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Oxygen Exposure Management

Today, community practices limit oxygen exposures to a PO2 of 1.2-1.4 bar for the working phase of the dive and 1.4-1.6 bar for decompression depending on circumstances, training agency recommendations, and platform. Things were not always this way. In the early days of tech diving, there were big fights over what was a ‘safe’ exposure—a carryover from the deep air diving days, where a working PO2 of 1.6-1.8 was considered “no big deal.” NOT. In this 1993 story from the aquaCORPS archives, Diver Alert Network’s former and now deceased research director Dr. Richard Vann, explains the biochemistry of oxygen toxicity, reviews the data, and weighs in with recommendations for the tech community.

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By Richard D. Vann
Header image courtesy of Sean Romanowski
The article was originally published in aquaCORPS #7, C2
, December, 1993

Richard Vann. Courtesy of the Dick Vann / DAN Archives.

Knowledge of central nervous system (CNS) oxygen toxicity is unnecessary in order to breathe oxygen underwater safely at a partial pressure of one bar or less. Considerably more knowledge is needed at higher partial pressures or when the oxygen pressure changes with time. The real questions are; how much oxygen can be used safely given our current knowledge, and how can oxygen be used more effectively without sacrificing safety?

The Biochemistry of Oxygen Toxicity 

Oxygen metabolism is the primary energy source in higher life forms. Because heat energy produced by oxygen reactions, such as fire, would damage tissue, metabolic pathways have evolved that safely capture small packets of reusable chemical energy. This energy is stored in molecules called adenosine triphosphate (ATP).

Figure F1 illustrates some features of ATP production during the breakdown of sugar at normal oxygen partial pressures. The biochemical processes, known as glycolysis, use no oxygen and produce relatively little ATP. The major product of glycolysis, pyruvic acid, enters the Krebs cycle which releases carbon dioxide and supplies electrons needed to form ATP. Most ATP is produced in a series of electron transport reactions called the respiratory chain.

Oxygen usually does not enter the respiratory chain until the very end, where it reacts with hydrogen to form water. Should oxygen enter the respiratory chain prematurely, molecules like the superoxide anion (O2-) and hydrogen peroxide (H2O2) can form. These reactive species of oxygen are potentially toxic but are deactivated by protective enzymes such as superoxide dismutase and catalase.

When the oxygen partial pressure is raised (Figure F2), the production of reactive oxygen species increases and may overwhelm the protective mechanisms. This can initiate biochemical and physiological changes that interfere with normal function and cause signs and symptoms we know as oxygen toxicity. 

Convulsions are the most spectacular and objective signs and symptoms of CNS oxygen toxicity, but there is no evidence they lead to permanent damage if the oxygen exposure is discontinued promptly. This assumes, of course, that drowning or physical injury are avoided. Experimental oxygen exposures are often terminated by less specific symptoms including abnormal breathing, nausea, twitching, dizziness, lack of coordination, and visual or auditory disturbances. These symptoms do not necessarily precede convulsions. Factors which elevate cerebral blood flow, thereby augmenting oxygen delivery to the brain, appear to increase susceptibility to oxygen toxicity. These factors include immersion, exercise, and carbon dioxide. Carbon dioxide may be present in the inspired gas or may be retained due to inadequate ventilation. Inadequate ventilation can be caused by high gas density, external breathing resistance, or poor ventilatory response to carbon dioxide by “CO2 retainers.”



Oxygen Exposure Limits

Oxygen exposure limits, like those of Figure F3, were established to decrease the risk of convulsions for divers breathing pure oxygen or oxygen in mixed gas. Figure F3 shows three sets of pure oxygen limits and two sets of mixed gas limits. The U.S. Navy limits from the 1973 Diving Manual (USN 1973) were published in the 1 979 NOAA Diving Manual (NOAA 1979). The Navy has since modified its pure oxygen limits while NOAA has modified both the pure oxygen and mixed gas limits for its 1991 Diving Manual (NOAA 1991). Compared with the 1973 Navy/1979 NOAA limits for pure oxygen, F3 shows that the 1986 Navy limits are less conservative, while the 1991 NOAA limits are more conservative. For mixed gas, the 1991 NOAA limits are less conservative than the 1973 Navy/1979 NOAA limits.

The changes to the exposure limits of F3 reflect uncertainty concerning which limits are best and suggest an examination of the type of data upon which oxygen limits are based. These data are shown in Figure F4 and represent most of the CNS toxicity episodes that have occurred in U.S. experiments during wet, working dives at a single depth for pure oxygen or for oxygen in mixed gas. The squares represent convulsions, and the triangles represent symptoms. 

The 1991 NOAA limits are shown for comparison. While the discussion below is confined to U.S. data, Donald (1992) has recently published a large body of British data which will be very important.

The mixed gas incidents occurred at lower oxygen partial pressures than the pure oxygen incidents. Ed Lanphier, who conducted oxygen research for the Navy in the 1950s, postulated that high breathing resistance during deeper mixed gas dives caused carbon dioxide retention which potentiated oxygen toxicity by increasing cerebral blood flow. This led him to propose more restrictive limits for mixed gas than for pure oxygen. In subsequent studies, the lowest partial pressure and shortest exposure time at which a mixed gas convulsion occurred was 1.6 bar for 40 min. The corresponding exposure for pure oxygen was 1.76 bar for 72 min.

Richard Vann with Neal Pollock (L) and Petar Denoble at the Rebreather Forum 3 (2012). Courtesy of Dick Vann / DAN Archives.

The mixed-gas convulsion occurred after 40 min at 100 fsw during a wet, working nitrox chamber dive with a 1.6 bar oxy-gen set-point in a rebreather. Heavy exercise and high breathing resistance appeared to be contributing factors. Upon decreasing the breathing resistance and reducing the oxygen pressure to 1.4 bar, 110 dives were conducted with no further oxygen incidents during 60 min exposures at 100 and 150 fsw with both nitrox and heliox.

Is an oxygen partial pressure of 1.4 bar sufficiently conservative given the potential for depth control error, the unpredictability of carbon dioxide retention, and the minimal mixed-gas exposure data? The Navy is leaning towards a set-point of 1.2-1.3 bar for rebreathers where the oxygen partial pressure fluctuates during control around a predetermined set-point.

The data shown in F4 suggest a need for separate mixed gas and pure oxygen limits but are insufficient to conclusively prove this need. As a convulsion underwater is potentially fatal, however, a cautious diver might wish to use separate oxygen and mixed gas limits until further data firmly establish they are unnecessary.

Open-Water Experience 

What can we learn about oxygen toxicity from open-water diving with mixed gas and pure oxygen? The incidents described below took place within the past year.

A mixed gas fatality occurred in a southeastern U.S. cave where two divers breathed air for 15 min and EAN 40 (40% O2, balance N2) for 45 min at depths of 80-105 fsw. The oxygen partial pressure was mostly 1.4 bar but occasionally reached 1.5-1.7 bar. After 45 min of hard swimming on enriched air nitrox, one diver convulsed and lost his regulator. His buddy could not reinsert the regulator, and the diver drowned after a failed attempt to swim him out of the cave. The oxygen exposure was, for the most part, less than the 1991 NOAA limit of 1.6 bar for mixed gas diving.

Another enriched air diver who drowned after an apparent convulsion had told friends that the NOAA limits did not apply to him. His oxygen partial pressure was estimated at 1.7-2.0 bar for a bottom time of 45-50 min.

An incident involving pure oxygen occurred in a southeastern U.S. lake. After an 8 min exposure at 300 fsw on a trimix 14/33 (14% O2, 33% He, and 53% N2) a diver decompressed on EAN 32 to 20 fsw/6msw where he switched to pure oxygen. Prior to breathing oxygen at 20 fsw/6msw  (1.6 bar PO2), his PO2 was 1.4 bar except for 7 min at 1.5-1.7 bar. After 20 min on oxygen, he unclipped from his decompression line to visit a nearby diver but drifted down to 35 fsw (2.05 bar PO2) and dozed off. (An Emergency Medical Technician, he had slept only two hours the previous night.) He was awakened by abnormal breathing and the onset of convulsions but inflated his buoyancy compensator before losing consciousness. He recovered from near drowning after rescue on the surface.

It is commonly assumed that convulsions do not occur at oxygen pressures of less than about 1.6 bar, but this assumption depends on a normal seizure threshold. Figure F5 shows the depth-time profile of an 80 fsw dive that terminated with a convulsion at 34 min. The diver breathed EAN 33 with an oxygen partial pressure of 1.26 bar. After rescue, he was found to have an unreported history of convulsions and to be on anticonvulsant medication. While such a situation is rare, it emphasizes the uncertainty of our knowledge, the need to expect emergencies such as oxygen convulsions or decompression illness, and the necessity for emergency management plans.

Statistical Modeling

Do these open-water incidents over-emphasize rare events? What is the risk of a rare event? We can estimate this risk by statistical modeling of oxygen exposure data.

Suppose the risk of oxygen toxicity increased with the concentration of the reactive oxygen species produced during hyperoxic metabolism (F2) and represented below by “X”. Suppose also that the rate of change of the local concentration of X were equal to its production minus its removal. If X were produced in proportion to the local oxygen tension (c•PO2) and removed at a fixed rate (k), its rate of change would be:

where c and k are constants. When integrated, this first order differential equation gives:

The risk of toxicity is specified by a separate function of X.

Equation 1 defines a family of rectangular hyperbolas proposed empirically for the pressure-time relationship of pulmonary and CNS oxygen toxicity. Statistical modeling derives this relationship theoretically and finds the constants c and k directly from experimental data. This allows the risk of toxicity to be estimated for any oxygen exposure. 

Figure F6 shows three rectangular hyperbolas for 2%, 5%, and 8% risks of either symptoms or convulsions. These were estimated from data on 773 pure oxygen exposures. The convulsions, represented by black dots in Figure F6, occurred at estimated risks of 2-8%. In a context of risk, an oxygen exposure limit is the depth and time at the level of risk which is judged to be acceptable. In F6, for example, the limit for a pure oxygen exposure at 25 fsw (1.76 bar) would be 49 min if a 2% risk of either symptoms or convulsions were judged acceptable. The level of acceptable risk for a chamber dive where immediate rescue is possible after a convulsion is greater than for an open-water dive where drowning is the likely outcome.

Statistical modeling can track the resolution of risk as well as its development. In Figure F7, for example, a pure oxygen diver spends 120 min at 20 fsw, 15 min at 40 fsw, and 105 min at 20 fsw. His risk increases gradually to 0.2% while at 20 fsw and rapidly to 4.1% at 40 fsw. The maximum risk of 4.3% occurs just before surfacing, after which the risk resolves in 10 min.

Unfortunately, the accuracy of the risk estimates of Figures F6 and F7 is uncertain, because human oxygen exposure data are limited and their results variable. This uncertainty encourages conservative exposure limits, at present, instead of permitting the oxygen exposure to be adjusted continuously such that the estimated risk never exceeds the risk judged to be acceptable. For mixed gas, even less data are available than for pure oxygen, and the potential for carbon dioxide retention introduces further uncertainty, which makes modeling of mixed gas risk even more problematic.

What Are “Safe” Oxygen Exposure Limits?

The choice of “safe” oxygen exposure limits depends upon the risk of convulsions that one is willing to accept. This subjective judgment is rendered all the more difficult because so few data are available from which to estimate risk and because there is so much variability in the response to oxygen exposure. Variability can be due to exercise, carbon dioxide retention, gas analysis error, oxygen set-point control, and susceptibility to oxygen toxicity from inter- and intra-individual differences.

For air or enriched air diving, a maximum exposure limit of 1.2 bar would appear to be conservative while allowing a “cushion” for oxygen partial pressure increases due to unplanned depth excursions. Perhaps 1.4 bar would be acceptable if depth could be carefully controlled.

For air or enriched air diving, a maximum exposure limit of 1.2 bar would appear to be conservative while allowing a “cushion” for oxygen partial pressure increases due to unplanned depth excursions. Perhaps 1.4 bar would be acceptable if depth could be carefully controlled. On the other hand, there are those who testify to diving safely at 1.6 bar. This may well be true, but skepticism is appropriate until these divers document their claims in the form of computer-recorded depth-time profiles with certified breathing mixtures (F5). Denoble et al. (1993) describe a project and data acquisition software which might help to provide such documentation.

For pure oxygen, commercial and scientific experience suggests that at least 30 min of in-water oxygen decompression may be possible at 1.61 bar (20 fsw) with little risk of CNS toxicity. Experimental data (F4) also suggest a low risk at 1.76 bar (25 fsw), but a small depth excursion can cause large increases in oxygen pressure. Pure oxygen diving at depths below 20 fsw is more hazardous.



Improvements in our ability to manage oxygen exposure are expected as basic studies illuminate the fundamental biochemistry and physiology, as additional exposure data become available, and as statistical modeling methods develop. Basic studies have already led to pharmacological methods for extending oxygen exposure in mice, but further work is needed before such methods are applied to humans. The diving community itself can provide some of the necessary exposure data should it adopt a rigorous approach to data collection.

Statistical modeling and computer tracking of oxygen exposure may eventually lead to guidelines for variable oxygen partial pressures to supplement single stage oxygen limits (F3). A particularly important advance that might eliminate much of the current unpredictability would be a mouth-piece sensor for measuring end-inspired and end-expired carbon dioxide. In the meantime, a patient and conservative approach to oxygen exposure management is appropriate to minimize the frequency of mishaps such as those of the past year.

Richard Vann performing a Flicker Test. Courtesy of Dick Vann / DAN Archives.

Additional Recources: 

Underwater Physiology VIII. Eds. A.J. Bachrach and M.M. Matzen. UMS, Inc., Bethesda.

Butler F K Jr., and Thalmann E D 1986. Undersea Biomed. Res. 13(2): 193-223. Clark J M 1974. The toxicity of oxygen. Am. Rev. Resp. Dis. 110:40-50.

Clark J M 1993. The physiology and medicine of diving, 4th edn., pp. 121-169. Ed. P.B. Bennett and D.H.Elliott. London: W.B. Saunders.

Donald K 1992. Oxygen and the Diver. The SPA Ltd. Worcs.

Proceedings of repetitive diving workshop. M.A. Lang and R.D. Vann (eds.). American Academy of Underwater Sciences. AAUS DSP-RDW-02-92. Pp. 45-54. Decompression in surface-based diving. Eds. I. 

Nashimoto and E.H. Lanphier. 36th UHMS Workshop. Pub. No. 73(DEC)6/15/87. Pp. 90-100.

Lanphier E H, Dwyer j V 1954. NEDU Report 11 -54.

Lanphier E H 1955. NEDU Report 7-55. Lanphier E H (Editor). 1982. 

Undersea Medical Society Publication Number 52WS (RC) 1-25-82.

Lanphier E H 1992. The story of CO2 build-up. aquaCorps j. 3(1): 67-69. Technical Diver 1992. 3(2): 2-10. NOAA Diving Manual 1979, 1991 2nd edn., 3rd edn. U.S. Department of Commerce, December 1979.

Oury T D, Ho Y S, Piantadosi C A, Crapo J D 1992. Proc. Nat. Acad. Sci. 89: 9715- 9719.

Piantadosi C A, Clinton R L, Thalmann E D 1979. Undersea Biomed. Res. 6: 347-356. Schwartz H J C 1984. NEDU Report 1 3-84. U.S. Navy Diving Manual, Change 1, Tables 9-20 and 13-1.

Vann R D 1988. Oxygen toxicity risk assessment. Final Report on ONR Contract N00014-87-C-0283. 

The Physiology and Medicine of Diving, 4th edn., pp. 376-432. Ed. P.B. Bennett and D.H. Elliott. London: W.B. Saunders. Warkander D E, Norfleet W T, Nagasawa G K, Lundgren C E G. 1990. Undersea Biomed. Res. 17(6): 515-523.

Dr. Vann passed away on 17 APRIL 2020. See: Richard D. Vann: Legendary researcher and champion of dive safety by Frauke Tillmans, Ph.D., and Petar Denoble, M.D., D.Sc.


Be A Part of History: To access our treasure trove of dive history and become a member, visit us at: www.HDS.org. We are also on Facebook: Historical Diving Society USA

Cave

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.

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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.

Santa Ana Cuauhtémoc.

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.

The team posing during a quick pit stop on the way from Tehuacán to Santa Ana.

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.

Sunrise view from the field house.

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.

Sleeping quarters in the field house.
Andreas organising gear for the first dive day.
Steve packing the truck while brushing his teeth.

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.

The 4×4 squeezing out of the narrow driveway on the way to the canyon.
The road down the mountain was often shared with cows and horses. Or broken down cars.
Ben and Jon scrabble over boulders towards the cave.
The second river crossing.

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.

Rob preparing to go under.

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”.

Andreas and Jon during a clean up dive hauling gear back over the waterfall.

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.

SJ trying to stay sane while knotting line and being stuck at the field house during the first week.

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.

Squeezing sugar cane into juice before the fermenting process.
The team watches with great interest.

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.

Steve organising his dry suit in front of the cave entrance.
Andreas’ pre-dive excitement.

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.

Jon getting dressed before a not-so-great dive.
Steve getting in the zone.

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.

Steve and Zeb making an aguardiente offering to the cave for good fortune.
Zeb and Steve mixing cave dinner and packing dry tubes for the first camping attempt.

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.

Steve watching as Andreas and Rob descend on the way to camp 1.
Jon swimming out of the cave though the first arch.
Jon swimming though the double arches in Sump 1.

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.

A festive parade walking down the mountain through town.
Tacos were hugely enjoyed by the team.

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.

One of the very many o-ring failures during the 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.

A very broken bracket.

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.

Team discussions in the field house.

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.

Zeb stuffing a dry tube with Jon posing questionably in the background.

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…”

Ben swimming above river pebbles in the cave.
Ben ascending one of the many ups and downs in Sump 1.

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.

Ben showing the small cavern and explaining that the three missing team members are camping in the cave right now.
Late lunch on the way back at the family’s camp.

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.

Jon swimming into the cave through the first arch.

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…

Jon’s HUD glowing while swimming through Sump 1.

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.

Visibility got worse with the water levels dropping towards the end of April.

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.

Jon swimming through a tunnel in Sump 1.

Additional Resources:

Wikipedia: Sistema Huautla
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. 

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