Zen and the Art of Mexican Cave Navigation
Do you know your arrows, cookies, REMS, TEMS, presence, personal and team markers? Singley or in combination? Jump protocols? If your answer to any of these is ‘Nada’ you may find yourself lost in a Mexican cave. Fortunately, underground explorer and instructor Natalie Gibb has agreed to provide safe navigation through the watery wilds of Riviera Maya. Grab your markers.
Photos and text by Natalie Gibb
The most idiotic cave navigation mistake I ever witnessed happened on a dive in Cenote Chan Hol about eight years ago. Exiting the cave, my buddy and I approached our jump line, but were pushed out of the way by a solo diver who elbowed his way over our jump line and into the side passageway.
I stared in disbelief: the diver had not installed his own jump spool, he had just swum over mine. Had he even noticed my jump spool? Did he realize he was swimming into a side passageway? My buddy and I both had personal markers on the main line, as well as markers on our spool. My jump spool had a neon green line, and all of our markers were clearly marked with reflective tape and our names. It was not subtle.
I signaled with my light to the solo diver, and gestured, That’s my jump! He shrugged nonchalantly, turned around, swam back over my jump spool to the main line, and trundled on down the principal cave passage. If I hadn’t had a regulator in my mouth, my jaw would have dropped open.
Since that day, I have viewed cave navigation in Mexico as a two-part responsibility:
- Don’t confuse yourself or your teammates.
- Don’t confuse other divers.
As one can imagine, the latter is the more difficult to accomplish.
Cave Training Agency Differences vs Regional Navigation Differences
This article began as a comparison between different cave training agencies’ navigational standards. Interestingly, there is not much to compare! From my research it seems that TDI, IANTD, NSS-CDS, RAID, PSAI, and GUE agree on a basic philosophy: specific navigational markings and protocols are often region-specific and even cave-specific. Every instructor from every training agency I have spoken with has stated more or less the same thing: While agencies may have general guidelines, navigation procedures are left up to the instructor.
As a TDI instructor, my students have mentioned that it’s frustrating not to have exact navigation protocols written out in the textbook. I can understand their annoyance, but from an instructor’s standpoint, I prefer it this way. Not having rigid operating procedures published in training manuals allows cave instructors to teach to their local protocols.
While GUE is perhaps the clearest of all agencies, most training agencies agree on basic best practices, including the need to use personal markers to mark the exit side of intersections, the acceptance of cookies as a navigation tool, and the need to maintain a continuous guideline to the open water by using jump spools and reels.
If regional peculiarities, as opposed to dogmatic navigation standards, dictate protocols, the question becomes: What makes Mexico cave navigation different? How should you navigate there?
What Makes the Caves in Mexico So Confusing?
Several factors make the navigation in Mexico challenging. I believe one of the major factors is that Mexican caves usually lack strong flow. Divers who have grown up navigating caves in other regions are often used to the orienting movement of the water, providing a physical sense of directionality; it’s unlikely to become truly confused about your direction of exit if the water is pushing you “out.” Of course, there are exceptions to this rule, but in general I find noticeable flow is an excellent, almost subconscious indicator of directionality, and it’s missing in most Mexican caves.
The cave systems in Mexico are very complex, with multiple cenote entrances and exits. This may sound like a benefit, but in reality, it’s not possible to exit the water in all of the cenotes, and even if you can get out of the water, you could find yourself deep in the jungle without a road or the ability to call for help. Several years ago, a lost diver perished from exhaustion after blindly following the cave arrows to an alternate exit, then wandering lost in the strong jungle heat.
The arrows in Mexico’s caves are likely to alter directions throughout a single dive, changing to indicate whatever the person who placed the arrows felt was the nearest surface or exit. That person could have been wrong. The route to the “nearer entrances” might involve 16 jumps and a no-mount restriction. The lines could have been cut or changed since the arrows were placed. There are no warnings, and there is no system in place, although we are working on one!
Additionally, Mexican cave navigation follows its own logic, different from that of other regions. We use multiple arrows, directional changes, and pretty much zero distance markers. Areas far back in the cave, or areas that were explored in past decades, may have “reach gaps,” often unmarked jumps that end centimeters from the main line, as well as blind T’s, and other navigational monstrosities. My favorite recent bizarre navigation find was an “H” intersection.
Don’t Depend on the Arrows!
It’s best to view cave lines and arrows as secondary navigational tools. What’s your primary navigational tool? The cave itself.
Plan this out ahead of time if you have a map, and confirm it during the dive by looking back and actually checking your compass once you have completed a jump or T. If you know your directions, you won’t have to rely on the pesky arrows. The magnetic field of the earth is unlikely to change during a single dive. In a pinch, caves north of Chan Hol Cenote are typically upstream to the northwest and downstream to the southeast. This changes a bit south of Chan Hol, due to fracture zones.
Cave tunnels often have a general direction, even if they twist and turn a little. Carry a compass and keep track of which direction you are traveling, in many places it is more or less uniform throughout the dive. You don’t need a degree heading; northwest-ish in and southeast-ish out will be enough to determine your exit direction if you get confused. If you make a 90 degree turn, you should notice the heading change. If you make a jump off the mainline or navigate a T intersection, it’s essential to know the exit direction along the mainline once you return to that point.
Natural navigation is also important. A diver can observe many key features of a cave without a doctorate in hydrogeology. Is the cave big and wide, or small and restricted? What color is the floor sediment? Are there any unusual speleothems that catch your eye? Do you feel water flow? Was there a major depth change? Maybe you swam over a collapse? If you swam into the cave through a small, restricted tunnel, and you turned around to exit but passed no restrictions, you are going the wrong way.
This simple observation would have saved multiple dive teams who all made the same mistake. They entered Cenote Kalimba and made a jump from an arrow pointing away from their exit towards Grand Cenote. When they retrieved the jump spool, they swam toward Grand Cenote instead of Kalimba, through enormous cave passages instead of restrictions. They blindly followed the jump arrow, which was not pointing toward their exit; they did not correct themselves, though the cave passageway they were in was very different from the cave passageway leading to their original exit. The more you can learn about geology, the easier it is to read a cave, remember it, and notice if you are going the wrong way.
Mexican System Markers
Of course, lines and arrows are important to understand, and they give divers information about the general layout of the cave. While there is no standardized system of marking cave lines in Mexico, most caves follow a somewhat logical method. Again, a diver should never assume that the arrows are actually correct or that exits indicated by the arrows are accessible.
In Mexico, jump lines are usually indicated by a single arrow on the main cave line. This is different from Florida, where jumps are often marked with two arrows (a double arrow). Interestingly, double arrow jumps do exist in Mexico, but they are rare and have a special significance – they indicate a particularly important jump, such as a jump to complete a circuit or a traverse. Divers familiar with popular Mexican caves may have noticed this in the double arrow jump to the Death Arrow Passage in Cenote Maya Blue, or the double arrow jump to complete the circuit in Cenote Minotauro.
There are also a great number of secret or hidden jumps that are not marked at all on the mainline. Examples include the jump to the Chinese Garden at Cenote Tajma Ha, or the jump to the Room of Tears in Cenote Carwash.
From a practical standpoint, the lack of double arrow jumps means that line-to-line jump connections are generally not used in Mexico, and a better protocol is to tie into a line marker (more on this to follow.)
As mentioned above, arrows will often change directions along a single cave line. If you swim far enough into a Mexican cave, it is common to encounter a set of two arrows pointing in different directions. These are commonly referred to as a directional change or opposing arrows. The purpose of opposing arrows is to indicate that there is an exit equidistant in each direction, and to draw the diver’s attention to the fact that the line markers past the opposing arrows will point away from the diver’s original exit.
Similarly, a set of three arrows, with two pointing in one direction and one pointing in the opposite direction is intended to indicate two exits, with one being closer in time or distance. However, keep in mind that system arrows may also flip direction without opposing arrows as a warning. It’s your job to notice this. Common practice is to leave a line marker (cookie or REM) on your team’s exit side of the opposing arrows or first flipped arrow to confirm your direction of exit.
T Intersections. T intersections are generally rare in Mexico. Most navigation is accomplished with jump lines or gaps. T’s are most often present at major intersections, when the cave splits into two equally sized tunnels. At a T, one or more exits are indicated by system arrows pointing away from the T toward the exit. T’s are also common close to cenote entrances, with a double arrow indicating the presence of a cenote off the main tunnel. As with opposing arrows, good practice in Mexico is for the cave team to mark their exit side of the T with a personal marker.
At this point, an important clarification must be made: never blindly follow system arrows toward an exit that you have not personally confirmed. Cave arrows may point toward an exit that requires multiple jumps, a no-mount restriction, or that is blocked by a collapse that occurred after the original line was laid. Sometimes line markers are simply wrong. Always return to your original proven exit regardless of the arrow direction, and you will stay safe.
Three main types of personal markers are commonly used in Mexico cave diving, and many divers now 3D print or craft their own, very unique styles. The most common markers are Arrows, Cookies, and REMs (referencing exit markers). It’s important to consider directionality of personal markers and where in the cave they can be used.
Arrows. Personal arrows are commonly used to anchor jump spools. However there are a few situations in which I feel that arrows should not be used. In accordance with Rule #2: don’t confuse other divers, it’s generally frowned upon to place personal arrows directly next to system/permanent arrows. A personal arrow, no matter how nicely marked, may look like a system marker to another, less observant dive team, and placing a personal arrow very close to a system marker may lead other divers to believe a double arrow is present. Remember, double arrows have a special significance in Mexico.
At jumps, placing a personal jump arrow within a few inches of a system arrow can make it look like a double arrow jump. Instead, the team should separate the personal jump arrows from system arrows by 0.5m/1.6 ft (if possible) to avoid confusion.
At T-intersections or directional changes, using a personal arrow to mark the team’s exit may confuse other teams, as it may look like a double arrow indicating a nearer exit. For these applications cookies or REMs are preferred.
Cookies. Cookies were a Mexican cave diving innovation invented by explorer/instructor Daniel Riorden in the late 1990s, in accordance with Rule #2. Cookies are round, or non-directional, and are typically not used as system markers. The shape makes them clearly personal markers, which simplifies marking intersections and directional changes as all other cave teams know that a cookie indicates nothing about general cave navigation. Cookies may also be used to mark a cave team’s reels.
Cookies are appropriate for most uses, with the exception of anchoring jump reels or spools. It’s inappropriate to tie a reel or jump spool onto a lone cookie, as the cookie alone does not indicate a direction of exit at the intersection, a clear violation of Rule #1: Don’t Confuse Yourself or Your Team Mates.
REMs. REMs (Referencing Exit Markers), invented by Bil Phillips, are a common sight in Mexico and uncommon in most other regions. They are rectangular markers, with slats for line attachment closer to one end. The longer end points towards the team exit.
REMs are interesting in that they are directional, but can not be confused with arrows. They can be used for the same functions as both arrows and cookies, eliminating the need for a diver to carry a variety of personal markers. In my conversations with Bil, he told me this was not his original intention for the markers, but he liked that people were getting creative with them.
REMs have one other useful feature, which is that the exit side of the marker has enough space to write a serious note. This can be left on the line for notes to a buddy or for personal notes, in place of wetnote pages or other methods.
Creating Your Personal Markers
No matter what style of markers a diver chooses to use, their markers should be clearly personalized. All organizations teach divers to write their names, nicknames, or an identifying word on their personal markers to indicate who the marker belongs to. This helps to fulfill both Rule #1 and Rule #2, making the markers easy to identify.
An additional method of personalizing cave markers that I recommend is to make them touch contact identifiable. I ask my divers to modify their markers in a way to make them uniquely identifiable in zero visibility, such as cutting a corner off, punching holes in them, or adding something as simple as a cable tie in order to physically identify them if visibility is lost.
Many divers also like to number their markers, which allows divers to account for all markers and refer to a certain navigational decision in the debriefing or in their notes – for example, “the jump where the diver put down marker number three.”
A word of caution here – it is possible that some system or permanent markers have a diver’s or dive team’s name on it. These are exploration markers. They typically have the team’s names, the date or year of the project, and maybe even a keyword or number written on them, and are not marked with touch or physical identification cues. Exploration markers are so much fun to find! Every time I run across a historic marker with an explorer’s name on it, I feel a kinship to the original explorers and imagine what the person must have felt like, being the first human in this cave. If a diver has a question about the cave, the presence of the explorer’s name indicates who to contact with questions.
Team vs Individual Markers
Now we are getting into the great debate! Which is better, team or individual markers?
When cave divers are using the team marker approach, the diver in the front of the team places all markers for the team, marking and placing jump lines, marking T’s, and placing any other markers that are deemed necessary. Other team members carefully observe the diver in front, and confirm that the markers are correctly placed. The advantage of this style of marking lines is that there are fewer markers on the line, and that it is slightly faster than having each diver personally mark intersections. UTD’s Andrew Georgitisis rather infamously premiered a REM-style “TEM (Team Exit Marker)” in a Facebook video last year advocating the use of team markers.
If a cave team uses the individual marker approach, each diver places a personal marker to mark the team’s exit at every point of navigation, including jumps, T’s, and directional changes. The advantages of this system are that each diver physically participates in the cave navigation, which helps to fix it in his or her memory. Additionally, more markers are easier to see, and add an additional degree of personalization to the navigation. If my team is using all REMs, and your team is using arrows and cookies, just the fact that a jump is marked with arrows and cookies means that it is not mine.
However, the most important argument for this method, is that individual markers create a level of redundancy in navigation. If a diver accidentally ties into a jump arrow that points away from the team’s exit (and no one notices), but the other divers mark the exit side of the intersection correctly with their cookies or REMs, it is clear that there is a navigational discrepancy upon exit. Divers then know to refer to their compasses and natural navigation clues to determine the correct direction to exit, instead of blindly following the arrow in the wrong direction when leaving the cave. This can be life saving. GUE uses this approach, though most training organizations do not state a preference.
Finally, some instructors teach that in an instance of team separation, divers leaving the cave remove their personal markers, while leaving the jump lines and other team member’s markers in place. This indicates to the team who is still in the cave, and who has made it out. This method of dealing with a team separation is debated in the cave community.
The advantages of individual markers as far as clarity, redundancy, and problem solving make the individual marker method my choice.
Presence markers are personal markers that are placed at the beginning of a cave line when the line starts in open water and no primary reel is required, or on the primary reel line when a reel is run from the open water. Presence markers can be team markers or individual markers, and indicate the presence of the team in the cave. When individual presence markers are used, they additionally indicate the number of divers in the cave.
If a team must exit in complete zero visibility all the way to the end of line in the open water, personalized presence markers allow the divers to confirm that they have navigated correctly to the open water and may safely surface. No agencies seem to have a firm stance on presence markers, nor is this an established local protocol in Mexico. However, I quite like presence markers and use them in my courses and personal diving.
When cave divers swim from the main cave line to a secondary line (jump line) in a side passage, they have made a jump. As all modern training organizations require cave divers to maintain a continuous guideline to the open water, the cave teams install a jump reel or spool to connect the mainline to the secondary line. This is a visual reference upon exit and allows the team to navigate out of the cave in zero visibility by touch.
How should a team install a jump line? Good question! Once again there are many options, and again, there is no “right” answer as long as the marking is clear to other divers, the team that installed the line, and has some sort of marker indicating the direction of exit. It’s helpful to use colored line on jump spools as opposed to white line, which is typically used for permanent cave lines, as this makes the temporary nature of the line obvious to other teams. Here is a non-exhaustive list of options.
Line to line connection. When a team loops a jump spool directly around the cave line (as opposed to tying the spool into a line marker), the jump is a line-to-line connection. This sort of navigation is more commonly observed in Florida and other locations where jumps are indicated by two arrows. The team can tie the jump spool between the two jump arrows without risk of the jump line sliding out of position. Using this method, two arrows are present (if they are pointing in the correct direction) to indicate the team’s direction of exit.
In Mexico, where jumps are indicated by a single arrow which may be pointing away from the team’s exit, this method is generally frowned upon. Lines that are not anchored by a line marker are likely to slide out of position, particularly in zero visibility.
Jump from a system or permanent marker. One common method of installing a jump line in Mexico is to loop the jump spool’s line around the system arrow. This fixes the jump spool’s line in place and avoids the problems mentioned above. However, it is important that the team carefully observes the arrow’s direction. Divers should never attach a jump spool to an arrow that points away from the team’s exit. Doing so has been implicated in numerous fatalities, including several well-known accidents at Cenote Kalimba. If the team chooses to use individual markers, team members place cookies or REMs on the exit side of the intersection created by the mainline and jump line. Jumping from a system marker is not possible when the arrow has already been used by another team, or if the jump is unmarked, and it’s a terrible idea if the arrow points away from the team’s exit. Always be prepared to use the final jump method.
Jump from a personal marker. To jump from a personal marker, the team leader places an arrow or REM on the line indicating the team’s exit direction, and ties into the personal marker. If individual markers are used, each member of the team places a cookie or REM on the exit side of the intersection created by the jump spool line and the main line. Again, this can be used as standard protocol, in the case where the jump arrow is pointing away from the team’s exit, in the case where the arrow is already used by another team, or in the case where the jump is not marked by a system or permanent arrow. This is my preferred method because it avoids uncertainty, and I use the same exact protocol for every jump, regardless of what is present in the cave.
If a team chooses to jump from a personal marker when a system arrow is present, what side of the system arrow should the team jump from? I prefer to install my personal marker at least arm’s length away from the system arrow to differentiate my navigation from the system navigation. I like to jump from behind, or from the cave side of the direction the system arrow points, because this leaves the permanent marker on the exit side of my intersection, allowing other teams already in the cave who have noticed this arrow to have an unobstructed reference to the exit.
Marking Reels and Spools.
When tying into the main cave line or a jump line, many divers, myself included, like to place a marker on the reel or jump spool line. On a primary reel line, these markers serve as presence markers. The markers also help to visually and physically identify the diver’s line in zero visibility, especially in the event that there are multiple spools or reels tied into the cave line. Finally, this helps to unambiguously identify similar types of spools and reels, helping to avoid removing a different team’s reel or spool by mistake. Not everyone does this, and some consider it redundant and unnecessary, as realistically divers should be able to identify their own reel or spool. I feel like it is extra clear, so I like to mark my spools and reels.
Navigating Around Multiple Cave Teams
A brief note is warranted to mention protocols for navigating when there are multiple teams in the cave. The general rule is to attach any spools or reels cave side of another team’s spools, unless there is a very large space exit side. The same rule applies to the placement of jump spools. When navigating a T, place your markers on the exit side of the other teams markers, so that their markers are the first encountered during the exit.
What’s the Number One Navigational Mistake I Should Avoid as a Tourist Cave Diver in Mexico?
The number one navigational mistake to avoid is to blindly follow arrows and lines to your death. Nearly every Mexican cave fatality I am aware of involves navigational errors exacerbated by camera use. Mistakes include divers jumping off system markers pointing away from their exit and subsequently going the wrong way when returning from their jump line, or teams getting turned around when taking photos. Mark all intersections methodically with personal markers, understand the overall compass heading of your planned dive, and be aware that arrows do not always point towards your team’s exit, or even an accessible exit. Notice if arrows change directions and mark them accordingly. In the event that you become confused, use your compass heading and natural navigation in conjunction with the cave lines to find your way home.
Mexico Cave Navigation Is an Art
Navigation in Mexican caves is subtle and often confusing. My shop teaches the system of navigation that we like the best, but there is a wide variety of protocols used in the area, and I wouldn’t say that any one way is necessarily incorrect. I am probably in the minority with this opinion, but I would say as long as you and your teammates stay oriented, and you don’t confuse other cave diving teams, have at it. No matter what you do, someone else is going to think you are wrong. Chin up. Did you confuse your team? Did you confuse other teams? No? Good enough.
In fact, I actually think it’s great that there are slight variations in the way people mark their lines. If I use REMs, and you use arrows, then I can easily and quickly identify my markers simply because they are different from yours. Nice!
One cannot be too strict with navigational protocols in Mexico, because while a team can have a standard way of placing and marking lines, the established lines in Mexico do not have standardized systems of marking. Maybe you like to anchor your jumps on personal markers behind the system markers? I do. But it’s not always possible based on the layout of the cave. You must adapt your navigation to the environment, and that’s why it’s an art! As unsettling as this is to many people, there are no absolutes.
So Many Options! What to Do?
Yes, it can be confusing, and determining your personal or team navigation style takes some thought and discussion within the dive team. There are options, and as grown up adults, you get to choose what is clearest and easiest for you. When evaluating a navigational method, just make sure it doesn’t violate the rules of safe cave navigation:
- Don’t confuse yourself or your team mates.
- Don’t confuse other divers.
Based on the previous discussion and general cave training organization guidelines, we can add two additional points.
- Maintain a continuous line to the open water using spools and reels as needed.
- Have a marker on the line indicating your direction of exit at key points of navigation, including jumps, T intersections, and directional changes.
In addition, I would urge dive teams to be consistent. Come up with a protocol, whether it’s team or individual markers, anchoring your line on system markers or personal markers, and use the same protocol on every dive. This makes in-water decision making easier and helps to avoid confusion. Review your navigational protocol with new team members, and agree to a protocol before entering the water the first time.
No matter what navigational procedure your team chooses to use, keep in mind that while important, plastic markers and the cave line are secondary navigational clues. Your first source of navigation is the cave itself: directionality, formations, physical features, and flow if it exists. Unlike lines, cave features are unlikely to be removed or changed. Learning to observe and read the cave will increase your safety and enjoyment!
Natalie L Gibb’s passion in life is underwater cave exploration and conservation. With her exploration partner Vincent Rouquette-Cathala, she has led her team to discover over 20 previously unknown cave systems in Mexico’s Yucatan Peninsula, mapping more than 80 kilometers/nearly 50 miles of cave passageways. She is a public speaker, author, photographer, and videographer, and a member of the Woman Diver’s Hall of Fame. Natalie is co-owner of Under the Jungle, a cave diver training center in Mexico, and a TDI Full Cave Instructor.
N=1: The Inside Story of the First-Ever Hydrogen CCR dive
This Valentine’s Day, Dr. Richard Harris, aka ‘Dr. Harry,’ and the Wetmules made the first reported hydrogen (H2) rebreather dive to a depth of 230m/751 ft, in The Pearse Resurgence, New Zealand. The 13 hour dive, which was nearly two years in planning, was a field test to determine the efficacy of using hydrogen to improve safety and performance on über-deep tech dives. Harris’s dive was the deepest “bounce” dive in approximately 54 experimental H2 dives—the majority SAT dives—that have been conducted over the last 80 years by military, commercial and, yes, a group of technical divers. Now in this first published account, InDEPTH editor Ashley Stewart details the inside story behind the dive, a dive that will arguably be remembered 100 years from now!
By Ashley Stewart. Images courtesy of Simon Mitchell unless noted.
On March 11, a little more than three weeks after completing what is believed to be the first-ever rebreather dive with hydrogen as a diluent gas, Dr. Richard “Harry” Harris convened the group of scientists and researchers who had spent years helping to plan the attempt.
He started with an apology. “All of you had the sense that you were party to this crime, either knowingly or suspecting that you were complicit in this criminal activity,” Harris, an Australian anesthesiologist and diver known for his role in the Tham Luang Cave rescue, told the group.
The apology came because the dive was dangerous—not just to Harris who was risking his life, but for the people who supported him were risking a hit to their reputations and worried their friend may not return home. Harris and his team put it all on the line to develop a new technology to enable exploration at greater depths.
A significant challenge to deep diving is an increased work of breathing and CO2 buildup as breathing gas becomes more dense at greater depths. This can not only culminate in fatal respiratory failure but also increases the risk of practically everything else divers want to avoid, like inert gas narcosis and oxygen toxicity. For this reason, helium is favored by divers for its low density and non-narcotic effect. However, at such great depths, helium increases the risk of tremors and seizures from High Pressure Nervous Syndrome (HPNS). This can be ameliorated by keeping a small amount of narcotic nitrogen in the mix. The problem is that even small amounts of nitrogen makes the mix too dense past 250 meters.
Harris’s experiment would determine if divers can turn to an even lighter gas: Hydrogen, the lightest in the universe. Hydrogen is about half the density of helium. It’s also slightly narcotic and hence thought to ameliorate HPNS, thus allowing elimination of nitrogen from the mix.
The addition of hydrogen into a breathing gas, however, comes with one small technical uncertainty—the extremely explosive nature of hydrogen. History confirmed this reality with the 1937 Hindenburg disaster in which the hydrogen-filled dirigible airship burst into flames. As Harris tells it, he set out to dive hydrogen in his diluent gas while avoiding the nickname “Hindenburg Harry.”
Hydrogen in the Mix
Why would anyone attempt to breathe hydrogen? Harris and his colleagues have spent more than a decade and a half exploring the Pearse Resurgence cave system in New Zealand. This extremely challenging, cold water cave system (water temperature is 6ºC/43ºF) has been explored by Harris and his team, who call themselves the Wetmules, to a maximum depth of 245 meters/803 feet in 2020. Their gas density at depth was 7.2 g/l, significantly above the recommended hard ceiling of less than 6.2 g/l.
Diving past this point introduces increased risks, not only of CO2 buildup, but narcosis, decompression sickness, HPNS, cold breathing gas, having adequate gas supply or bailout, and isobaric counter diffusion (ICD) in which different gasses diffuse into and out of tissues after a gas switch causing bubble formation and related symptoms, cold breathing gas, and having adequate gas supply or bailout.
Divers have been examining hydrogen as a breathing gas for decades. The Swedish Navy was the first to experiment with hydrogen as a possible deep diving gas during World War II. The U.S. Navy in a 1965 paper proposed replacing helium with hydrogen due to projected helium scarcity. Later, beginning in 1991, researchers at the Naval Medical Research Institute (NMRI) in Bethesda, Maryland spent a decade studying hydrogen’s potential physiological impacts and biochemical decompression. French commercial diving contractor Comex (Compagnie maritime d’expertises) launched its hydrogen program in 1982, and the Undersea Hyperbaric Medical Society (UHMS) held a workshop “Hydrogen as a Diving Gas,” in 1987.
Even technical divers considered hydrogen. Legendary cave explorer Sheck Exley considered hydrogen in the early 1990s to mitigate HPNS symptoms, which are ultimately believed to have contributed to Exley’s death at Zacatón in 1994. Nearly all of the experimental hydrogen work up until this point used surface-supplied systems and saturation diving versus self-contained diving, and none of it, as far as we know, has been done with a rebreather.
The primary objective of Harris’ hydrogen experiment was to address the issue of increased work of breathing. Harris’s team had previously encountered CO2 incidents at the Pearse Resurgence. In one incident, while at 194 meters/636 feet, explorer Craig Challen—Harris’s primary dive buddy since 2006—lost buoyancy but was unable to find his buoyancy compensating button quickly. He kicked up a couple of times to stop his descent and immediately got a CO2 hit. Challen was able to grab the wall, calm down, slow his breathing, and survive. Based on such incidents, it’s clear to the team that they have reached the limits of the gas. “I feel we are on the knife edge all the time,” Harris said, in terms of physiology and equipment.
While hydrogen in the diluent breathing mix was expected to address increased work of breathing, the rest of the issues associated with deep diving were “major unknowns,” and some (such as respiratory heat loss) were potentially even made worse by hydrogen.
“At what depth do the risks of introducing this new technology outweigh the risks of carrying on with trimix?” Harris said. “That’s a very difficult question to answer. At some point we are going to have to consider different technologies and, at this point, hydrogen is perhaps the only one available to us.”
H2 Working Group
In 2021, the year after Harris completed his deepest dive at the Pearse Resurgence, InDepth editor-in-chief Michael Menduno was taking a technical diving class and reading about the government looking at hydrogen as a diving gas again. “Technical divers should be at the table,” Menduno said he thought to himself at the time, “our divers are as good as anybody’s.” He called John Clarke, who had spent 27 years as scientific director of the U.S. Navy Experimental Diving Unit (NEDU), and discussed setting up a working group. Menduno’s next call was to Harris, who had shared his troubles with gas density at the Pearse Resurgence. Harris had also, separately, been thinking about hydrogen.
The so-called H2 working group met for the first time in May 2021 and included many of the top minds in diving medicine and research, including Clarke, NEDU’s David Doolette and Greg Murphy, research physiologist Susan Kayar who headed up the US Navy’s hydrogen research at the Naval Medical Research Institute (NAMRI), along with her former graduate student Andreas Fahlman. There was diving engineer Åke Larsson who had hydrogen diving experience, deep-diving legend Nuno Gomes, decompression engineer JP Imbert who had been involved in COMEX’s Hydrogen diving program, and anesthesiologist and diving physician Simon Mitchell. The group was later joined by Vince Ferris, a diving hardware specialist from the U.S. Navy, and explorer and engineer Dr. Bill Stone, founder of Stone Aerospace.
The working group met regularly with the goal of figuring out how one might possibly operationalize hydrogen for a deep technical dive using the Resurgence as an example. During one of their meetings, Clark used a breathing system simulator built for the Navy to predict how hydrogen would affect gas density in a closed circuit rebreather at depths to 300 meters/984 feet.
To Doolette, who has known Harris for decades and supervised his Diploma of Diving Medicine project in 2001, it was immediately clear this was not a hypothetical discussion. “Unlike some of the scientists, I was under no illusion that the question before the working group was fiction, I knew that Harry was likely to try a H2 technical dive in the Pearse Resurgence,” said Doolette, a cave explorer in his own right, who has laid line in the Resurgence.
By fall of 2022, it was clear to many in the group that Harris was going to attempt the dive. The group had mixed feelings ranging from cautious optimism to comments like, “My friend is going to die.”
Doolette was concerned Harris and Challen would not survive the dive due to either ignition of hydrogen—in the worst case, inside the rebreather at depth—or a serious adverse response to respiratory heat loss (the latter was especially if Harris attempted diving beyond 245 meters/803 feet as he had originally planned) he said. “I have known Harry for longer than most in the group. I encouraged him to take up cave diving, so I felt a personal responsibility toward him,” Doolette said. “I have a lot of experience in operationalizing new diving technology. My goal was, if unable to discourage him, to force him to focus on the important issues.”
Leading up to the dive, Menduno scheduled Harris to give the banquet talk about the expedition at the Rebreather Forum 4 industry meeting in April. The outcome of the dive, of course, was uncertain, and the two had to make an alternate plan in the event that Harris did not return. “We had to say we were going to talk about your dive one way or another,” Menduno said. “If you don’t make it back, Simon Mitchell is going to have to give a presentation about what went wrong. Harry made some typical Harry joke like, ‘Well, as long as you don’t stop talking about me.’” Harris’s lighthearted tone betrays how seriously he took the dive and its preparation, people close to him said.
While no one involved was taking as big a risk as Harris and Challen, they were risking a hit to their professional reputations by being associated with a controversial dive, especially in the event of a tragic outcome.
“At heart, I’m an explorer, and that was pure exploration,” Mitchell, who was the diving supervisor on Harry’s dive, said when asked why he would take such a risk. “Exploration in the sense that we were pioneering a technique that hadn’t been used for quite some time and never in technical diving, not deep technical diving.” He also emphatically added, “I was more worried about my mate dying than about my professional reputation.”
Later, in planning Harris’s trip to the RF4 event, Menduno had occasion to speak to Harris’s wife, Fiona who brought up the dive.
“She said to me ‘I hope Harry is going to be OK’,” Menduno said. “I had no idea how much Harry told her, what she knew and didn’t know. All I could say was he’s got the best people in the world on his team, and if anybody can do it, he can.”
“We all held our breath and waited,” Menduno said.
‘Hydrogen Trials’ at Harry’s House
Ahead of the dive, Harris was preparing at home. The first thing Harris said he had to get his head around was—no surprise—the risk of explosion, and how to manage the gas to mitigate that risk. The potential source of explosion that Harry was most concerned with was static ignition within the CCR itself, plus other potential ignition sources like electronics, the solenoid, and adiabatic heating. Industrial literature—or “sober reading” as Harris calls it—suggested that the tiny amount of static necessary to initiate a spark to ignite hydrogen is .017 mJ, 400 times less than the smallest static spark you can feel with your fingertips and several hundred times less than required to ignite gasoline. “It ain’t much, in other words,” Harris said, noting that counterlung fabric rubbing against itself could generate just such a spark.
Ultimately, Harris came across research that suggested that static decreases with humidity. “I started to feel like there was no source of ignition inside a rebreather, but then again I said to myself, ‘Harry you only need to be wrong once’.”
The other concern was whether he could actually fill hydrogen safely while decanting, or filling one tank from another at the same pressure, and boosting the gas to reach higher pressures.
“I decided there is only one way to actually resolve this and that is to retire to the shed, order a sneaky bottle of hydrogen, and without telling my wife what was going on down the back of the house, start to actually have a bit of a play with this,” Harris said.
First Harris had to make his own DIN fitting (though not out of the ordinary for the anesthesiologist who built and tested his own rebreather before buying a commercial one in 2002) to decant the gas. Next he took his dual Megalodon rebreather with 100% hydrogen in one diluent cylinder and 100% oxygen in the other to the “test bed” in his backyard—his pool—and started to introduce hydrogen into his rebreather.
“Putting an explosive device into water was perhaps not the most logical approach because it becomes more like a depth charge than a bomb, but I thought, ‘Well, at least it might contain the blast somehow into the pool.’ I knew if I broke the back windows in the house or worse, my life wouldn’t be at risk just from the hydrogen. There would be bigger trouble afoot,” Harris said. “I left the lid of the rebreather unclipped in the vain hope it would spare me and the pool and the dog, who was helping with this experiment.”
He pressed the button of the Automatic Diluent Valve (ADV) on his rebreather, introducing hydrogen to the loop, and finally activated the solenoid before he started breathing from it. The first breaths were pleasant, he said. “It did feel very light and very slippery, and the hydrogen voice is even sillier than the helium voice, as you would expect,” he said. “I don’t want people to rush away thinking this is a safe and sensible thing to do. I’m under no illusions I’ve produced any evidence for you to see, but this is an honest account of the hydrogen trials at my house.”
The unit had not exploded with a fill of oxygen from zero to 70%, and very low humidity. “Harry, dog, and CCR survive,” as Harry wrote in his report of the trials. “Nothing bad had happened, so it was reasonable to move to the next step,” he said.
Harris, Challen, and other members of the Wetmules, arrived at the site of the Pearse Resurgence on New Zealand’s south island in February 2023. The cave system is so remote they needed around 10 helicopter trips to transport the team and all of its equipment. Mitchell, the diving physician, ran surface operations with “mixed feelings,” as Harris put it.
The group stayed for two weeks at a campsite, complete with a gas-mixing station, an electronics shelter for charging gear, and a “big green army tent where we meet and drink a lot of coffee and try and put off going back into the water each day,” Harris said.
The expedition was plagued with an unheard of number of problems, Harris said, “Every time we got in the water, something popped or blew up or failed.” The campsite is where Harris boosted hydrogen for the first time, from 100 to 150 bar. He flushed the booster and all the whips with hydrogen prior to boosting to make sure no oxygen was left in the system, but it was an anxious moment.
On dive day, Harris and Challen set out on what would be a 13 hour dive to 230 meters/754 feet—a “comfortable depth,” as Harris put it. Due to some problems during the expedition, it was decided that Harry would dive hydrogen, while Craig would dive trimix. At 200 meters/656 feet depth, Harris pivoted the switch block to introduce hydrogen into the loop. “The first cautious sip of hydrogen just to activate the ADV was satisfying,” he said. Gas density was not subjectively improved, but Harris noticed an obvious benefit—the HPNS-induced hand tremors he typically experienced after 180 meters/590 feet disappeared. Harris kept his setpoint at .7 during the descent and working portion of the dive, careful not to reach a fraction of oxygen above 4% which would make the mix explosive, and proceeded to the 230-meter test depth.
After completing their time at 230 meters, the team began their ascent. Harry shut off the hydrogen feed to the active loop of his dual Megalodon rebreather back at 200 meters, and then conducted a diluent flush every 10 meters/33 feet to remove the hydrogen from the loop until reaching 150 meters/492 feet. At that point, Harris boosted his PO2 to 1.3 from his set point of 0.7 (Challen remained at 1.3 throughout the dive), and they continued their ascent decompressing on a trimix (O2, He, N2) schedule, treating hydrogen as if it were helium. The complete technical details of the dive will be published in a forthcoming paper in the Diving and Hyperbaric Medicine Journal.
As soon as the team were helicoptered back to civilization, Harry called Michael from the road. “Michael, we did it!,” Harris said.
“Harry, you’re alive!,” Menduno responded.
At that March meeting with the H2 working group, Harris presented his findings from the dive. “I’m not sure what to conclude to a highly scientific, analytical, and evidence-based audience like yourselves,” he told the group. “Conclusions: N=1,” meaning it had been successful one time.
Doolette, who had been the most vocal in the group about his concerns, suggested Harris could add to his conclusions “the probability of survival is greater than zero.” Doolette, whom Mitchell contacted as soon as they reached civilization, said he “was relieved to hear that Harry survived this test dive” but remains disappointed with some aspects of the experiment, and concerned about possible future attempts. “For instance, I imagine among the engineers he consulted would have been someone with the ability and resources to do a computational fluid dynamic analysis of the Megalodon rebreather to establish the ignition risk, but instead Harry filled his rebreather up with hydrogen in his backyard.”
Overall, Harris said his findings are that hydrogen can be handled and boosted, hydrogen and CCR diving are compatible, a strategy to introduce hydrogen on descent was successful, a decompression dive was successful, a low setpoint at depth did not practically affect total dive time, strategy to reintroduce a high PO2 on ascent was successful, and HPNS and narcotic impacts were subjectively favorable.
“In introducing hydrogen we have addressed the issue of gas density, but we certainly have not established it is safe to use in terms of explosion risk, decompression of the thermal hazards,” Harris said.
Among his conclusions, Harris pointed out that he also managed to evade the nickname “Hindenburg Harry.” “Fortunately that was avoided,” he said, “but remains an ever-present risk.”
The Future of H2
Harris warns not to read too much into what his team achieved—a single data point that should in no way encourage others to repeat the dive. “David Doolette’s comment should be heeded,” Harris said. “All we have shown is that we got away with it on one occasion.”
Provided it can be safely proven and built upon, Harris said he thinks of his hydrogen dive as a window into the future that would enable tech divers to continue exploring into the 250 to 350 meter/820 to 1148 feet range. “Imagine the wrecks and caves that lay unvisited around the planet,” Harris said.
YouTube: Wetmules 245m Cave Dive in the Pearse Resurgence, New Zealand (2020)
InDEPTH: Hydrogen, At Last by Michael Menduno
InDEPTH: Density Discords: Understanding and Applying Gas Density Research by Reilly Fogarty
InDEPTH: Playing with Fire: Hydrogen as a Diving Gas by Reilly Fogarty
InDEPTH: High Pressure Problems on Über-Deep Dives: Dealing with HPNS by Reilly Fogarty
InDEPTH: The Case for Biochemical Decompression by Susan Kayar
John Clarke Online: Hydrogen Diving: The Good, The Bad, the Ugly (2021)
InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes.
Undersea Hyperbaric Medical Society: Hydrogen as a Diving Gas: Proceedings of the 33rd UHMS Workshop Wilmington, North Carolina USA (February 1987)
InDepth Managing Editor Ashley Stewart is a Seattle-based journalist and tech diver. Ashley started diving with Global Underwater Explorers and writing for InDepth in 2021. She is a GUE Tech 2 and CCR1 diver and on her way to becoming an instructor. In her day job, Ashley is an investigative journalist reporting on technology companies. She can be reached at: firstname.lastname@example.org.
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