Connect with us


Charting Sardinia’s Grotta del Fico

Swiss u/w photographer and tech instructor Gatien Cosendey takes us for a dive into PHREATIC’s recent “Map the Gulf” citizen science project. Their goal? Produce a digital map of Grotta del Fico, one of the major springs located along Sardinia’s Gulf of Orosei, using MNemo devices and Ariane’s Line software. For the citizen scientist in you!



By Gatien Cosendey. Images courtesy of G. Cosendey unless noted.

Central-eastern Sardinia’s Supramonte mountain range is one of the biggest karst areas of the island, covering a surface of about 170 km2. It is made of nearly a kilometer-thick layer of limestones and dolostones that covers a granite and schist basement and comprises more than 400 known caves, out of which only two percent contain important groundwater bodies (Cabras et al. 2008). This karst aquifer extends eastward up to the Tyrrhenian Sea and encompasses the Gulf of Orosei, whose 37 km/23 miles long shoreline makes up a succession of vertical carbonate rock cliffs reaching a height of 700 m/2,297 ft (Sanna and De Waele 2010). The main cave diving springs along the Gulf of Orosei are; Grotta del Bue Marino, Bel Torrente, Risorgenza di Cala Luna, Utopia, Euforia, Grotta del Fico and Macedonia.

Map of the Gulf of Orosei (D’Angeli et al. 2013).

Based in Cala Gonone, Phreatic is a nonprofit organization founded in 2014 by a group of explorers, researchers, and scientists who are mainly active in the Supramonte plateau and Gulf of Orosei areas. Their stated mission is to advance knowledge about groundwater resources and marine caves, and to insure their protection and conservation, with projects covering a broad spectrum of topics including environmental processes, geology, speleology, marine biology and conservation. 

We believe it’s important to raise public awareness of not only how important these fragile karst geo-ecosystems are, but also about the hazards related to these environments, such as sinkholes and flash floods. “Map the Gulf” is one of the ongoing projects from Phreatic, and it relies on citizen science to gather data and survey flooded caves of the Gulf of Orosei with the goal of making all the resulting data available to the public. 

Indeed, the available data on these cave systems is scarce outside of the dry passageways, and detailed and complete maps are missing. The latest Map the Gulf campaign started in March 2022 and focuses on Grotta del Fico. Operations are run from Base1 Sardinia, a dive center located in Cala Gonone delivering logistics for cave and wreck diving in the Gulf of Orosei (See PHREATIC).

Phreatic Exploration

Grotta del Fico, together with Bue Marino, are the two cave systems of the Gulf, which have dry passageways open to tourists in addition to underwater conduits for cave divers. Each summer more than 15,000 visitors come and admire the beauty of the speleothem formations within Grotta del Fico, accessing the cave through the main entrance located 10 m above sea level on a steep cliff. Two other entrances open underwater, one of them being used by cave divers. 

Cave exploration of Grotta del Fico started in the 1960s, and recent exploration revealed a total development of cave conduits to more than 2.5 km/8203 ft. These conduits are characterized by a main air-filled passage parallel to the coastline and a smaller mostly submerged branch running perpendicular to the coast (Sanna and De Waele 2010, D’Angeli et al. 2013).

Current map of Grotta del Fico showing both dry and flooded tunnels (D’Angeli et al. 2013). Note the question marks at the end of the submerged western end of the cave.
One of the two seawater entrances to the Grotta del Fico.
Restriction located at the start of Grotta del Fico main line.
Grotta del Fico submerged passages are often narrow and full of features.

I joined the second project session, which took place from June 19-24, 2022. I arrived one day early and teamed up with two fellow project attendees for a warm up dive in Grotta del Blue Marino It is worth explaining the logistics required to dive caves in the Gulf of Orosei—the aforementioned caves are underground rivers that spring directly into the sea, which means that the natural access to dive these caves is from the ocean.
As mentioned, the shoreline is mainly made of high cliffs; therefore, the Gulf of Orosei is only one of few locations worldwide where a rigid-hulled inflatable boat (RHIB) ride is the normal way to go cave diving. For the same reason, cave divers will experience both a halocline and a thermocline while transitioning from the warm seawater to the cool freshwater of the caves.

The line disappears behind the halocline in Blue Marino cave.
A diver is admiring a speleothem in Bue Marino cave. The halocline makes half the image look blurry.

The project week started with a full classroom day led by project leader, Base1 co-founder, and Global Underwater Explorers (GUE) cave Instructor Examiner, Andrea Marassich. The session included a general orientation about local geography and geology, an overview of the project, a statement of objectives, and detailed explanation of methods and procedures. As mentioned, this year’s Map The Gulf campaign is focused on surveying Grotta del Fico’s main flooded passageways.

The Digital Surveying Process

The survey process can be broken down into the following steps; First, stations are defined using numbered cookies, a.k.a. survey cookies, which are placed at each tie-off, tee, and jump arrow of the cave main line. These stations help to spatially reference the different types of data that will be subsequently acquired. The next step makes use of a MNemo device from Ariane’s line to acquire stick map data: the MNemo is clipped onto the main line, measures distances using an optical sensor while running along the line and acquires depth and azimuth angle at both the beginning and end of segments between stations. Afterwards, left-right-up-down (LRUD) distance estimates are collected at each station and written down in wetnotes and correspond to the available space for a diver to swim through the given cave section.

Both MNemo and LRUD data can be collected simultaneously by a buddy pair and will at the end of the day be loaded into Ariane’s Line software (or equivalent) in order to produce the cave stick map with polyhedral volume estimates. The resulting stick map and LRUD distances are then printed on water friendly paper, taped onto chopping boards and taken with a pencil into the cave. While one diver sketches details of the cave walls and features, his buddy records videos of the corresponding passage for further reference. This enables the divers to complete or correct the drawing outside of the cave while watching the videos with survey cookies referencing the video sequences with respect to the sketched map.
The afternoon was about a MNemo dry run with everyone practicing correct use of the device and subsequent data loading into the dedicated software. The day ended up with some of us going for a test dive from shore, the others enjoying the beach. Note that this first day was the only one to be light enough for us to have a bit of chilling time.

Andrea is drawing the Grotta del Fico stick map from memory during the briefing.
The MNemo from Arian’s Line makes cave surveying much easier and more accurate.

Teams were formed on the first day: a GUE Cave 2 (C2) team, a GUE Cave 1 (C1) team, and finally my team, the TDI “freak” team. Indeed, my buddy was diving sidemount and I was flying the Divesoft Liberty CCR, both of us being full cave divers. Previous teams had obtained MNemo stick map and LRUD data of a first segment of the cave of about 175 m during the first 2022 project week and started sketching it. Therefore, the one team would primarily focus on sketching and shooting referencing videos of this first segment, while the other teams would venture further into the cave and start surveying the next segment.

The C1 team’s plans for the week were quite straightforward. Diving on thirds of 2/3 of their total gas, their limited reach would not allow them to penetrate far into the cave. However, their skills would be precious to make use of the previously acquired stick map and LRUD data and sketch the first segment of the cave. 

Recalculating their gas after exiting the cave, they were able to perform two dives per twinset, bringing their total dive count per day to four as they were taking a second twinset on the boat. As the first segment of the cave is below 10 m/30 ft, they had plenty of time to carefully draw details of the cave map plan and profile views as well as the main cave features. Using videos shot during their dives, they were able to further refine their sketches during the data processing time at the end of each dive day.

The C1 team shares tasks. One diver sketches the cave…
…while his buddy shoots videos using a GoPro.

The other two teams took one, sometimes two, stages performing one long dive and extending the survey past the first segment. The tasks performed were, in order: drop survey cookies at all stations, data acquisition for the stick map with the MNemo, LRUD data collection, and finally sketch of cave walls and features. The mixed team also surveyed two bypasses within the first segment while the C2 team laid some new line in the far end of the first sump. In addition to the survey tasks, I also took my camera on four out of the five dives that we performed, sometimes juggling between the camera and the MNemo to fulfill all objectives. Well, to be honest, I screwed up my first MNemo measurement, which explained why I had to deal with both the MNemo and the camera on the next dive.

I would like to thank my buddy Vas for taking care of the navigation and most of the data collection, allowing me to play with my camera. We also took the time to stage a few shots on the last dive, which was pure bliss given the discipline and skill level of all my teammates. We spent a total of more than 13 hours underwater over the five project dives.

This is the Italian sign for “the MNemo is not operating as expected”.
A diver swims past survey cookie number 22.
The MNemo is run along the main line to measure distances between stations.
LRUP data is collected at each station.
Team C2 is sketching a remote segment of the cave.

Bringing Home the Data

Back from the dive, we met at the dive center and processed the newly acquired data. This was a decisive moment, where we would know whether the data collection was properly done, or not. We also planned for the next day’s dives. Setting up the gear in the morning, loading everything in the dive center van, transferring the gear into the RHIB, the half hour boat trip to the cave, and everything in reverse on the way back, plus rinsing the gear, taking care of the rebreather and the camera, charging all video lights and strobes, sorting and processing the daily pictures—the week was gone before I noticed.

The map plan view is completed once back at the dive center thanks to the videos.

During this week, our common efforts bring the stick map length and the sketched plan and profile views to a total distance of 410 m/1345 ft. We replaced 590 m/1936 ft of the old line and took many cool pictures. Most importantly, we all learned a lot and came back with new friendships and awesome memories! There is still a lot of work to be done, and the project will continue with additional survey sessions.

I would like to thank my teammates Joana, Sebastian, Oliver, Marco, and Vas for this great session!

Videos of each day of the June project can be found here:

Subscribe for free

This month we’re launching a survey panel on dive computing. Please help us by sharing your thoughts & practice at: Dive Computers-Exploratory Survey. 


Cabras S, De Waele J, Sanna L. 2008. Caves and Karst Aquifer Drainage of Supramonte (Sardinia, Italy): A Review. Acta Carsologica / Karsoslovni Zbornik. 37. 227-240. 10.3986/ac.v37i2.148.

D’Angeli IM, De Waele J, Ruggieri R, Sanna L. 2013. Pleistocene sea level changes as revealed by flank margin caves in telogenetic limestones in Sicily and Sardinia (Italy). Proceedings of the 16th International Congress of Speleology, Brno 19-27 July 2013. 3.

Sanna L, De Waele J. 2010. Karst landscape and caves in the Gulf of Orosei (Central-East Sardinia): a scientific and cultural resource. 10.13140/2.1.2783.0728.

Dive Deeper

Website: Phreatic: Citizen Science and groundwater research. 

Website: Ariane’s Line


InDEPTH: Data for Divers: Mexican Explorers Go Digital to Chart Riviera Maya by Michael Menduno

Gatien Cosendey is a Swiss underwater photographer and technical/CCR instructor. When not traveling across Europe to enjoy caves, mines, and wrecks, he mainly dives the cold and dark Swiss lakes around his hometown. He loves shooting photos in all sorts of environments, ranging from deep caves to frozen mountain lakes. Besides diving, Gatien holds a PhD in Photonics and works as an optical engineer in the field of augmented and mixed reality.

Subscribe for the InDepth Newsletter


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:

Subscribe for free
Continue Reading

Thank You to Our Sponsors

  • Fathom


Education, Conservation, and Exploration articles for the diving obsessed. Subscribe to our monthly blog and get our latest stories and content delivered to your inbox every Thursday.

Latest Features