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A First Exploration of the Flooded Mines of Thailand

Thailand-based, Finnish explorer Mikko Paasi reports on the first known exploration of the Sahakorn Nikon mine, which he and his team recently conducted in late 2022. Located in the Kanchanaburi province in northwest Thailand, the mine complex, which was abandoned and flooded some thirty years ago, is believed to be the largest in Asia. Working with local authorities, the team is planning to conduct a 3D photogrammetry survey of the complex, which lacks maps or blueprints. Paasi, who was recognized by the King of Thailand, and president of Finland for his efforts in the Thai cave rescue, believes the discovery heralds a new era of local exploration.



By Mikko Paasi. Header image: Bottomline Projects exploration team ”The Filthy Few” descending to the Song Toh mine in Kanchanaburi province to conduct the first ever mine dive in Thailand. Photo: Karn Romyasai

I’ve always been passionate about mine diving. Actually I started my professional dive career in Ojamo mine in 1998, which is where I went through a year long instructor training. During that year, we conducted nearly 300 dives in the mine. Back in the 90s there was no proper training available, so we poked our heads around the first chambers with our c- battery-loaded 100 lumen bulb lights. 

Today, with all the developing technology in the diving industry, overhead environment diving has developed to a totally new level. DPV’s, rebreathers, and powerful handheld torches help us penetrate further than we could have even dreamed a few decades ago. Even if mine diving might be popular in Europe, in Thailand it is a totally new privilege, and according to my knowledge, no one had dived here before until a couple of years ago, when we decided to start scouting old, flooded and abandoned mine sites.

Like many exploration projects, this one started a long time ago over a pint of beer and a drawing board. After a long while scouting around, we finally began to locate serious mine options that were worth checking out. This is a story of the first two flooded mines that we discovered and dived in Thailand.

Into the Mines of Moria, err Thailand

Thailand has a rich mining culture. The main minerals have been lead, tin, zinc, gold, and other heavy metals. Our first exploration target, the Sahakorn Nikon mine, is located in Kanchanaburi province near the Mayanmar border in northwest region of Thailand. It is believed to be the largest mine complex in Asia. For three generations and about a hundred years the Bhol family extracted lead minerals from under the stone mountain the locals call the “Everest of Thailand” for the shape of it. The mining stopped here over 30 years ago and the underground section has been submersed ever since. 

Our dive team is based on a small island in the middle of the Gulf of Thailand, called Koh Tao, where I have been running my dive center for the past 25 years. Koh Tao Divers (KTD) is where we train our Bottomline Projects foundation team and other like-minded divers. I was the expedition leader and the rest of our dive team consisted of two full cave and KISS Sidewinder rebreather divers, Naomi Allen and Pasi Laihanen, a couple who both work as instructors at KTD. 

To get to the mine area we first needed to load up all our gear into a high speed catamaran that took us to the mainland where a minivan took us on the second leg of the journey, a 800 kilometer drive up north. On the way, we picked up two dry cavers, David Templeman and Karn Romyasai, who had been in these mines before and knew the coordinates and the location of the possible diving spots inside the mines.

04:00 am at the entrance of Song Toh mine. We’ve just cut truough a kilometer of thick jungle in a wild elephant territory and ready to explore the underground. From left: David Templeman, Naomi Allen, Mikko Paasi and Pasi Laihanen. Photo: Karn Romyasai

We arrived on the site around 04:00 AM in the heart of the night. This area is known for its herd of wild elephants that kill many members of the local community every year. Cutting away the jungle with our machetes in the dark, pushing our way toward the hidden entrance, we made ourselves very vulnerable to the wild life surrounding us. 

Eventually, after a downhill journey of a half kilometer transporting our equipment through the slippery terrain, we found the entrance and started to descend down a spiraling drive way that led us to the water. Once we arrived at the water level, we realized that the mine was a monster, both in size and the hostile conditions such as quicksand that could suck you down to your hips, mud, and rusty, fragile structures to be navigated. We couldn’t help imagining how it would be under the surface..

Encouraged by the fact that we had come all the way here and that we were actually about to do the first mine dive in the Kingdom, we began our way toward the shaft that led down to what was unknown to us. My tool for the job was a mechanical ccr, KISS Sidewinder with two aluminum 40 cf bailouts side-mounted on both sides, my colleagues, Pasi and Naomi, were both on open circuit sidemount. While gearing up, I had torn my drysuit wrist seal completely off, and since I didn’t have a spare with me, I had zip-tied my right sleeve to minimize the leak. Luckily, the water was 20ºC/68ºF.

On our first dive we traversed to this shaft that leads all the way to the ground level. You can tell by the amount of the plumbing pipes and other structure, that we are talking adout a giant of a mine. Photo: Mikko Paasi

Since these mines do not have maps or blueprints that would have given us some idea of what was waiting for us down there. By the time I got my reel line attached to the super structure, the visibility at the shaft had dropped from five meters to zero. We started our dive by dropping down and feeling our way in until we reached the first tunnel at just 10 m/33 ft depth. Everything was covered with a thick layer of fine dust, and we were lucky to manage a few tie offs on the way until we popped into another vertical shaft with massive amounts of plumbing and rusty ladders going in multiple directions. We had just traversed to another vent shaft that led straight away to the ground surface and had exited the mine about 50 m/164 ft above us. Unfortunately, when calculating the risk versus the reward, this experience was not worth risking any more exploring with the given equipment and gasses we had with us. Yet again, we think the shafts will keep going and, given the amount of structure there was, it must go for quite some way.

At sunrise we surfaced back to the ground level and out from the darkness of this monster of a mine. The next site was approximately a two-hour drive away. We had time for some breakfast and to focus on the next dive in another abandoned, flooded mine. 

Author, Team leader and the founder of Bottomline Projects foundation, Mikko Paasi. Photo: Krista Paasi

At 11:00 am we arrived, and the approach was a bit easier since we had the daylight with us. Once we found the entrance, we started walking in, following David, who was leading us to the spot where the tunnel submerged. Our hopes were realistic, because we had just dived a massive and fairly potential mine a couple of hours before. We knew that the main tunnel where we were now walking on the ground level was over two kilometers long, and we couldn’t help thinking about what could lie underneath our feet.

At the water’s edge we started to get excited. The color of the water and the surroundings reminded us of the Mexican cenotes, except these tunnels were man-made ”rusty caves” with all sorts of machinery in them.

We slid in, tied off to a pipe that led down, and off we went. The walls were bright and everything was covered with fine white silt that settled down shortly. Visibility exceeded our expectations, being between 10-20 m/33-66 ft from time to time. This tunnel was shallow, just barely under water, and after a few junctions and turns, we popped up in a big chamber with daylight shining in. We’d just discovered an alternative entry/exit to the mine and accidentally traversed into it. We ended our first reel there and tied it on permanently to a plumbing pipe. 

We took a quick look around the chamber and noticed that opposite to where the day light was coming there was a massive driveway with multiple pipes leading deeper down toward the heart of the mountain. The visibility was crystal clear, and the main tunnel had constant junctions and dead ends to the left and the right. We knew we had struck gold and would be back again soon with a bit more gear and a clearer idea of what we were up against.

At the bottom of the mine where the diving starts. Thailand’s first mine divers (from left) Pasi Laihanen, Naomi Allen and Mikko Paasi. Photo: Karn Romyasai

Our Second Push

Back on our little island, we had time to get prepared for the second push into the mines but with out-dated information. At least we had a family contact when we needed to get permission for further exploration. We had also decided to concentrate on only the second mine since we felt it was well worth the risk. From the first trip, we had many questions:, will the tunnel extend, or will it pop back to the surface? Are there other, deeper levels under the first one? Is the water quality safe to dive in? How big is the complex down there?

The second push dive team consisted of two of my friends, Cedric and Fan, who were both full mine CCR divers and top-notch blokes with which to hang around.

This time we arrived at 03:30 AM after a boat ride and a night in the van. We immediately went scouting and carried the first load of equipment to the starting point. During the three hours of scouting around the dry mine tunnels we found two new entry points which then made four different entries to the underworld. Would they all be connected? 

Cedric Chan deploying a belay to one of the very rare tie off points in the south section of the Boh Yai mine complex. Photo: Mikko Paasi

With the sunrise on our backs, we entered the mine and began to prepare for the first push dive. This time, we had brought with us a kilometer of cave line and we were vigorously spooling it into our reels while keeping an eye out for the scorpions, snakes, and other critters that might be crawling into our Sidewinders mouth pieces. The first question was, would the main tunnel, the one that we found the last time, surface or would it go deeper. Tying off to the piping on the sides was easy, and we soon figured that we would have work to do for years in this place. There were constant crossroads, and the driveway kept going on, deeper and straight into the mountain.

We emptied our reels and turned the dive. The scenery, while diving back, and watching Cedric and Fan gliding in the distance, was breathtaking and I could tell what they were feeling at that moment, knowing that when you’ve just discovered something enormous, your mind races with so many possibilities.

Helmet and a rebreather are mine divers fundamental tools to add safety and to avoid perculation or roof collapses. Photo: Mikko Paasi

This time we were all on Sidewinder CCRs, so moving across the dry sections was much easier, and we were able to do longer dives. We traversed to the other side of the mine and carried our equipment to one of the new entries we had found in the morning. This entry was teeming with bats, but we were in adrenalin-fueled exploration mode with the first glimpse under the surface. Checks and primary tie offs, and we were once again on our way to the unknown. This time the tunnel was spiraling down, and at the 15 m/50 ft level we’d done a full 360 degrees,meaning we were right under where we had started. Up right there was a familiar-looking tight passage that we recognized as the other entry we had found earlier. Down right there was a tight restriction that dropped down to the next deeper level. On the left we could see a massive iron gate that was just barely open and that led to a ventilation shaft. We decided to follow the main tunnel that kept going deeper and curving in again. 

Tying off in this section was almost impossible, since the tunnel was cut smooth with nothing to grip, and at the bottom there was no machinery nor even stones to tie onto.

Eventually, we ran out of line and turned back, and this time we ascended through the small passage to mark it and then back in and down again to take some images and water examples for later studies.

We had now laid about half a kilometer/1640 ft of new line in, and everywhere we turned there seemed to be an endless complex of tunnels going in all directions. We were starting to realize what a 100 years of mining might look like. 

Norrased Palasing exploring what lies behind one of the many iron gates along the main driveway ”Soi Cowboy” in Boh Yai mine number 7. Photo: Mikko Paasi

The Future of Thai Mine Diving

Discovering something this big and beautiful being hidden and forgotten for decades is the sole reason why we train so hard everyday. The fact that there might not be any blueprints for this mine just makes the adventure so much more appealing, and it becomes true exploration. Not knowing what lies around the next corner or when you might be at the end of the last corridor, and being the first one to visit the century-long history of the three generations of the Bhol mining family’s underground enterprise is well worth the elevated risk that comes with exploratory mine diving. 

With such world-class diving possibilities, the surrounding natural caves, rivers, waterfalls, and hot springs, this place needs to be taken seriously. Were the caves to be carefully surveyed, the local community could welcome more adventurous visitors to the region–explorers who would enjoy Thailand’s longest underground tunnels as well as the warm and crystal clear flooded sections of this beautiful monster of a mine.

We have decided to keep the exact coordinates amongst just a few of us to hopefully avoid less-experienced adventure seekers from rushing in without proper training. Meanwhile, we will try to negotiate the terms of the use, and survey the site properly using the 3D photogrammetry method as our main tool to understand the magnitude of this complex.

Until today, we have managed to take various water and dust samples from different locations, and so far we haven’t found any elevated traces of lead or any other toxins. The local community has given us exclusive permission to keep exploring deeper into the mines, and the next big push is scheduled at the end of April 2023. This time, we will have to start mixing helium in the blend because it is now obvious that the mine goes way past air diluent depths. Another tool I will add to my artillery next time is the helmet. We had a friendly reminder from a dive we did with our Thai team Por Parasu Komaradat and Norrased Palasing where we experienced a couple of small collapses in the side passages and that put the whole team on alert. I plan to also consider if parts of the mine are too fragile to dive, especially on open circuit. As safety is our number one goal, these mines obviously will only be open for divers with proper (mine diver) training and site introduction. 

So far, all the interviews with the old miners haven’t brought us any credible data, as they all recall things slightly differently. We have located a fourth mine in the same region, but no permission for exploration has been granted yet because the locals are too worried about us getting hurt by the wild elephants living in the area. We believe that there will be real exploration and new projects around these mines for years to come and would like to encourage CCR exploration-minded teams or individuals to reach out and get involved in the upcoming series of expeditions.

All and all, a new era of overhead environment diving in Thailand has begun, and we are extremely excited to seewhere this all will lead and how many more diveable mines there will be once the word is out. 

This series of expeditions is supported by XDEEP Exploration Support Program, SEAL dry suits and Big Blue Dive Lights.

Dive Deeper

Great Mines!

InDEPTH: Out of the Depths: The Story of British Mine Diving by Jon Glanfield

InDEPTH:They Discovered an 11,000-year-old Submerged Ochre Mine by John Kendall

InDEPTH: I See A Darkness: A Descent Into Germany’s Felicitas Mine by Andrea Murdock Alpini

inDEPTH: Mine Over Monitor: Iconic dive sites with a shared history by Edd Stockdale

Mikko Paasi is a passionate explorer, ccr cave/mine Instructor Trainer and an underwater camera operator. His soon 30 year long professional diving career started at Ojamo mine, Finland, in the 90’s. Today he runs his dive center, Koh Tao Divers, in Thailand and conducts constantly new diving related projects and expeditions through his foundation called Bottomline Projects. Mikko has been involved in multiple documentaries and movies both, behind and in front of the camera, latest being a film called Ghost Ships which is now premiering in the Berlin Film Festival. He played a key role in the Thai Cave Rescue 2018 from where the King of Thailand awarded him with a 1st class Knight Grand Cross.


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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