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In The Line of Duty: Surveying 12.7 km of New Cave Passage

You may think explorers are coming up empty handed after nearly 40 years of scooping booty in Maya Riviera cave systems. You’d be wrong. Explorers Bjarne Knudsen, Emőke Wagner and László Cseh, aka Team BEL, discovered and surveyed 12.7 km /41,600 ft of new underwater passageway, found a new cenote, two new connections, and a prehistoric bone site during the last 12 month season. Here’s how to go where no one has gone before!



by Bjarne Knudsen, Emőke Wagner and László Cseh, aka Team BEL. Note that Bel also means “path” in Mayan. Images courtesy of BEL unless noted.

Photo by Tom St. George

Our first project last season (2021) started from a well-known cenote called Yax Chen at the southernmost corner of the Ox Bel Ha cave system. The prior year, we needed additional survey data from the region and, on a regular survey dive, we found the beginning of a new, potentially unexplored, area. Although it was close to another new recently discovered section of the cave called, “The Swampland,” we didn’t believe the two were connected, so they decided to wait for this season to proceed.

The project began by surveying existing lines on the downstream section of cenote Yax Chen. The most successful line was laid by early Yucatan pioneer Gary Walten in the downstream area after multiple restrictions and cenotes took a turn to the north and headed toward historic sections of Ox Bel Ha. After 1.6 km/5,250 ft of distance into the cave, his line ended in front of a massive collapse. 

Observing the area, we were able to navigate between fallen boulders as noticeable flow was still present. After a while they managed to climb on top of a more open hill, which had a light source high up. It was a cenote; however, in addition to the opening being sidemount size, a piece of a fallen tree blocked the small exit. With some effort, the team was able to shift the tree slightly so that, after unloading all their gear, they were able to exit into the cenote. At this time, the visibility was crystal clear, and the plan was to drop back down into the cave on the other side of the seemingly small cenote. With further examination, it was determined that the cenote was not as small as previously thought, and it made more sense, instead
of another tight and silty squeeze, to follow the path of the water in the cenote. We did that and, as in some areas, had to cut our way through dense mangrove roots, the cenote descended back into the cave after 57 m/187 ft of distance. 

Discovering “The Roots”

The small new opening had a line already very close to the cenote; we knew we had arrived at the desired historic section, and another connection between Yax Chen and Ox Bel Ha was made while discovering a new cenote that the team named “The Roots.” Later, this name was used for the entire new section. It is unknown why Gary or members of the ​​Grupo Exploración Ox Bel Ha (GEO)* never traversed the cenote from either side of the cave; one likely reason could be the conditions—with heavy rainfall, the cenote and the entire region turns almost undiveable due to tannic conditions (as our BEL team soon learned).

New leads discovered in the historic section all pointed into a seemingly open area, which made future exploration very promising. Unfortunately, we had to pass through the new ‘sidemount’ restriction at the cenote on every dive in this area, since no other path was found around it from either side of the cave. This meant unclipping everything, putting it on the line, squeezing through, and then the other diver still inside the cave had to hand the gear piece-by-piece to the diver already in the cenote. After this, another squeeze was conducted by the second diver, followed by gearing up in the cenote. 

As time progressed, we became better and more efficient with the process, but it was always tedious and time-consuming. On the other side of the restriction, the dive continued, and the goal was to head as far northeast as possible, since the cave seemed to be open in that direction. We had to navigate in small, low ceilinged areas, where the bottom was very densely filled with thick layers of black sediment. In this area at least, percolation was minimal. 

After some dives towards the northeast, all of the small and irregular tunnels started to become even smaller, and eventually they stopped. Another complication in the area was the constant and intense rains we had during the summer of 2021. Because the Roots area was very shallow, the rain washed all the tannins into the majority of the tunnels and made visibility close to zero. With slow but determined progress, a more defined east tunnel started to form, but because of the bad conditions in the overhead, we had to delay our dives. The conditions did not improve, but the team was curious where the new east tunnel went. The problem with this tunnel was that it was quite small in its beginning, and a better route had to be established to push more comfortably toward the east. When we finally arrived at the above-mentioned area, the conditions were still bad, but we were determined to establish a new tunnel towards the east. Going from wall to wall we managed to lay a line through an area which was very dark, but bigger and, in the end, looped back to the east line. This allowed opportunities for future dives with more equipment.

Going to the Sea

After many months, the conditions improved slightly, and the team was back to finally check where the new east line went. Having a better route for travel, we arrived at the desired area in good time and kept pushing. Unfortunately, a few hundred meters past the end of the line we arrived at another sidemount restriction at about 2.3 km/7,540 ft of penetration. There was seemingly no other way around this one either. 

Pushing through the restriction, the cave continued, but the team had to navigate collapsed and silty areas. The cave started to change slowly, and the bedrock became more solid. The cave opened up more regularly into huge rooms, and some of them had metal or PVC tubes in them, as we were traveling under a touristy area with lots of buildings. Loud noise, probably from large generators powering the hotels, could also be heard. 

Checking the survey data at home, the new lines were headed toward the sea. This encouraged us to return to try to connect this part of the system to the sea. Unfortunately, this never happened, as the closest we got was 50 m/164 ft. Flow was present, but the cracks in the wall became too small to pass, and the tunnels turned more parallel to the shore than perpendicular. After this area had been checked thoroughly, the team wrapped up remaining leads and we were lucky enough to connect almost all of the newly discovered lines into one survey, which established the section called The Roots with 5.8 km/19,000 ft of new tunnels. The total exploration in the area was 6 km/19,700 ft, with a maximum penetration distance from Yax Chen of 3 km/9,800 ft. Average depth was 6.5 m/21ft, maximum depth was 12.6 m/41 ft. We were using the Global Underwater Explorers (GUE) sidemount configuration with multiple stages and SUEX diver propulsion vehicles (DPVs). 

The Cave East of Coka Ha

After the exploration in Yax Chen, it was time for the team to switch the scenery and focus their efforts for a while in other places. The 2019 exploration from Cenote Chuup Ich and Coka Ha resulted in new data, but some distant areas of the cave there were left unchecked. One of the most interesting parts of the area was a very defined, long, open tunnel heading straight toward the east from cenote Coka Ha. At the time, we were not able to fully explore the region due to landowner restrictions, but some possible leads made this place worth revisiting one more time. With enough preparation and planning, we had no choice but to prepare for regular, long distance dives, as access to closer cenotes was not available.

From Cenote Chuup Ich, the end of line from 2019 lay at a distance of 4.1 km/13,400 ft. Most of the trip was comfortable, but the cave was very dark there, had a lot of sediment, percolation, and complex navigation. As the dives to this area became more frequent, the team also increased its capacity to deal with the longer travel more efficiently; the shortest and easiest path was fully overhead, as it avoided any nearby cenotes, but at 3.6 km/11,800 ft of penetration, a major restriction was encountered. 

A bypass tunnel was found quickly, which made that part a bit easier to navigate. We also built and tested special batteries for our scooters, which made those dives possible with minimal gear load. When we reached the furthest lead for the first time, instead of following the line out into the two relatively new cenotes that we found in 2019, we took a northeast turn and tried to bypass the openings. There, the cave dropped slightly deeper, became smaller, and was even siltier. 

After a short distance of irregular tunnels, the cave opened back up again into its original size and continued due east. The cave was highly decorated with formations, had some small collapses on the sides of the main tunnel, and remained dark in color. After a few hundred meters, the passage hit a high, silty slope which had organic sediment. As we swam up to the top, a small dry cave with a little opening was observed while bats were flying inside it. Because of this, we named it the “Bat Cave” for future reference. There didn’t seem to be any obvious way to continue past the bat cave.

On a subsequent dive, we tried to find a way to bypass the Bat Cave area. The northeast side of the slope seemed to be a possibility if we dropped deeper. A smaller tunnel with heavy percolation allowed us to progress further. There the cave started to branch into more directions, but identifying the proper lead to move on was difficult as the low ceiling allowed us only moments of time to look around because sediment from the ceiling completely destroyed the visibility in seconds. After a few trials, it seemed the cave would head northeast rather than east, which seemingly could have been even better, as the north was in a fully open region. Unfortunately, as we tried to push from here, the cave either ended in collapse or became smaller and died. This part of the cave also never turned back to the south to continue in the original east direction again behind the bat cave. Finally, toward the end, other, smaller leads were finished, and the team managed to explore 3.2 km/10,500 ft of new cave in this area. Their furthest point of penetration was close to 5 km/16,400 ft and, depending on the environment, the team used both (open circuit) backmount and sidemount configurations with multiple stages and the DPVs. Average depth was 11 m/36 ft, maximum depth was 19.5 m/64 feet. 

Moving to The Bees

As the new year of 2022 unfolded, we hoped to complete one last small project following our exploration of Coka Ha. Fortunately, we received additional help from Dr. Mario Vallotta, who was interested in surveying a cave called Abejas, or “The Bees” in Spanish. The cave was last explored extensively in 2004 and belongs to the giant Sac Actun cave system.

As usual, with Mario’s help, we started our survey of existing lines from the cave entrance. On a regular survey dive, we found a few hundred meters of new tunnel close to the entrance. Analyzing the data back home, it appeared that this new tunnel created a big shortcut for divers wanting to travel to the northeast. The size of the tunnel also allowed equipment to be carried through it. 

As a result of finding the shortcut tunnel, our new goal became traveling further to the northeast to look around in that area, which hadn’t been visited much or at all in many years. As we kept surveying towards the north, we found more bypass tunnels along the way, which further decreased travel time and increased our efficiency. One of these new lines created the second connection between Abejas and the northeast area of Sac Aktun. The first open areas after this had good possibilities to the west, where the cave seemingly opened up. Unfortunately, all the leads in this initial area dropped quickly down into the halocline, where the small saltwater tunnels quickly ended. The freshwater sections of this cave were white, but these saltwater tunnels had black walls.

As the distance increased, we relied on our DPVs, and cave diver Cameron Miller joined us in exploring further upstream. Fortunately, we got lucky following the north lines and always keeping an eye open for possibilities to the west. Around 2 km/6,600 ft upstream from the original entrance, there was a low ceilinged area laying northwest that opened up into a bigger more defined tunnel with good conditions—a lot of sediment was present on the floor, but percolation from the ceiling was minimal. 

Finally, after another major restriction, the cave took a hard turn to the west and kept going in that direction for quite some distance. That part of the cave had bigger rooms with cave formations and still contained fresh water with mild flow, which was a good sign. After a few hundred meters, more restrictions had to be navigated and the conditions worsened. On the other side of the new small and silty section, small areas of the cave began to collapse more frequently. One of the collapses revealed an area with animal bone remains. Judging by their characteristics they must have belonged to different types of animals and they were most likely prehistoric. 

Past the collapsed section, the cave descended back into the saltwater layer. Slowly, after some branched tunnels, all the passages became progressively smaller and were impassable to the west and to the north. After checking the final leads, this new small section had 2.4 km/7,800 ft of new tunnels, bringing this entire project’s new exploration distance to 3.5 km/11,500 ft, with many kilometers of resurveyed old line. Maximum penetration distance was 3 km/9,800 ft. Average depth was 9 m/30 ft, maximum depth was 13.3 m/43 ft. We used backmount, along with stages and scooters for this portion of the exploration.

More To be Found

Needless to say, our BEL team had a good last year in terms of finding new cave passages in some of the largest cave systems in the world. In addition to many kilometers of resurveying of existing lines, the team managed to discover 12.7 km/41,600 ft of new tunnels, one new cenote, two new connections, and one prehistoric bone site. Their dive times averaged between six and eight hours. Efforts like these show that even after many years of great cave explorations, Mexican caves still hide some fascinating surprises and possibilities for interested and committed divers.

This was also the first time we tried to incorporate some more divers into the projects and put their skills to good use! Many thanks to Mario and Cameron for their support. Lastly, huge thanks go to the Cuzel Filling Station who supports our dives with tanks, standard gasses, excellent logistics, and their availability even in the evenings at the end of our long exploration days!

Footnotes:​​* Grupo Exploración Ox Bel Ha: a legacy cave exploration group that consisted of the late Bil Philips, Steve Boagerts, Chris Le Maillot, Daniel Riordan, Bern Birnbach and more.

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InDEPTH: Laying Line in Ox Bel Ha By Emőke Wagner, László Cseh and Bjarne Knudsen

InDEPTH: Data for Divers: Mexican Explorers Go Digital to Chart Riviera Maya by Michael MendunoWebsite/Blog: Emőke Wagner and Laszlo Cseh GUE Instructors

Bjarne Knudsen began diving in 1993, taking his first tech classes in 1997 and his first GUE Cave and Tech classes in 1999, so has been part of the GUE community since the early days. In the early 2000s he spent some years in Florida, where he was a part of the WKPP. During this time, he also pushed Sheck Exley’s end of line in the Cathedral Cave system with Todd Leonard (and lots of support from friends). Bjarne is currently on a slow world cruise with his wife on their sailboat. For the last few years, they’ve been a little stuck in Mexico and the surrounding countries, which offer so much nice diving.

Emőke Wagner is originally from Hungary and began diving at a young age. She has been an active instructor since 2014. After a couple of years spent traveling around the globe, she moved to Mexico with her husband in 2017. While living in Mexico, cave diving became her real passion, and she began exploring more of the local cave systems. Since 2016, Emőke has been working as a full-time GUE instructor and is currently teaching the cave, foundational, and recreational curriculum.

László Cseh is from Hungary and has always been fascinated with the underwater world. He became a recreational diving instructor in 2012 and began teaching and traveling with his wife, Emőke. After becoming a GUE instructor in 2016, he moved to Mexico to look for new diving challenges. Local cave exploration possibilities helped him achieve his GUE Cave instructor certification.

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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.

Richard ‘Harry’ Harris embarking on the first hydrogen rebreather dive on 14 FEB 2023.

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. 

Gas density is NOT a diver’s friend. Increased gas density above 6.1 g/l increases a diver’s risk of having an “event” during a dive. You do NOT want an eventful dive! Chart courtesy of John Clarke.

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.

Harry’s dive profile of their 245m dive at the Pearse Resurgence in 2020. The gas density is in the Red Zone!

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. 

Sheck Exley at Manté

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.

Cave explorer Craig Challen

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. 

Diving physiologist and explorer David Doolette in northwest Florida. Photo courtesy of D. Doolette.

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. 

Wetmules waiting for Harry and Craig to return from their 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.

Don’t try this at home kids. Photo courtesy of Richard Harris.

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.”

Dual Megalodon rebreathers connected via their BOVs. Photo courtesy of R. Harris.

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.

A gear intensive expedition that required 10 helicopter trips to ferry in all of the equipment.

The Expedition

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.

Wetmules camp along the river.

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. 

Wetmules camp chat.
Wetmules—Back row (left to right) Simon Mitchell, Dave Apperley, Craig Challen, Richard Harris, Dave Hurst. Front row (left to right) John Dalla-Zuanna, Ken Smith, Martyn Griffiths (with Colin, the team’s bowling ball mascot) and Luke Nelson.
Harry and Craig suiting up for the dive.
Harry’s secret sauce. Image by Martyn Griffiths
The entrance to the Pearse Resurgence
The 17 meter habitat.
Harry with Dual Megalodon and Seacraft scooter at the 17 meter habitat.

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.

Harry arrives at the surface following his hydrogen dive.

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:

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