derived index method that Arieli has published requires a bit of work. For PO2 greater than 0.6 bar, the following equation results in the POT index:
POT Index = T2* PO24.57
This yields a POT Index of 49. This value can then be used to estimate incidence of toxicity symptoms via the following:
Incidence (%) = 1.85 + 1.071 * POT Index
This value, as estimated by the model and the dataset from 16 HBO exposures performed at the NEDU, results in an estimated 54% likelihood of pulmonary toxicity symptom evolution. Within the limitations of Arieli’s dataset, the predictions appear to have a strong correlation to real-world results, and the calculated risk is similar to that found using Shykoff’s model. Bondone reported that though he has had pulmonary oxygen toxicity symptoms on dives, he did not experience any symptoms after diving the Brandenberg.
Pulmonary oxygen toxicity comes to the forefront of diver’s concerns as their expeditions grow longer, so there’s value in looking at a longer, slightly shallower dive as well. Dr. Andy Pitkin is a pediatric cardiac anesthesiologist and researcher who regularly participates in dives exploring some of the deepest caves in the United States. In 2019, he and his teammates performed a dive to a maximum depth of 124 m/404 ft in just over 12 hours while exploring the Weeki Wachee Springs system with the Karst Underwater Research (KUR) group.
The dive involved years of work-up and an innovative array of support personnel, equipment, underwater habitats, and emergency planning, but it’s the effect of the extreme exposure to elevated PO2 that we’re interested in here. Using a rebreather with a setpoint of 1.1 bar, Pitkin spent 725 minutes underwater. Ignoring the air breaks used to manage CNS toxicity near the end of his profile, here’s how each model breaks it down:
Here’s how Pitkin’s dive looks calculated via the original UPTD algorithm:
UPTD = t * [(PO2 – 0.5)/(1-0.5)]1/1.2
UPTD = (725 minutes) * [1.1 – 0.5)/(1-0.5)]1/1.2
UPTD = 843
Like Bondone’s dive, this exceeds the US Navy Diving Manual limit for daily exposure, this time by almost double (450 UPTD vs 843 UPTD). However, the allowable limit for a single extreme exposure of 1425 renders this technically within some limits, although those were intended only for medical applications in emergency situations. By UPTD standards, this dive would be significantly beyond any reasonable single-dive exposure.
Just as with Bondone’s dive, the REPEX calculations provide the same 843 OTU value as the UPTD calculations. In this case, the exposure is even further past the NOAA Diving Manual guidelines for a maximum single exposure (just 240 minutes at the working PO2 of 1.1 bar). It would not be possible to perform this dive via either REPEX or UPTD guidelines.
It’s impossible to calculate this dive within the confines of Shykoff’s method because she designed her model around a working PO2 of 1.3 to 1.4 bar. Attempting to calculate a dive this long at an artificially high PO2 results in dramatic numbers, but none that reflect reality. Because Pitkin chose to run a lower PO2 to attenuate the risk of CNS oxygen toxicity, this dive falls below the more common working PO2 used by rebreather divers around which Shykoff has designed her model.
This dive falls significantly beyond the exposure guidelines that Arieli has set forth, but his are less binary than those used by Shykoff’s model. Calculating POT Index as follows:
POT Index = T2* PO24.57
And theoretical incidence via:
Incidence (%) = 1.85 + 1.071 * POT Index
Results in a value of 240%, or 240% likelihood of pulmonary toxicity symptom evolution.
It’s results such as these that illustrate the fact that these models are entirely theoretical to those who are first playing with the data. Particularly as exposures deviate from the norm, their results will appear increasingly extraordinary, but none of those results may reflect reality in the first place.
Pitkin completed his dives with minimal symptoms, despite all four of these models indicating that it could be extremely hazardous to use such a profile. Pitkin explained that he and his partners frequently experience some low-grade symptoms of pulmonary oxygen toxicity following similar dives. These symptoms typically include “substernal tightness, mild cough, and the feeling of inability to take a deep breath,” which is why they now limit the PO2s on these dives. Symptom onset has become an expectation on dives such as these but severity is decreased by using a setpoint of 1.2 bar or less (interestingly, according to Pitkin, time to symptom onset is not improved by this change). “A bonus is you don’t feel as crappy afterwards,” he said.
Each of these four models has indicated the likely onset of pulmonary toxicity symptoms from this profile, and while symptoms have occurred they are perhaps milder than some models would suggest. This somewhat mixed result is indicative of the confusing nature of this type of modeling – Pitkin noted that, “there is a lot of inter-individual variation in susceptibility to pulmonary oxygen toxicity,” and this variability likely presents enormous challenges to accurate risk modeling.
We conferred with Arieli regarding these results. He noted that some recent research indicates the possibility of acclimatization to high PO2 exposures in technical and CCR divers that could explain lower than expected rates of CNS oxygen toxicity. This could potentially apply to pulmonary toxicity events as well.
The difficulty with covering a topic like this is the desire to educate but not instill undue confidence in divers. Knowledge is a powerful thing, and it’s easy to see research that agrees with what we’ve been taught and immediately apply it to our own diving. My hope is that you can take this article, use it to fill in the gaps in your own knowledge, and find the areas where you’d like to dig a little deeper.
UPTD and REPEX models may be inaccurate (wildly so in some applications), but many divers have been trained to use them in ways that promote safe diving practices and result in low injury incidence. Similarly, Shykoff and Arieli’s methods are exciting and appear accurate but lack the real-world testing to verify their efficacy. They are significantly hindered by their parameters, and we have yet to determine if they represent a more accurate model at large, or just a closer correlation to a limited dataset.
Like many elements of diving research, the potential here is enormous. While you may be left waiting for more research before you can change the way you plan your dives, it’s likely that these two models and those that will come from them will dictate how we dive in the future. Knowing how we got to this point, what the pitfalls of our current calculations are, and how researchers hope to remedy those pitfalls in the future is valuable and can contribute to your safety. Dig into the details wherever possible, see how each model calculates your most challenging dives, and think about how you can make your dives safer with all of the knowledge available to you.
A Special Thanks to Massimo Bondone and Dr. Andy Pitkin for sharing their dive data and Neal Pollock, Ph.D. for lending his expertise.
InDepth: How To Calculate the Risk Of Pulmonary Oxygen Toxicity by Ran Arieli
Frontiers in Physiology: Aviner B., Arieli R.,Yalov A. Power equation for predicting the risk of central nervous system oxygen toxicity at rest
Calculator For Estimating The Risk Of Pulmonary Oxygen Toxicity by Dr. Barbara Shykoff
Shearwater Research: Why UPTD Calculations Should Not Be Used by Barbara Shykoff, 2017
Spums Journal: Tolerating Oxygen Exposure by RW Bill Hamilton, 1997
RW Bill Hamilton’s Original REPEX paper: Tolerating Exposure To High Oxygen Levels: Repex And Other Methods by RW Hamilton, 1989An early 1985 review of the UPTD Model: Predicting Pulmonary O2 Toxicity: A New Look at the Unit Pulmonary Toxicity Dose by AL Harabin, L.D. Homer, PK Weathersby and ET Flynn
Reilly Fogarty is an expert in diving safety, hyperbaric research, and risk management. Recent work has included research at the Duke Center for Hyperbaric Medicine and Environmental Physiology, risk management program creation at Divers Alert Network, and emergency simulation training for Harvard Medical School. A USCG licensed captain, he can most often be found running technical charters and teaching rebreather diving in Gloucester, Massachusetts.
SUMP POTION #9
Located high in the Sierra Mazateca mountains in Oaxaca, Mexico, Sistema Huautla has captured the imagination of elite cave explorers for more than 50 years. Join photographer SJ Alice Bennett and cave/tech instructor Jon Kieren on Beyond The Sump’s recent March/April 2022 expedition to Sump 9.
Text by Jon Kieren. Images by SJ Alice Bennett.
🎶🎶 Pre-dive Clicklist: 붐바야 (BOOMBAYAH) by BLACKPINK curated by Steve Lambert
Sistema Huautla, in Oaxaca, Mexico, one of the most iconic and expansive cave systems in the world with over 30 entrances, more than 100.7 kilometers/62.5 miles of known passage, and reaching a depth of over 1500 meters/5000 feet, has been an obsession for cavers around the world for over 50 years. Every year, several groups such as Beyond the Sump (BtS) and Proyecto Espeleológico Sistema Huautla (PESH) mount expeditions to the region to explore. Surrounded by karst topography with several other gigantic systems, such as Chevé and Kijahe Xontjo are close by, there is surprisingly only one main exit point for the water flow (based on several dye trace studies), the Huautla Resurgence. Huautla is still being actively explored from the plateau to find the allusive connection with its resurgence. Terminating in a 9th sump at 81 m/264 ft depth, it is logistically extremely difficult to push the end of the line from there. This leaves exploration from the resurgence as the most likely tactic to make the connection.
Nestled deep in a canyon 1200 m/4000 feet below the sleepy little town of Santa Ana Cuauhtémoc in the Sierra Madre de Oaxaca mountains in Oaxaca, Mexico, is the Santo Domingo River. The Santo Domingo is fed by multiple water sources from various cave systems in the area including the Peña Colorada, Agua Frio Resurgence, HR Resurgence, and the Huautla Resurgence. The Huautla Resurgence was first explored in 1982, followed by expeditions in 1984 and 1995 led by Bill Stone. In 2001, Jason Mallinson and Rick Stanton pushed the cave to a maximum depth of 65m/215 ft and reached a sump pool where a dry cave passage heading off could be seen 10m/30 ft above, but with vertical muddy walls stopping the divers from being able to exit the water. Beyond the Sump, expeditions began exploring the resurgence in 2016 and 2017 where they found an exit from Sump 2 into a dry section, named “Passage of the Cheeky Monkey”, which was thoroughly explored and mapped, with several sumps found along the way. When time ran out for the 2017 expedition, several questions remained unanswered. Primarily, “where the hell does all the water come from?”, as the only source of water seemed to come from a small flowstone restriction affectionately named the “Squirty Hole”. A question that would need to wait five years to be answered.
In late March, 2022, Beyond the Sump set off on another expedition to Santa Ana to find the way on to “Sump 9”. The team consisted of Andreas Klocker (AUT/AUS), Zeb Lilly (USA), Steve Lambert (USA), SJ Alice Bennett (UK/GER), Ben Wright (UK), Rob Thomas (UK) and myself, Jon Kieren (USA), with logistical support happening remotely by Alejandra “Alex” Mendoza (MEX). Bios on the team can be found at: Beyond The Sump-Team. This is a log of our experiences and discoveries.
28 March, 2022
The entire group met for the first time in Tehuacán. Andreas, Zeb, and Steve had driven down from the US, while SJ and I had driven over from Tulum, and Ben and Rob had flown in from the UK. Everyone’s travel up to this point was relatively uneventful, except for SJ. She had managed to badly sprain her ankle the night before, leaving us questioning how the first couple of weeks of the expedition would pan out for her. Both trucks were packed tight, but room was made for the Brits and SJ’s swollen ankle for the remaining four-hour drive up through the Sierra Madre mountains to Santa Ana. The drive is spectacular, beginning on the north western side of the mountains where it is an arid desert filled with giant cactus and ending at an elevation of about 5200 ft in a lush green mountain forest.
After a quick stop for tacos and to grab a “few” bottles of mezcal, we arrived at our field house after dark. We quickly scrambled to unload the trucks into the concrete box we would call home for the next four weeks. We hastily set up our beds, and a bottle of whisky and mezcal made a few quick passes around the room to con- gratulate our arrival before lights out.
29 March 2022
Church bells rang at 5:30 am which woke both us and the surrounding livestock as the sun began to rise through the canyon below us, a truly remarkable sight that I highly doubted I would ever grow tired of. First order of business was to dig out the coffee pot and tea kettle. Once adequately caffeinated, we started organizing all of the equipment for base camp and diving. We set up a makeshift kitchen with two small camping stoves and a fold-out table. After a batch of scrambled eggs were devoured, everyone started tearing into the dive equipment and getting personal kit and team resources organized. We assembled a boosting station in the field house and set the compressor up outside. Regulators, cylinders, and rebreathers were scattered everywhere, and SJ was busy with camera equipment. Morale was high as everyone made predictions for what the cave was going to do.
Steve, Zeb, and Andreas were supposed to have a meeting with the town council to finalize permission to use the road leading down the canyon and set up operations in the cave. We had no doubts we would gain permission, but it was important to play the local politics and stay friendly with the community. The meeting didn’t happen, but we were assured “mañana” (which often means “later” as opposed to the direct translation of “tomorrow”). Instead of holding the meeting, Steve, Zeb, and Andreas were handed a bottle of moonshine made from sugar cane, called aguardiente. In an effort to be diplomatic, they graciously accepted a drink, and then another. Soon they were hooked into a few hours of hilarity trying to socialize in broken Spanish while the rest of us waited patiently for word on what our plan would be for the next day. We would need to wait until 5:30 am when the church bells rang to assess everyone’s energy levels and see what we thought about the “beg for forgiveness” tactic for finalizing permission before deciding to head down into the canyon or not.
30 March 2022
We decided to go for it and started to set up in the canyon. The 1219 m/4000 ft descent down to the canyon took about an hour by 4×4 truck and was absolutely breathtaking, second only to the hike to the resurgence. The hike was a fairly easy- going 1.2 miles, but took about 40 minutes each way with heavy loads and several river crossings. Luckily, we were able to keep most of the heavy kit in the cave for the majority of the expedition with only CCR bottles, the “cave cascade” (a few lightweight high pressure carbon cylinders we had set up in the cave to refill cylinders), and other little bits and bobs of personal kit needing to be transported in and out each day.
Andreas, Steve, and I did the first dive to reline and survey the first sump and rig the waterfall. Upon surfacing at the waterfall, a wave of “holy shit, this is remote” hit me quite hard, and the smile would not come off my face. While we dove, the rest of the group (minus poor gimpy SJ, who was stuck at the field house knotting line) did two more gear hauls from the truck. Everyone was pretty beat, but nothing a couple Victoria beers and a few liters of gatorade wouldn’t fix.
In the evening, we were able to meet with the local officials for formal permission to use the road and access the cave. We donated some pesos to fund their annual celebration of the anniversary of Emiliano Zapata, which we also had to promise to attend.
Despite Steve’s insistence on K-pop for our daily soundtrack, morale was high.
31 March 2022
Day 2 of diving was productive. Zeb, Rob, and Ben were able to set up the gear line for the far side of the waterfall to hang the deep bailouts, run line to 140 feet, and set the deep bailouts. Andreas, Steve, and I did a few gear hauls through the canyon. The next day, Steve and I planned to reline and survey the second sump out another 1200 feet or so at a max depth of about 215 ft. I was excited for the “real” diving to begin.
1 April 2022
Instead of the fiery red sunrise through the canyon, we were suddenly in the clouds and surrounded by cold mist, chugging coffee and tea but still struggling to wake up. The group appeared tired from the few days of intense hiking in the canyon, but moods lightened as the coffee hit, and we started to think about today’s dive. After today, we would likely begin pushing the leads left over from 2017 and searching for the way on to Sump 9. I was a bit apprehensive about making it over the waterfall with my Fathom CCR on, and felt a bit jealous of the side mount and chest mount units other team members were using. The waterfall was only about a meter high, but had high flow and razor sharp jagged rocks protecting it. I figured if it was a big hassle, I’d switch out my Flex2 side mount unit for future dives to make getting to sump 2 a bit easier.
On our drive down to the canyon, we were stopped by a group of enthusiastic locals. With big smiles on their faces, they insisted we get out of the truck and follow them up a small trail in the mountainside. As we followed, we could see smoke coming from a pit, and a strong scent of something sweet in the air. The group wanted to show us how they were processing sugar cane to produce piloncillo, an unrefined sugar commonly used in Mexican cooking. We were given a block of the piloncillo, which we later used to make syrup for pancakes and French toast when we started getting tired of scrambled eggs.
Steve and I had a great dive. We crossed the waterfall to sump 2. I made it over with my Fathom on, but it took a bit of effort. I was thinking that switching to the Flex for the next dive would make life easier, especially if we would be hauling more cylinders and scooters over the waterfall. Sump 2 was just a truly stunning, big passage with rolling hills all covered in silt. Our max depth was 56 m/183 ft on this dive, with about an hour of deco to do upstream of the waterfall. We laid another 365 m/1200 ft of line while swimming, setting up the next team to re-line all the way to camp 1 in the Passage of the Cheeky Monkey, and check what was thought to be the most promising lead discovered in 2017, referred to as the “11 meter lead”.
2 April 2022
SJ came down into the canyon for the first time today. Her ankle was still in pretty rough shape, but life in the box on the mountainside had become dull. She had been as productive as she could be by knotting line and photographing the town. She also managed to make no less than four new boyfriends, led by a 6-year-old who kept bringing his siblings and friends into the field house and proudly exclaiming “gringa!” while pointing at SJ. He then would lead them around the field house showing off all of the strange equipment we had scattered about.
On our way into the canyon, we were hailed by another group of farmers just a little down the road from the piloncillo farm. As they enthusiastically led us to their farm, we could smell the pungent aroma of fermenting sugarcane before we could see the still. They first showed us how they crushed the sugarcane plants to extract the juice, which we sampled. Rich, sweet, and syrupy, it was hard to get down with the thought of the hard hike through the canyon ahead. Next, they showed us where the fermentation was taking place in large tubs next to the still. We were offered a sample straight from the still, which we had to decline, as there was much work and diving to be done yet. So we promised to stop back at the end of the day to have a drink.
The diving for the day proved to be less productive. SJ was able to take some photos of the canyon and divers prepping to enter the cave, but the dive was called early due to a rebreather failure. The line was still extended a few hundred feet, so all was not lost. But the line still did not reach the Passage of the Cheeky Monkey nor had any leads been investigated. Morale was a bit low.
At the end of the day, we stopped back at the aguardiente distillery and were poured a fresh bottle to be passed around. Before taking a drink, Steve asked how much alcohol was in it. The man proudly proclaimed “22 grado,” which Steve interpreted as 22% and took a chug. His eyes went big, he handed it to me, and I took a big swig for myself and quickly realized that “22 grado” does NOT equal 22% as I handed the bottle on to the next person. Realizing we needed to leave ASAP or it would be unlikely any of us could drive the truck home, Rob (who seemed quite pleased with the aguardiente) offered to buy the bottle to take home with us. With the transaction complete, we headed back up the mountain to get to the bottom of this “22 grado” business.
3 April 2022
Another cold and cloudy day. I was tired, and my back felt broken when heading down the mountain. I needed a day of rest but knew we needed to push on. I switched to the Flex and headed in with Steve on DPVs to line the cave to camp 1 and check the 11 meter lead. I immediately realized I was overweighted with the Flex, steel side mounted bailout cylinders, and extra safeties and deco gas that were to be installed in sump 2.
Crossing the waterfall, I tore the right ankle of my drysuit, which I noticed as soon as I got back in the water on the other side. Knowing I had heated undergarments on and plenty of battery power for the couple hours of deco we might end up with, I decided I would be fine to continue the dive.
With each stage drop, I hoped my stability would improve, but it didn’t. I struggled on, Steve and I making it to the far side of sump 2 to search for the way on to Cheeky Monkey. We made our way up what we believed to be the correct path, doing our deco as we circled up towards an air bell. We did not find the 11 meter lead where we thought it would be, and realized we were in an area known as “Jason’s Eyes,” a dry section first discovered in 2001 by Jason Mallinson which had no way on. Steve asked if I wanted to surface to look around and chat about where to look next, and I reluctantly raised my thumb and pointed back toward the exit. I was super uncomfortable being overweighted as well as needing to dive back to 65 m/215 ft and have an hour or so of deco before the waterfall with a flooded drysuit. Plus, I knew that if we dragged this dive on much longer, I was going to start making mistakes. So we re-descended from our 3 m/10 ft stop and headed back toward Sump 1, when I was abruptly stopped at 9 m/30 ft as I could no longer inflate my wing or drysuit.
Grabbing the cave wall, I realized that the two liter cylinder I had dedicated for wing and suit inflation was dead, clearly a result of struggling with being overweighted and unstable. I got Steve’s attention and communicated the problem, and we started to inventory resources with an LPI connection. We had an O2 bottle, which would not be great for suit inflation considering I was already shivering in the 18º C/65º F water and would desperately need to use my heat during deco. The 50% bottle we were to drop at the deco station heading to camp 1 only had a QC6 connection, which would be no help to me. And that left only my side mounted bailout, which was 15/55 trimix. Certainly not ideal for suit inflation, but better than starting myself on fire. I plugged in and filled my suit with the icy trimix as we started to exit. I had to constantly switch the hose from my suit to wing as we scootered out but managed to make it back to the waterfall with only an hour of deco, which was manageable with my heated vest on full blast.
We were unsuccessful in completing our tasks for the day, and I was in a world of self-pity from my poor decision to change configurations without a shakedown dive. We went back to the field house to conduct some experiments regarding the actual alcohol content of the Aguar. Tomorrow we would rest and re-group. Morale was low.
8 April 2022
The past few days had been challenging. Several attempts at exploration in Sump 2 had proven unsuccessful. We had scoured the deep section, and the fabled 11 meter lead, and others like it, which all pinched off quickly. While there was significant flow coming out of these tight passages, they were simply Swiss cheese that was not passible by humans. Maybe after a few hundred thousand years or so, they would be big enough so we could jam Steve in there to take a look, but for now, we were going to have to focus our search in the Passage of the Cheeky Monkey and the sumps within it to find the way on.
Logistically, this would mean exploring from camp 1 to avoid having to pass a waterfall and do a 65 m/215 ft dive prior to surfacing and hiking gear a couple thousand feet through dry cave to the Sump each day. We would be using the next day as an opportunity to rest and get the first camp team, Rob and Andreas, ready to set off for a couple of days in the Cheeky Monkey.
SJ’s ankle was feeling well enough for her to dive, so we’d done a couple of shakedown dives to test the ankle and get a feel for the cave before starting to shoot the next day.
9 April 2022
SJ and I entered the water for a photo dive shortly after Andreas and Rob pushed off for camp 1. About 30 minutes into shooting, we noticed lights and the sound of scooters buzzing toward us. It was Andreas and Rob, obviously having had some sort of problem and aborting early. We decided to exit with them to see if we could assist somehow. Turned out they had a dry tube failure when they made it to the end of Sump 1, drowning most of the camping equipment.
With only 9 days of diving left, and time starting to run out, we couldn’t afford any more mishaps if we were going to figure this cave out. A serious team discussion was had to decide on the schedule for the next few days to prioritize exploration, as well as to ensure that we would have opportunities for documentation. We planned to prep and re-group again the next day, then Steve, Zeb, and I would head into camp 1 for a very long day of poking around in the Cheeky Monkey to determine what the objectives should be for the first camping team.
In the evening, we had a chance to meet up with Bill Stone and his team who were exploring a nearby dry cave. It was pretty surreal to be in Oaxaca with Bill, hearing him tell stories of exploration in the area, as well as to discuss what we had found and what we thought the cave might do. Bill was convinced the Swiss cheese we had found could not be the only water source, as it was rumored that during the rainy season the resurgence produced a geyser several meters tall. We discussed what our plans were moving forward, and Bill seemed to agree that the sumps in the Cheeky Monkey must be hiding something.
10 April 2022
A day of rest and prep for a long day tomorrow. There was a celebration in town for Emiliano Zapata with parades, fireworks, and lots of mescal and aguardiente. It began last night and never really ended. We were supposed to attend the festivities that evening, but hopefully only for a short while as we were planning to leave the field house at daybreak to be in the water early morning.
The music and festivities in town added a joyful feel to the somewhat mixed emotions in the field house. Excitement, stress, and anxiety. Morale was pretty high considering the pressure we were under.
11 April 2022
A long but successful day. Steve, Zeb, and I pushed off early in the morning and spent the majority of the day in Cheeky Monkey. From the beach where we surfaced, it was about a 30-minute hike through fairly rough terrain, but no serious climbing required. Hauling dive gear did create some challenges, though. We checked Surprise Sump first, which had not been dived before, and it turned out to be the biggest discovery we’d had the whole month. Immediately upon descending, Steve noticed darkness beyond the duck under in front of him. As he shouted for joy through his loop and descended with a line peeling off the reel, hearts started pounding as we realized what might have been right under the team’s noses during the 2017 expedition.
After a hundred feet or so, it surfaced, followed by a short hike and another sump which had an upstream and a downstream, and then another waterfall on the upstream side. Not the borehole we were hoping for, but there was more cave here than we knew about the day before, so that was a huge plus, and it seemed to be heading in the right direction–toward Sump 9. Logistics would definitely get more interesting, but we had a good idea of what resources would be needed for the first camping trip. We exited the water a little after 6 pm with rejuvenated spirits and confidence that we were on the brink of breaking this thing open.
13 April 2022
SJ, Andreas, and I were supposed to do a photo dive today. On our way down the mountain, Zeb’s truck’s suspension started making some terrible noises. When we inspected it, we noticed the leaf spring hanger bracket had torn in half, leaving the leaf spring pressing up into the bed. With no option, we slowly drove the truck back up to the field house to start the process of finding parts and tools. After a quick team meeting, we made new plans based on best and worst case scenarios. Best case would be that the truck was fixed today or early tomorrow morning, and we could get a camping team in to push from Surprise Sump for a few days while SJ and I got as many photos as possible. Worst case, we wouldn’t have time left for camping and would have to do the best we could with a couple of day trips.
SJ and I drove down to Tehuacan to pick up a new bracket while the team tried to get the old one off. At the suspension shop, I was struggling to communicate with the woman at the parts counter. She seemed to know what we needed, I was just trying to verify the part number to be sure we weren’t about to make a 7-hour round trip and return with the wrong part. A kind man waiting in line asked us in decent English what we needed, and I explained. He said, with a sly grin and a wink pointing at the woman behind the counter “she knows”. The woman looked at me and smiled. I shrugged and nodded as she grabbed the bracket and darted off while saying something to our new friend. He told us she took it in back so the shop could press the bushing into the bracket for us. While we waited, we chatted with the man about what we were doing there. He seemed intrigued, was enjoying the stories of our adventure, and I was showing him some photos of caves in Tulum on my phone, when the woman returned with the bracket. As I was paying her, two young men were trying to give SJ a couple sandwiches and pepsis. When she tried to refuse, the woman behind the counter got very excited, gesturing for us to take them. Apparently, when we said we had come 3.5 hours down the mountain to get the part, they were empathetic to our situation. And based on my ragged clothes, matted hair, tired face, and sand-fly covered body (SJ looked great as always), they must have assumed it was quite the journey and refused to let us go away hungry and thirsty.
When we got back to Santa Ana, the team let us know they were unable to pull the old bracket, and that we’d have to take the truck to the nearest town with a mechanic first thing in the morning to try to repair it.
14 April 2022
It took until about 2 pm to get the truck fixed, but determined to get some work done, Steve, Zeb, and Andreas decided to push off for camp 1. They were in the water by 6pm, planning to reemerge on the 18th at 4pm.
While driving the truck back up the mountain, I noticed the brakes seemed a bit soft and the steering a bit stiff. However, this was my first time driving Zeb’s truck, and without much other choice, I kept making our way back up to the field house. When we arrived at the house, it was noticed that power steering fluid was leaking below the truck. By then it was after 8 pm, and there really wasn’t much we could do about it at the moment anyway, so we all promptly crashed out so we could get up early and try to sort out the problem in time for SJ and I to finally get in a proper photo dive.
15 April 2022
We topped off the hydraulic fluid but were unable to determine the source of the leak. Ben and I drove the truck around on the more benign roads at the top of the mountain with no noticeable leaks or ill effects on the steering or brakes. So we made the decision to head down the canyon and take some pictures. SJ on the camera, me on lights, and Ben as a model. All went off without a hitch, and the truck made it back up the mountain with still no signs of a leak. I was happy about that, but quite wary. As my dad says, “Problems don’t usually just fix themselves…”
16 April 2022
A day for surface photos. SJ had plans to photograph the canyon as well as take some simple shots in the cave entrance. It was a light and easy day that should have wrapped up quite early. However, as we started to pack up and leave the cave for the day, two by two, our entire host family, all 13 of them, started coming around the corner walking toward the cave. It was surreal, we hadn’t seen a single other human in the canyon for weeks, and there out of the blue, was the whole family. Dragging half sleeping children, the happy and excited adults hastily climbed the rocks up to the cave entrance. They were amused to hear that our friends were several kilometers underground and wouldn’t return for a few days yet. After a bit of climbing around, we all started to make our way back down the canyon toward the truck. After the first river crossing, SJ noticed one of the young mothers struggling to carry Liam, the two-year-old. She gave me a nudge, and I turned and offered to carry the little guy. At the next river crossing, we noticed they had a whole camp set up at the edge of the river. As we approached, the young mother offered us a drink, took Liam back from me, and before we knew what was happening, they had reignited the campfire stove and were preparing a late lunch for us. We ate some of the most amazing refried beans on the planet while the kids played in the river until the abuela (grandmother) started packing up a few things. I looked at her and asked “vamos?” (We go?), to which she loudly exclaimed “VAMANOS!” (Let’s go!) With a smile on her face, as everyone scrambled and had camp packed up and were hiking again within moments.
After encountering the family, our day suddenly became much longer than we had anticipated. We got home after dark, exhausted from another hot day hiking in the canyon, yet rejuvenated from the experience we had just had. It had been hard to keep morale up with the never ending issues we encountered, as well as less than stellar productivity, but to be able to share a bit of what we were doing there with our caring and supportive host family was truly an experience. They thought what we were doing was truly remarkable, which it really was; it was just hard to remember that when facing failures and adversity. So, a little reminder by way of the smiles on the faces of our new friends gave us quite a boost. We ate dinner quickly and settled in as early as we could. One more shot at cave photos the next day. Before Steve, Zeb and Andreas come out and mess up the vis hauling all of their camping gear out.
17 April 2022
SJ and I were able to get in a nice long photo dive. As we were packing up to head out, we saw lights flicker below the surface. Steve, Zeb and Andreas were back a day early, not necessarily a good thing…
As they emerged, one by one, there were no high fives or cheers of joy. Just a content look on Zeb’s face as he calmly stated in his mild southern drawl, “she doesn’t go”.
Arriving at camp 1 after 8 pm on the 14th, they had set up camp and prepared for the following day’s explorations. Over the next two days, they scoured the Passage of the Cheeky Monkey and the sumps within.
They dived Surprise Sump, the newly discovered Gold Star Sump, as well as checked the stream way beyond the new waterfall, and searched every corner of the dry cave. The downstream section of Gold Star Sump pinched off into swiss cheese where there was a significant amount of flow. The stream way beyond the waterfall also pinched off into another flowstone restriction, similar to the Squirty Hole. No new sections of dry cave were discovered. Based on observation of the amount and direction of flow exiting downstream Gold Star Sump and the small restrictions in Sump 2, the team estimated it is approximately equal to the flow coming over the waterfall in Sump 1 as well as exiting the resurgence. Concluding that all water sources have been discovered, none of which will allow a human to pass, and no passable dry cave is accessible.
Disappointed, but content that every corner of the Huautla Resurgence had been checked, they decided to close the book on the project and head out a day early.
The next few days were dedicated to more photos and cleanup. With 12 safety and deep bailout cylinders remaining in Sump 2, scooters staged at the waterfall, several safeties in Sump 1, six shallow bailout cylinders, rebreathers, and personal gear for seven divers left in the cave entrance, there was a lot of work to do. However, with teamwork, we managed to get everything out of the canyon in just three days. Our backs a bit sore, and our dreams of big going borehole passage beyond Sump 2 unrealized, moods were a mix of relief to be finished and a reluctance to leave, knowing we would likely never have a reason to return to this truly remarkable site.
Completing a project is a bittersweet feeling, of course. While sad there’s no more cave, there’s also a feeling of content completion. We did everything possible to find the way to connect the resurgence to Sump 9 of Sistema Huautla, and we are probably the last team to ever see the inside of the resurgence for the foreseeable future (or ever), which is pretty damn cool. We also had the opportunity to spend time with new friends in a truly remarkable place with extraordinarily gracious hosts. So, in all, I would certainly call this year’s Beyond The Sump expedition a success.
|Explorers Club:||Sistema Huautla, Mexico – the 50-year original exploration and study of the deepest cave in the world|
|NatGeo:||One of the Deepest Caves in the World is Even Bigger Than We Thought|
Exploration groups involved with Sistema Huautla:
|Beyond The Sump | www.facebook.com/CaveDive|
|Proyecto Espeleológico Sistema Huautla (PESH)|
|United States Deep Caving Team|
SJ Alice Bennett has been photographically documenting the world around her since she was a kid. After completing a diploma in Graphic & Communication and a B.A. in Visual & Motion Design and moving to Quintana Roo, Mexico in 2017 she’s turned her focus on the underground rivers of the area. Her documentary style of shooting is well known for capturing the emotions of the moment and creating a sense of being there with her. She has a passion for documenting exploration and has worked as a freelance photographer and graphic designer around the globe and just joined the InDepth team. Watch this space.
Jon Kieren is a cave, technical, and CCR instructor/instructor trainer who has dedicated his career over the past 13 years to improving dive training. As an active TDI/IANTD/NSS-CDS and GUE Instructor, and former training director and training advisory panel member for TDI, he has vast experience working with divers and instructors at all levels, but his main professional focus resides in the caves. In his own personal diving, Jon’s true passions are deep extended range cave dives (the more deco the better), as well as working with photographers to bring back images of his favourite places to share with the world.