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I See A Darkness: A Descent Into Germany’s Felicitas Mine

We join Italian explorer and tech instructor Andrea Murdock Alpini on an poetic exploration of the Felicitas Mine in Germany, as he and his teammates ponder the life of the German miners who once occupied its passageways. Be sure to check out his video documentary below!



Text and video by Andrea Murdock Alpini. Photos courtesy of PHY Diving Equipment.

The more time passes, the less is the distance that separates you from the object of your desire—in this case it’s a place. Watching, observing, studying, writing, and pinning drafts of questions that are waiting for an answer. 

Deciding to follow a line, understanding its feasibility, its esthetical beauty, and the historical meaning. Few hours separate us from the place where German miners had been working for centuries, we are going to dive into Felicitas Mine where hundreds of workers lived and dreamt for decades covered by a soft, sooty layer of slate. Advancing across tunnels and caressing the black dream of ancient slate rocks, coexists with personal loneliness of cold water, rock, writing and breath. Descending below the edge of earth, to move closer to the surface of life. Our story will be only a layer of dry walls who built Felicitas in the past. “The anonymous history is stratified”.  

Black Blade Feelings

In the morning we moved out from our base camp in Hutten. In the last few days, we had measured more than a thousand meters of cave line. We clarified our main targets and the areas of the mine I wished to film. We fixed our checkpoints by dropping off cylinders and spots where we placed our directional markers. My Cave-Van was filled with nineteen deco cylinders and twinsets ready to be used in Felicitas Mine. 

Fifty kilometers separated us from the arrival. Now the houses’ façades were black, as well as the roofs. The houses were built with the pure German Style “Fachwerkhäuser”. Looking out the car window, it seemed to be on top of Golgota mountain during Easter Sunday. The sky was obscured by deep black clouds, the color the same as the surroundings. The black is more than a shadow, was authoritarian. It was fascinating and absorbed all the rays of opaline light.  

Approaching the mine and reading the old billboard “Abela Heilstollen” was exciting. I wished to be there, inside the mine, in front of the water. As soon as I arrived at the spot, I started walking around the ancient barracks. We were surrounded by corn fields. Far away, at the end of the field on the left, I saw a cement turret rising over the corn plants. This was the end of the left branch of the mine: one of our main targets. Watching it and estimating the distance that we would have to fill later, swimming in open circuit, was impressive. The right branch, also known as “The Old Mine” was closer—530 m/1740 ft from the starting point of the dive. On this day we wished to reach the end of it. 

When we got inside the dry part of the mine, everything was the same as it was left a few decades ago, when the mine expired its vocation and was sold to a new society for another business. Slate machinery has been abandoned inside the hangar. Our team started to unload the heavy diving equipment from the van. We split it into three groups: mine, Gianni’s area, and finally Flavio’s one. He was the surface assistant and gas mixing supervisor, checking regulators, tank MODs and, last but not least, our interpreter, Mr. Wolf. 

Our daily plan consisted of three different dives, each one with a different final goal. 

The very first dive was focused on setting down our strong main-line, a 120 m/394 f length of solid 8mm rope. During the second dive, we were to carry six cylinders of safety gas (in the end) and EAN 50 plus Oxygen at 21 m/70 ft and 6 m/20 ft depth to complete our decompression procedures. At the end of the main-line there is a “T”, and turning left brings you directly into the NEW Mine, on the opposite side we had the OLD part of Felicitas Mine.

During our second dive we explored 270 m/886 ft of old tunnels, leaving directional markers with distance and we clipped safety gas cylinders along the way. We also visited the Santa Barbara, a real bunker where explosives were safeguarded. A layer of concrete separated the TNT from the slate tunnel. Inside, the room looked like a bank caveau, and what I found in front of my eyes was not so different from watching Mecca’s Kaaba. We had planned the basis for our third dive of the day, and it was time to start decompression. 

7:10pm—The Endless Path

We put our heads underwater, again. We left Flavio’s “OK” signal and swam the first 120 m/394 ft of the mine; we had to be quick and save time for the following part of the dive. We wanted to reach the end of the Old Mine’s tunnel. When we arrived at a fork, we got inside an old brick tunnel that was stunning. Below us, the ancient rail track slid away. More or less, we were 350 m/1148 ft  from the entrance, 170m/558 ft separated us from the “touchdown.” Along the way, we observed that many parts had collapsed, sometimes there were walls, and some others were debris that had fallen from the rooftop. 

Large ruins marked out the area, and at around 430 m/1411 ft, we had to swim for 100 m /328 ft more before reaching the farther part of the old branch. The ambience sometimes appeared scary and gloomy. This part was very tricky and precarious. Visiting the right branch was a great adventure, definitely a “must see” place of Felicitas.  

We were back then to the main “T”, where the path split. I was in front of an iron sacred shrine—a holy place where miners used to pray every day before starting their jobs, as well as before leaving the mine. On my left I could see our floating deco cylinders, clipped on the line. I was struggling between a “holy and profane love.” 

Barrel of Black Powder

At fifteen minutes to six pm, the third dive of the day was waiting for us there in Felicitas.  On this morning we placed the emergency line along the left branch. Large, empty spaces and huge machinery left inside the modern part of the mine characterized this area: a main tunnel with added side rooms. Felicitas Mine had closed the extractive industry in 1997.

My mind goes back to this morning’s dive, when we dove at around noon. The planned bottom time was fifty mins, just enough time to  drop our cylinders for further progression. On this day, we would not explore the mine to the end, so we decided instead to simulate different diving scenarios. We wanted to be ready for the main dive on the next day. so. We spent a lot of time below the Steel Barrel Tunnel, always upwards, the steel seemed to be very fragile. On top of the barrel, some massive and huge stones covered its roof. 

Out of the Steel Barrel Tunnel on the right side, 10 m/33 ft ahead more or less, there was the first of the large-scale empty rooms where slate had been mined. On the left of the main path there was a small storage space, and we used its rooftop as cylinders pick-up/drop-off stations. All around was muddy and sometimes foggy. Probably the silt had been stirred up by unstable rocks that fell down.

We moved forward and later we went back. Passing the same spots again and again helped me to memorize this place wrapped in darkness. At the end of the day, we calculated the full length of penetration (return): 1500 m/4900 ft, each of us always carrying from 4 to 6 cylinders, moving ahead by fins only, without the aid of a DPV. Afternoon ran to its end; the last dive of the day was calling us, the most demanding one.

We needed to drop the heavy 20 liter tanks filled with Helitrox 30/10 (30% oxygen, 10% helium, balance nitrogen), on the farthest checkpoint on the map. This was our “home plate.” For the first time, we passed over the final fork. There the tunnelwais narrow and we went ahead, as slow thoughts passed through my mind and brought me back to the main “T”. Another tough scuba diving day in the mine was over. 

Check out The author’s documentary of their mine exploration

I see a Darkness

The Big Wednesday came. We were submerged up to our hips. I switched on my powerful video lights. No video shooting on our way in, we had spread our stage tanks with extra gases on the main line during the last two dives, now it was time to swim quickly. On this day we wished to reach the final target: the end of the left branch. Felicitas Mine was awaiting. We had to go west!

Thirty-five minutes passed, when we reached the planned checkpoint: the “anvil” 20 liter tank of trimix. After we reached it, I thought we were not too far from the End. 

Staying focused on breathing, being calm and relaxed, was what we had to do. This was a blind tunnel with no way out and no chance to find a different way back. We had to pay attention while we swam and moved forward, because a wrong frog-kick and visibility would drop to zero in no time. 

At the end of all the  black shadows we left behind, the slate was simply amazing.

A stunning atmosphere surrounded Gianni and me. I filmed the moment, and I wanted to live it again and again, in time. The rocks of Felicitas gave us crispy feelings. We are enthusiasts. Colors of rocks turned from black to yellow, gray, fire red, and finally light, bright blue. Awesome! I was breathless and without words. 

In front of me there were two stairways to heaven. The first one was a wooden stairway, the second one made of fragile steel. Climbing them were the only fast exits that this mine had. The entrance was 650 m/2133 ft far away, and reaching it during an emergency would be impossible on foot, let alone swimming. 

It was time to go back.

Shattered Ground

The beginning of a mine corresponds with its end. Miners or divers must walk the same steps before reaching the surface, again. When you turn in one direction and leave the black shadows behind you, the darkness swallows everything in the path. Only human memory can preserve the spirit of the life of someone who lived in that manner. The mine doesn’t care how powerful the lights are that you bring inside its rocky belly; it will always give you dark and obscurity in return. 

In July 1969 the Man left a footstep on the Moon. One of the most beautiful memories I collected from Felicitas Mine was looking at the worker’s footsteps on the ancient ground. Along the tunnels, the paths and traces of anonymous miners, pickers, and serial drinkers will remain forever, frozen in time. 

Coming back to Felicitas to learn more  about its stories and secrets, to discover more forks and find an even more beautiful place to film, this was my mood when I left Germany during the  summer of 2020. Diving inside the German’s lode was a human journey between history, economics, and anthropology. 

A Last Shot for the Miners

The last night we spent at Felicitas Mine I decided to honor its working class with a tribute. Each dive we did in the mine had been made possible only because the hands of mine workers had dug the slate for centuries. We had visited incredible places who were stolen by the rock with TNT, pickaxe, sweat, and blasphemies. No dive we did could have been made without the tortuous  job the miners did. Most people know how the working-class men love drinking strong alcohol at the end of a hard day of work.

I had forbidden my team to drink during dive days, even though, by the way, we had been used to having a tiny taste of a traditional Italian bitters every night before bedtime for digestion. When I was finished writing my daily report from the darkness, I proposed to the team: “Why don’t we offer a last shot from our bottle to the miners?”

The following day, before leaving the mine, we dedicated our last dive inside the black slate tunnels of Felicitas to the miners. I put a message in the bottle and we left it inside the Mine, and we offered a last shot to the workers amongst the black slate’s powder. 

Well, you’re my friend
Many times we’ve been out drinking
But did you ever, ever notice
Well, you know I have a love
And you know I have a drive
But can you see this opposition
That it’s dreadful imposition
And then I see a darkness

Diving Team: 

Andrea Murdock Alpini, Gianni Cecchi, Flavio “FlaK” Cavalli, Luigi “Mangiafuoco” Parolo, Michael “Flower” Forenzi, Marco Setti

Team’s Sponsor:

PHY Diving Equipment

Team’s Partners:

Scubatec, Tecnodive Booster, Big Blue Lights, TEMC Gas Analyzers. 

Dive Deeper:

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

Divernet: A TALE OF TWO MINES by Stefan Panis

Other stories by Andrea Murdock Alpini:

InDepth: A Baltic Elegy: Åland Islands and the Wreck of Nederland

InDepth: My Love Affair with the MV Viminale, the Italian Titanic

InDepth: Stefano Carletti: The Man Who Immortalized The Wreck of the Andrea Doria

InDepth: No Direction Home: A Slovenia Cave Diving Adventure

InDepth: Isverna Cave, Diving An Underground Dacia

Andrea Murdock Alpini is a TDI and CMAS technical trimix and advanced wreck-overhead instructor based in Italy. He is fascinated by deep wrecks, historical research, decompression studies, caves, filming, and writing. He holds a Master’s degree in Architecture and an MBA in Economics for The Arts. Andrea is also the founder of Phy Diving Equipment. His life revolves around teaching open circuit scuba diving, conducting expeditions, developing gear, and writing essays about his philosophy of wreck and cave diving. Recently he published his first book entitled, Deep Blue: storie di relitti e luoghi insoliti.


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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