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The Taming of the Slough: Crypt Edition

Rumors of a deep connection between the Crypt, a collapsed tunnel in Peacock Springs discovered by Sheck Exley in 1975, and lower Orange Groove, both located in Wes Skiles Peacock Springs State Park, Florida, only served to entice explorer Steve Lambert, who was determined to find the truth, after his team’s success extending Peacock III. Here is his story featuring a well-known cast of underground characters including Woody Jasper, Frank McNamara, Andrew Johnson, Thai Paul and greybeard Paul Heinerth who were involved in the find. Scoop that booty!



Text and images by Steve Lambert.

Jeff Dobbertien helping divers out of the water at Orange Grove

My name is Steve, and for the past four years I have had a somewhat unhealthy obsession with Peacock Springs. I couldn’t believe it when I heard that there was an alleged connection between the Crypt and Lower Orange Grove through a “deep section.” That rumor included the tale of a near-death experience—dive buddies frantically trying to get someone in the cavern to help, even though they had three hours of decompression to complete the dive. There were even furious police officers involved in the tale. Naturally, I had to go investigate. Over about half of a year in 2022, I was able to track down all of the pieces of this outlandish puzzle and put the story together as well as see the connection for myself. Nearly six months after that, I was able to begin an article on the subject of my desires.


The deep section of the Crypt was originally explored in the mid-80s by Woody Jasper. In a time before most people had access to trimix or trimix dive planning tools, divers had to be resourceful to reach the far recesses of caves. When I asked Woody about diving the Crypt in the early days he told me “ I used a travel mix of half oxygen and half air [60% O2] so I didn’t start my bottom timer until I went on air in the upper Crypt, then I would stop my bottom deco time when I went back on the hot mix. Saved a shit load of needless deco. Max depth on the hot mix was 52 feet, so pushing O2 a bit on the return trip, but Nitrox hadn’t been invented then so we still thought two atmospheres (PO2=2.0) was safe.” 

Using this method, Woody was able to shave off large amounts of decompression time from his dives. Keep in mind that this was at a time when “nitrox” hadn’t been invented and divers thought a PO2 of two atmospheres was within “safe” limits. It was also a time when you could count the number of sidemount divers on one hand, and only on a few fingers for the number of sidemount divers making deep dives.

Missed Connections

Frank McNamara started looking at Lower Orange Grove after Ted McCoy told him that he thought it connected to Lower Hendley’s in PIII (ironically, it was that same connection I was looking for when I began pushing PIII). Ted told Frank about some no-mount exploration he had worked on down in the “Little Sister” jump in the past, and he wanted to have a look.

Frank tied into the jump and headed off into a low flow, silty area. After accidentally making a wrong turn at a T, he ended up at a rock-on-clay restriction which appeared to be a dead end. “I went through it, he said, “and placed a silt stake and, after a turn, it popped out in a large room at about 46m/150 ft of water. This room had several breakdowns on the floor off to the left. After about 90m/300 ft, it got much bigger and dropped down to about 56m/185 ft deep and turned into a very wide and low tunnel covered in goethite with a decent amount of clear water flow.”

Overlay of Lambert’s survey and an area map. The loop closure was less than 24m/80ft.

Frank continues: “I continued down this tunnel for a bit and came upon a breakdown with two different possible paths up (both nasty restrictions). I took the one that looked slightly bigger and went bottle off to get through. It popped into a big room going up a hill and after ascending a little bit, I found what appeared to be a very old line. I tied in and left my reel (my favorite one at that), assuming I’d be back to survey (vis was completely cooked from passing the restriction). I then turned to head out. At this point, I tried to go through the restriction without taking a bottle off and got properly stuck (it required exhaling fully to get enough slack to release my bottle).”

After the dive, Frank realized he hadn’t laid enough line to connect to Lower Hendley’s and assumed he had connected back into another line in Lower Orange Grove. About six months later, he went back to repossess his reel, but on this dive he went in on the line he thought he had tied into on his last dive. To his surprise, his line was nowhere to be seen. He left defeated, and it was over a year before he went back to recover his reel. He retraced his old line but, once again, bad visibility from passing through restrictions prevented him from getting the survey data and he still had no idea where his line had tied into.

Deep Upstream section of the large room in the Crypt.

“Years later, I heard about Andrew’s near miss down there, so Luke and I went back to follow the line that I had tied into back up. When we saw the little skeleton in the Crypt we knew I had actually connected Lower Orange Grove to the Crypt in 2016,” McNamara said.

Thai Paul

In 2020, Piyajit Pratipasen or “Thai Paul”—as he is known by many cave divers—was staying in Luraville for a few weeks and took up an interest in the Crypt. At this time, nobody knew that Frank had connected the two sides of the cave, not even Frank. Generally, when people say something “doesn’t go”, most people take it as gospel and never bother trying. Particularly if it requires a bunch of extra work and expensive gas to go see, and even more so if you have to swim any notable distance. These challenges didn’t bother Paul, and he spent the summer of 2020 making many dives down into the Crypt.

I joined Paul on one of his first dives into the area. Having heard many stories of collapse and instability in that area, I was intrigued and a bit nervous. Diving our CHO2ptimas, we swam in from PI along the Nicholson Tunnel and into the room at 70 feet. I followed Paul, who had been down through the breakdown before, into a small spiraling maze of breakdown. Eventually, around 120 feet deep, we hit a rock-on-rock restriction that I didn’t quite fit through. Paul popped off his CHO2ptima like it was nothing and slipped through the crack, but it had been several months since I had been on a rebreather due to my construction hobby, and I decided that low vis, depth, and a rebreather-off restriction took me to my limits for that dive.

Over the next few weeks, Paul continued diving the Crypt with anybody he could hoodwink into going with him. Seeing as how there was no available data for the area, Paul decided to turn it into a survey project. As he mapped, he noticed it trending toward Lower Orange Grove. Eventually, his survey data showed the two locations trending towards each other, with minimal distance between them. He continued working on his survey for several weeks, until a dive incident stopped him in his tracks.

Justin decompressing at Peacock 1

“The Dive”

Rumors about this dive were the hot topic of cave country for months. Sensationalized versions included broken equipment, breathing bailout down to the last PSI and tears of relief and near death were shared among divers, who were very surprised to hear that the Crypt went anywhere other than into a dead-end breakdown pile. When I finally got the chance to talk to Andrew over dinner at High Springs premier Mexican restaurant, I learned that the story was neither as dramatic nor as ridiculous as it had been propagated to be. I was both relieved and slightly disappointed that the chaotic ending to the dive had much more rational and controlled roots.

On Paul’s last dive into the Crypt, he went with Andrew Johnston. Paul was on a CHO2ptima and Andrew on a KISS Sidewinder. They snaked down the vertical breakdown through restriction after restriction and ended up in a gorgeous room deep in the Crypt. Paul was giving Andrew a tour and showing him all of the different lines in the area. He pointed him to the path suspected of connecting the two areas (which happens to be a restriction and the smallest, deepest place in the whole cave) and motioned for Andrew to have a look. Andrew wiggled into the small rock-on-rock restriction, visibility became worse and worse. After some struggle, he popped through into the bedding plane beyond.

Paul had taken Andrew into that section of Lower Orange Grove on a previous dive, and Andrew recognized where he was. Now in very low visibility, through a bottle-off restriction, Andrew was in a tight spot. All puns aside, he was quickly racking up deco sitting at 56m/184 ft, unable to communicate with Paul through the silt-out. He had to quickly decide whether to take the risk of trying to feel his way back through a restriction that he wasn’t comfortable with, or going to a known exit while leaving his buddy behind.

The entrance to the bedding plane connecting Lower Orange Grove to the Crypt.

Paul was in the Crypt frantically waiting for the silt to clear. Worried that Andrew had gotten stuck, or confused by arrows from two different lines contradicting the exit, he waited at depth as long as he could before deciding he needed to get help. Paul swam briskly back to the entrance they had come from at PI, but had over three hours of decompression to do before he could safely ascend and ask for help. He was able to give a wet note explaining the situation to another diver in the cavern and had them take it to the surface while he finished the most stressful decompression of his life; he didn’t know whether his buddy was dead or alive.

Paul Heinerth was at Peacock Springs teaching that day and heard news of the situation. The IUCRR was alerted and a search began. Divers went to the Crypt to search, but Paul was still underwater and nobody else really knew their way around the Crypt. A local family swimming at Orange Grove spotted a diver in the water and alerted Kate Swanson. She jumped in where she met Andrew decompressing. Everyone involved was extremely relieved to hear that he was okay and he was able to exit the water without any major issues. By this point in the day, cave divers were all talking about the incident, shocked to find that the Crypt was connected to Lower Orange Grove, and even more shocked to hear the story of its accidental confirmation.

The team watched Nat Geo’s documentary of Bill Stone and Sistema Huautla during their decompression.

Getting the Data

In the summer of 2022, I was having trouble recruiting people who hated themselves enough to suffer through long PIII dives, so it was time to work on the next project on my list until PIII was back on the menu. By then I had heard so many unreal versions of the story about the Crypt connection, I decided to go see the real thing for myself. I recruited my coworker and fellow CHO2ptima diver Justin Judd to go check it out with me. I figured if we were going to put the time into diving it, we might as well come out with something useful, so we decided to make it a project to get the data confirming the connection, as Thai Paul had misplaced his and there were no copies. Aside from that, we were testing prototypes for the Nomad Ray and this project would be the perfect proving ground.

Thanks to sponsorship from Dive Rite, Justin and I were able to spend six dives and nearly thirty hours on our CHO2ptimas completing a tape survey from the dock at Orange Grove to the surface at Cisteen. We worked our way in from both ends, slowly working through to the connection point in the middle. Surveying from surface to surface gave us the advantage of being able to determine our own loop closure without needing to rely on the accuracy of any previous survey of the system.

Having said that, it was Michael Poucher—the leader of the original Peacock Springs survey himself—who helped us plot our data, so I can hardly imagine he would have any trouble tying our data into the existing survey.I had expected the two areas to connect as described by Paul and Andrew, but what I had not expected was how incredibly beautiful the deep section of the Crypt was. The clean white limestone ceiling polka-dotted with goethite formations in one of the bigger rooms was absolutely stunning. We were intrigued to find a deep water source at the far end of the Crypt, and water flows out of this Northern source up into the PI side, and siphons off into Lower Orange Grove.

It was also very cool to see twisted, knotted line in the deeper section (with Woody Jasper’s signature three-knot ending) still in perfect shape after nearly fifty years. Another unexpected bonus of slowing down and looking at the cave in detail as we surveyed was finding more passage; we were able to lay around 152m/500 ft of line and found a more stable route to get to the deep section.

Frank’s arrow tying his new line into Woody Jaspers old line. 

Making the Traverse, On Purpose!

After completing our survey work and having seen both sides of the connection and studying the “crux” of the traverse several times in great detail, Justin and I were ready to make the first dive by intentionally entering Lower Orange Grove and leaving from a different exit. We decided to enter from Lower Orange Grove, so we would be swimming into the flow as we progressed, making the deep silty restrictions easier to pass. With our intended route from Lower to Upper Orange Grove, no matter where we had to bail out, we would end up in the same place. We left two 80s of oxygen and a 50% bottle in the Orange Grove basin as we descended, carrying cave-filled sidemount LP85s of 18/45 as our bailout. On our last survey dive, we had staged two bottles of 50% at 70 feet in the Crypt in case we needed to bail out in the shallow section after passing through the restrictions.

The difference between exploration and adventure is data!

This made the traverse a much more pleasant dive than we had expected. All of our other dives had been spent tediously pulling a tape measure and trying to read a compass wedged in awkward places in bad vis. This was the first dive where we could really take our time to enjoy the incredible cave passage. Because we didn’t need to spend as much time at depth, it was the shortest out of all of our dives on the project. Everything went well, and both of us thoroughly enjoyed the relaxing dive from Lower Orange Grove to Upper Orange Grove.

A video of the author coming out after pulling the tape through the crux of the traverse

After completing the dive as planned, Justin, Grace Winfree, and I celebrated with a Luraville classic, the chili-slaw dog. While choking down the delightful eccentricities of rural North Florida, we engaged in a philosophical debate on whether the journey we had just undertaken was a traverse or a circuit. It did not fit any of the traditional definitions, and none of us had heard of anything similar in other caves. As the dogs were downed and the warm slaw dripped, it was decided upon that this novel course was most befitting of being titled a traverse. We all agreed that the new path would be known as the American Airlines Traverse; a title with a deeply personal meaning.

You will be hearing from this guy again!!

In Conclusion

The connection between the Crypt and Lower Orange is quite a significant one. Aside from being an enjoyable technical challenge and stunningly beautiful, it shows us that water is moving under the park in ways that we did not realize, and on more than one level. We now know there is more than one water source for Peacock Springs, and that Lower Orange Grove is taking water from both the Crypt and Upper Orange Grove. I highly suspect this water makes its way to PIII, but for now that remains to be seen …

Thank you to Woody Jasper, Frank McNamara, Thai Paul, and Andrew Johnston for sharing their pieces of this story with me, and to Dive Rite for sponsoring the survey project.


InDEPTH: The Taming of the Slough: P3 Edition by Steve Lambert

InDEPTH: SUMP POTION #9 by Jon Kieren

Steve Lambert lives in North Florida where he works at Dive Rite and is actively involved in the exploration and survey of underwater caves with Karst Underwater Research. He frequently joins expeditions with Beyond the Sump and Hole Patrol. His hobbies include quitting when things get too hard, residential construction, and compiling Kpop playlists.


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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