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U-1105: The Black Panther, The World’s Most Accessible U-Boat

Explorer and maritime historian Erik Petkovic details the development, history and eventual demise of Germany’s U-1105, which represented the cutting edge of the arm’s race between subsea hunters and the hunted during WWII. Resting in the brackish waters of the Potomac River near Piney Point, Maryland, USA, the U-1105 is the most accessible sunken U-boat along the US East Coast.



by Erik Petkovic

All photos by Erik Petkovic unless otherwise noted.

U-1105 was a highly modified Type VIIC nicknamed the Black Panther.

In the infancy of 1942, the American shoreline was ablaze and its waters ran red with blood and black with oil. Operation Paukenschlag (Drumbeat) had just commenced. The mastermind of Paukenschlag, Admiral Karl Donitz, authorized five Type VII U-boats to unleash hell in American waters. 

Reinhard Hardegen, commander of U-123, and his fifty-one man crew were in one of the U-boats assigned to Paukenschlag. Although unknown at the time, the world would know who he was in short order as he took aim at a tanker from 4 km/4,400 yards. U-123 hit the tanker with multiple torpedoes and sank Norness off New York. When Americans awoke to the headlines the next day, America would never be the same, and neither would the American psyche. This was the second time in five weeks (Pearl Harbor was attacked on December 7, 1941) and the second time since the War of 1812 that America was attacked on its home soil. 

Paukenschlag was a massive success from the Kriegsmarine perspective. From the Allied perspective, it was massively destructive. Paukenschlag was just the initial onslaught. The drum continued to beat long after Donitz’ five U-boats returned home. In the first seven months of 1942, 109 ships were lost in American waters along with 2,081 mariners. 

To combat the significant loss of life and vessel, the Allies—through ingenuity and engineering—developed three keys to turning the Battle of the Atlantic. The first was High Frequency Direction Finder aka “Huff-Duff”. Although this technology was not new (as the ability to locate low frequency bearings had been used for years) the British were able to tune the technology to be able to take bearings on high frequencies when U-boats surfaced to transmit weather reports or to coordinate wolf pack attacks. 

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The second advancement—breaking the secret code of the German Enigma machine—was arguably the most significant. The painstaking work by Alan Turing and those at Bletchley Park not only turned the tide of the Battle of the Atlantic, but of World War Two.

The further development of ASDIC, named after the Anti-Submarine Detection Investigation Committee, proved an effective weapon against the Grey Wolves. Known as SONAR to the Americans, ASDIC evolution was war-changing. The hunters became the hunted. 

These advancements were so effective that while 109 ships were lost in American waters in the first half of 1942, zero vessels were lost in the second half of the year. As a result, U-boats were being obliterated at an alarming pace. The horrific U-boat losses caused a change in tactics: Donitz recalled the U-boats from the western Atlantic, the Kriegsmarine shifted back to lone wolf operations from wolf pack attacks, and the Allied advancements necessitated new U-boat design and engineering. Enter U-1105.

A view off the port bow of U-1105 in dry dock. Alberich tiles can be seen attached to the hull.


When U-1105 was launched on April 20, 1944, (Hitler’s 55th birthday) World War Two was a year from being over. The “Happy Times” had long since disappeared. The wolves were being slaughtered in high numbers. By the end of the war, the casualty rate for Kriegsmarine submariners was nearly 75%—upwards of 28,000 sailors and nearly 800 U-boats would be lost. 

The Germans built seven varieties of Type VII U-boats. Of the 709 Type VII U-boats completed, 577 were Type VIIC, including U-1105. However, U-1105 was not a typical Type VIIC. U-1105 was a highly modified Type VIIC with the latest in German engineering.


The Kriegsmarine attempted to counter the effect SONAR was having on their disappearing U-boat fleet. The Germans developed a synthetic rubber material which absorbed and dispersed sound waves—in effect, they engineered an acoustic camouflage dubbed Alberich.

An early form of stealth technology, Alberich tiles were engineered with different patterns of holes perforating the rubber which allowed the rubber to absorb the sound pinging them by SONAR from Allied ships. Similar to the tiles on the space shuttle decades in the future in which tiles were specifically engineered for a certain part of the space shuttle, specific hole patterns were engineered for specific parts of the U-boat.

Studies showed Alberich could reduce the likelihood of detection by up to sixty percent. Despite the technological and engineering innovation further enhanced by successful trials, only thirteen U-boats were outfitted with Alberich. U-1105 was one of only four U-boats outfitted with Alberich that went on a war patrol. 

A tile of Alberich from U-1105’s conning tower. Each tile contained a unique set of holes to absorb sound waves – an early form of stealth technology.
A sheet of Alberich can be seen peeling away from U-1105’s hull.

GHG Balkon

Early U-boats were outfitted with a passive sonar array called Gruppenhorchgeraete (GHG). The GHG was placed just aft of the torpedo tubes on either side of the bow containing a dozen hydrophones which helped the U-boat listen to underwater sounds and decipher Allied propellers.

Limitations with GHG lead to the development of the GHG Balkon—an advanced listening system with 48 hydrophones. The GHG Balkon increased the effective range of the U-boat’s listening capability by an astounding 70 percent. The system was so effective sounds could be heard over the horizon. U-1105 was one of nearly two dozen U-boats to be equipped with GHG Balkon. 

The GHG Balkon – an active sonar array containing 48 hydrophones – was a significant upgrade which allowed U-Boats to hear sounds over the horizon. The GHG was so effective it was immediately copied by the Allies and placed on submarines.


U-boats had two ways of operating – either by electric, battery powered motors or by diesel engines. U-boats could not use the diesel engines while submerged; otherwise, the engines would suck all the air in the U-boat into the engines and the crew would suffocate.. Installation of a schnorkel allowed the U-boat to operate submerged while using the diesels.

The introduction of the schnorkel was more successful than any navy would have imagined. At the beginning of the war, U-boats could stay submerged for three days. By the end of the war, schnorkel technology had been perfected to allow a U-boat to stay submerged in excess of sixty days. This remained a record until the US Navy broke it in the 1970s, but only by use of nuclear power and after decades of Cold War engineering. 

A US Navy sailor testing a U-Boat schnorkel in the Chesapeake Bay in 1956. 

Black Panther

U-1105’s conning tower emblem depicted a black panther sprawling over the world. U-1105 commanding officer, Oberleutenant zur see Hans-Joachim Schwarz explained how U-1105 received its nickname: “The name and the heraldic figure were the result of a competition in the crew. A black panther is a beast of prey in the open country. In wartime, a U-boat is also a beast of prey that attacks ships on the surface. Because the boat was covered with a black rubber coat, we thought that the name was very suitable for U-1105.”

A starboard quarter view of U-1105’s conning tower and upper and lower wintergartens with automatic weapons prior to sea trials with the Royal Navy.


On April 12, 1945, U-1105 ventured out in the name of the Fatherland to the western approaches of Ireland—U-1105’s hunting grounds. Careful transit of the North Sea saw U-1105 arrive west of the Outer Hebrides on April23. Schwarz surfaced briefly to send a transmission to Befehlshaber der U-boote (BdU)—U-boat headquarters. The message was short and concise: “My position is AM 0213. Weak defense and patrol.” The U-boat was so close to shore the men could see advertisements, lights, and vehicles. No one knew U-1105 was there. 

U-1105 commanding officer Oberleutnant zur See Hans-Joachim Schwarz.

On April 27, 1945, HMS Conn, HMS Redmill, and HMS Rupert, members of the Royal Navy’s 21st Escort Group, were in Irish waters carrying sonobuoys to detect U-boats. U-1105 heard the noises of the three ships. At this time, HMS Conn picked up a sonar contact which was believed to be a U-boat, but was in fact an incoming torpedo from U-1105. Fifty-seconds after firing the T5, a massive explosion rocked HMS Redmill. U-1105 remained undetected.

U-1105 attempted to descend to 100 m/330 ft to escape what was believed would be an imminent and ferocious depth charge attack. The emergency descent quickly became uncontrolled with two tons of water in her ballast tanks. U-1105 hit bottom at 570 feet. U-1105 turned off its engines and sat on the bottom. Schwarz counted 299 depth charges dropped on U-1105. After 31 hours on the bottom while avoiding detection, U-1105 surfaced.

The crew of U-1105 poses on the conning tower and upper and lower wintergartens. The command staff stands on the main deck. Commander Schwarz in third from the right in the white hat. 


Six days later U-1105 received the order to cease all combat. World War Two was over. U-1105 surrendered at Lisahally. En route, U-1105 destroyed all classified material, communications, and paper, including her log book. All torpedoes in the tubes were discharged overboard. Additionally, all ammunition and seals for the submarine’s weapons were dumped overboard. Ironically, U-1105 surrendered to the 21st Escort Group – the same group U-1105 attacked and killed 32 of its men when it torpedoed HMS Redmill

A previously classified drawing showing the placement of the 250 pound MK6 depth charge suspended beneath U-1105’s hull.

Testing & Evaluation

All U-boats surrendered at Lisahally, with the exception of U-1105, were destroyed per Operation Deadlight. Intelligence personnel knew from previous prisoner interrogations that Alberich existed and that the Allied forces were keen to start testing and evaluations. U-1105 was the one prize the US, British, and Soviet forces were interested in testing. The Americans were eager to get their hands on U-1105, but would have to wait until the Royal Navy completed their own trials and testing.

In January 1946, after a harrowing trek across the angry North Atlantic, U-1105 finally arrived in the United States and was authorized for salvage and explosives testing. At the conclusion of the proposed tests, U-1105 “shall be finally disposed of by sinking in waters of such depth as to assure a swept depth of at least fifty feet.”

U-1105 at detonation in the Potomac River.

Thirteen months of testing saw U-1105 sink five times. On September 19, 1948, at 1229 hours, a 113 kg/250 pound MK2 depth charge suspended 9 m/30 ft below U-1105’s conning tower was detonated. U-1105 sank in less than one minute for the sixth and final time. 

US Navy personnel arming the MK6 depth charge.

Diving U-1105

U-1105 rests at a depth of 28m/91 ft in the brackish waters of the Potomac River off Piney Point, Maryland—the most accessible sunken U-boat along the US East Coast. U-1105’s bow points south toward the Chesapeake Bay. The wreck sits in an area with heavy commercial shipping traffic. Not only is the wreck easily accessible, but from April to November the wreck is easy to locate—a buoy marks the site.

Original drawing of U-1105 embedded in the bottom of the Potomac River by James Christley in 1991. Starboard bow orientation. Courtesy NPS. 

The buoy is marked with several advisories including: POOR VISIBILITY, STRONG CURRENTS and US NAVY PROPERTY OBJECT REMOVAL PROHIBITED.

A small diver ball is tied directly to the top of the conning tower at a depth of 20 m/65 feet. The line is usually secured around the base of either the attack periscope or sky periscope. Free descents or hot drops are not advisable on this wreck. With the limited visibility and a seemingly ever present current, one would not be guaranteed to see the wreck if a free descent was performed. It is recommended to dive the wreck based on tide tables, with the best conditions being at or near slack high tide. 

The fall and winter months allow for better visibility, less pleasure boat traffic, and fewer jellyfish. Visibility can double or triple in the winter months (2-3.5m/6-12 ft) compared to the summer months (1-1+m/3-4 ft).

The gun mounts are all that remain of U-1105’s 2cm Flak.

The Potomac quickly turns various hues of mundane color – greenish-brown, then brown, then shades of black. Regardless of color, it is murky. Vertical visibility is almost always better than horizontal visibility. As you approach the 18 m/60 ft mark in the black water, the faint outline of the top of the conning tower comes into view. The most prominent feature is the sky periscope housing and mount which projects above the conning tower.

Other features in the conning tower include original teakwood, attack periscope mount, diesel engine air intake, and housing for the FuMO Hohentwiel radar. The large 2 cm watertight ammunition container is located at the entrance to the upper Wintergarten. The Flak 38 gun mounts can be seen bolted to the original wood decking. 

The hinge and cap are still in place on the storage tube which once housed U-1105’s colors. 
U-1105’s 2cm ammunition container at the aft end of the conning tower.

The diver will continue aft along the starboard side of the upper Wintergarten and drop onto the lower Wintergarten. Immediately to the diver’s left is the external watertight storage tube. This is where U-1105’s flags were stored. Two 3.7mm watertight ammunition containers and the M42 automatic machine gun mount can be viewed.

Teak decking can still be seen on the lower Wintergarten. 
U-1105’s sky periscope mount protrudes from the conning tower.

There are some interesting features to see while dropping down along the main deck and circumnavigating the superstructure. Below the sky periscope, flaring out from the conning tower on the right (while looking aft), is the rectangular base for the schnorkel. This is where the schnorkel would clamp into the conning tower when deployed. Along the starboard side of the conning tower is the exhaust trunking. This intricate piping is a very unique piece of maritime history, as this is one of very few sunken U-boats in the world where this can be seen. 

Alberich can be seen in a multitude of places on and around the superstructure including the conning tower, both Wintergartens and saddle tanks. In 2009, the U-1105 Black Panther Historic Shipwreck Preserve was included on the inaugural list of the National Oceanic and Atmospheric Administration’s National System of Marine Protected Areas. 

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Dive Deeper

US Naval Institute: “To Hell, By Compass: The Remarkable Wreck and Rescue of USS S-5”
by Erik Petkovic

Osprey Publishing: German submarine U-1105 ‘Black Panther’ THE NAVAL ARCHAEOLOGY OF A U-BOAT

Wikipedia: German submarine U-1105

Erik Petkovic is an explorer, author, maritime historian, shipwreck researcher, and technical wreck diver. Erik is the author of multiple wreck diving and maritime history books. Erik has been featured in publications worldwide and is a consultant for various production companies. Erik regularly presents at the largest dive shows and museums and is a sought after presenter due to his unique storytelling and in-depth research. He currently lives in Southern Maryland with his wife and sons.


Über-deep with Cave Explorer Xavier Meniscus

Xavier Meniscus is one of a handful of elite cave explorers who consistently push sub-200 meter plus depths; he holds the record for the deepest cave dive. To accomplish these dives, the former French military and commercial diver utilizes three rebreathers, and is known to possess unusual physiological tolerances that have allowed him to exceed limits without adverse consequences. InDEPTH chief Michael Menduno reached out to Meniscus to discuss his penchant for exploration and his unique approach to cave diving.




By Michael Menduno, with help with translation by Didier Drageuiev. Photos courtesy of Xavier Meniscus.

Fifty-five year old French cave explorer Xavier Meniscus is one of a handful of elite divers making sub-200 meter cave dives and holds the record for deepest cave dive at 286 mfw/938 ffw, which he conducted at Font Estramar in 2019. Meniscus, who was trained at the Institut National de la Plongée Professionnelle (INPP) in Marseille, is a former army combat diver, and a commercial saturation (SAT) diver with COMEX. He started cave diving in 2000, while still a SAT diver, and has explored many caves around the world such as the Goul de la Tannerie in France, the Pozo Azul in Spain, and Boesmansgat in South Africa. Meniscus is also a cave diving instructor with the Fédération Française d’Etude et des Sport Sous Marin (FFESSM). Meniscus recently published a book on his cave exploration work, Les mysteres de l’eau en pays Gervanne entre l’emergence de Bourne et les Fontaigneux, [Translation:The Mysteries Of Water In Gervanne Country Between The Emergence Of Bourne And The Fontaigneux], which is available in French. InDEPTH sat down with the ardent French explorer to discuss his approach to deep cave exploration. Here’s what he had to say. 

InDEPTH: Xavier, I would like to discuss your record deep dive to 286 meters at Font Estramar, but before we do, I would like to get a little background on you and your diving. How many sub-200 meter dives have you conducted?

Xavier Meniscus: I have done more than 2,000 hours of rebreather diving in caves and have made around 50 sub-200 meter dives. 

Amazing. What is it about deep diving that you find alluring, that draws you?

Mike, it’s not the depth. The depth doesn’t matter. There is no specific target. My goal is to explore and to find what is behind the next cave, and the next. That’s the only thing that matters. There is no sense to go deeper just for the record. If you go deeper, if you take risks, it’s for exploration. Because exploration matters, and the record—it doesn’t matter. It’s completely not the point. This is the reason why I don’t want to have the Guinness record. It doesn’t matter for me. My motivation is exploration.

That’s good. I understand. How do you manage the challenges and risks on deep sub-200 m cave dives? How do you prepare yourself?

Xavier Meniscus

I have been a military diver and commercial diver, a saturation diver working with COMEX, and have worked with researchers and hyperbaric doctors, and so I have specific knowledge and understanding of the phenomena and risks. So, because of this knowledge and experience as a professional diver, I have constructed my own decompression and diving procedures. 

About the risks, as a military diver I am very, very rigorous about following procedures. I prepare and think out everything step by step ahead of time in order to be very precise on the dive. And it doesn’t matter if it’s a 50 m/164 ft dive or a 200 m/656 ft dive, I follow the same checklist and procedures.

Yes, that makes perfect sense. There is little or no margin for error.

I’m very rigorous, and I review all the aspects of the dive before-hand and review them in my mind as I am conducting the dive. Okay, step one is here, I check this. Step two, I check that and then I continue. And of course, there is always the awareness, call it adaptation to the depth and the cave, of where I am, and what is really the most dangerous thing specifically in the moment. I try to keep all the specific things in mind as I proceed. 


Let’s talk about your equipment. I understand that you use a triple rebreather.

That’s right. I took an example from famous cave explorer Olivier Isler, who had been diving since the 1990s using a triple rebreather. I worked with Frédéric Badiér, who had designed a homemade semi-closed recycler called Le Joky. I worked with him to modify the unit to a mechanical closed circuit rebreather with a constant mass flow at 0.7-0.8 liters per minute flow. It’s a very simple unit with a 2.5 kilo canister, breathing bag in the central part of the rebreather with a mechanical KISS valve. The unit has three sensors connected to a Divesoft dive computer. It’s a basic rebreather.

And you use three of them. How do you position them? 

I wear two rebreathers on my back—one on the left side, the second one on the right side—and one on the front. The recyclers are the same, exactly the same. I breathe on one, the second is a spare and there’s a third in case I have problems during decompression. So, I can switch from the first, second or third one if I need it. There are interchangeable connections for oxygen and diluent of course. 

My protocol is that I only use my main rebreather. I test my second rebreather at strategic points during my dive, for example before leaving, before the deep or narrow zone. The third rebreather is placed at the safety landing, connected to its diluent. The dive computer of my second rebreather has the PPO2 display permanently displayed and I take a look at it very regularly. If it was not waterproof, it would fill with water and become negative, so I would notice this and stop my dive. Also, on long dives, I switch to my second rebreather after 9-10 hours. The third recycler is always just the security. Which is really the worst case, you know. 

So, you always dive them as a trio?

When dives are extremely long and very demanding, having three rebreathers is essential. When the dives exceed 7-8 hours, I take a third rebreather.

And just to clarify, the units are all independent. They are not connected by a bailout valve (BOV) or other means.

No, it’s completely independent; there is no connection between the left and right rebreather and there is no BOV as well. It’s just a simple mouthpiece. So if you want to switch to the second one, you turn off the first one and you go to the second one. You need to do that manually. 

Have you ever had to bail out on a dive for real on your rig, and how was that?

I only had to use my social security recycler once. It was at Baume des Anges. When crossing an inter-siphon, I dropped my main recycler. The counter lungs were damaged, and when passing the next siphon at a depth of 100 m/328 ft, my rebreather filled with water. I was then forced to return with my safety rebreather.

Gotcha. And just out of curiosity, do you wear a rebreather safety strap i.e., a ‘gag’ strap? 

Yes, I’m a former military diver. It’s completely mandatory for us, even for a dive of three meters, you know. This thing can save your life. This is clear.

I understand that you run your oxygen levels very high on your rebreather. Talk to me about that. 

During my descent, I keep my PPO2 (partial pressure of oxygen) at 1.4, just to have a little margin if I descend very, very quickly, in order to not completely blow up the PPO2. After the descent, I try to keep my PPO2 at 1.6 for the entire rest of the dive. 

Wow, that’s surprising! Why do you run your oxygen that high? I mean, it might decrease your decompression obligation, but aren’t you worried about getting a CNS toxicity hit and convulsing? 

So, first of all, I tolerate the high oxygen pressures very well. I haven’t any issue with that. My experience as a military and professional diver taught me, step by step, what the human body is capable of and that my body tolerates high levels of oxygen.

[Ed. Note: Do NOT Try This at Home!!! The overwhelming community consensus is to maintain PO2s below 1.6 atm during the working portion of the dive, and the trend has been towards lower oxygen levels, particularly on big dives. GUE for example, sets its maximum at 1.2 atm during the dive with the option to manually boost PO2s to 1.3-1.4 during shallow decompression. CNS oxygen toxicity convulsions have been recorded at PO2=1.4, and can occur at PO2s less than 1.4 atm in the presence of heightened CO2, which occurs when gas density is high, whether or not it is felt by the diver. Susceptibility varies among individuals and within the same individual at different times. There’s no evidence that tolerance to oxygen toxicity can be improved through practice.]

Meniscus’ profile to a max depth of 286 m/938 ft with a 22 min bottom time at Font Estramar (2019).

You may have a high tolerance. But do you ever worry that you are just lucky, and one day you might convulse? Have you ever had an oxygen convulsion? 

I’ve never had any toxicity or convulsions with oxygen. My longest exposure was a dive lasting over 13 hours during my last exploration of the St Sauveur chasm in 2013. At that time, we weren’t using a double DPV or taking into account the PPO2 measurement from my computers. I did a total of nine hours at 1.6 bar and the last four hours at 1.9 bar in a bell at a depth of 9m/30 ft.

It’s true that when I came out of the water, my lungs were burning a bit for several hours but it disappeared the next day. To avoid too much oxygen toxicity, I take ‘air’ breaks after seven hours of diving, and breathe air or bottom mix for 5 minutes every half hour.

Do you worry that others will try to follow your example and run their working PO2 setpoint at 1.6 and get hurt? I’m sure you know that PO2=1.6 is outside of current accepted community practice except possibly during shallow water decompression, particularly for CCR. What are the standards for oxygen levels in the Fédération (FFESSM)? What do you teach your students? 

In France, we recommend a PPO2 of 1.4 for progression [the working portion of the dive] and 1.6 for decompression. That’s what I teach. The use of high PPO2 for me is a necessity at the depths I go to in order to reduce the inert gas load and limit my very long decompression times as much as possible. It’s an approach that I’ve validated through my training, my long experience and my personal physiological resistance. It is also obvious that on such extreme dives, we push the sliders to the maximum.

[Ed.note: According to V-Planner calculations, boosting PO2s to 1.6 during the working portion of a 286 m dive for a 22-minute bottom time vs maintaining a PO2=1.0-1.3, results in some, but not a significant decrease in overall decompression time. It does however greatly increase the risk of CNS oxygen toxicity. ]

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I understand. How deep you run your END (equivalent narcotic depth)? I understand you are using trimix. 

I always use trimix with a high level of helium. I maintain my equivalent narcotic depth to around 30 m/100 ft. I always keep a little nitrogen fraction into the mixture and do not use a pure heliox because of the high pressure risk. 

Not only that, I think it’s better to have a very, small narcotic sensation during the dive. A very small, very tiny sensation of narcosis. This acts as a reminder to me that says, “OK I am very deep and I need to be careful.” So, it’s a kind of alert. These are the reasons I don’t use pure heliox. 

That’s understandable on deep tech dives because of High Pressure Nervous Syndrome (HPNS). Has HPNS been an issue for you? 

Interestingly, during the descent, I often have the sensation of cold, above 200 m/656 ft, which could be the trimix, which is very near to heliox, in fact, with high levels of helium. Of course, the automatic diluent valve (ADV) is injecting the very cold diluent, and that could be the reason for my shakes. Then, for 20-30 seconds,when I reach 230-240 m/755-787 ft. I experience shaking, trembling, and some eyesight problems. However, when I continue my descent, the effects begin to disappear, and when I reach 260 to 280 m/850-920 ft, there is absolutely no effect. It’s over. It’s just 30 seconds or so at 230-240 meters. 

Wow. It would seem that you are not affected much at all by HPNS. Again, it sounds like you have some unique physiology. 

That’s exactly what I think. 

Has anyone studied you to figure out how you can do what you do? Are you aware of any reason why this is? 

It’s just practice and training, practice and training, that’s all. Year after year, dive after dive, I gradually began to have a low sensitivity to gas toxicity. I think that’s really the most important thing. The key is really the practice and the training. 

I am not aware that divers can modify and or improve their gas tolerances through practice and training. I’m pretty sure that’s the case with nitrogen narcosis, despite what some have to say. Have you had any problems with carbon dioxide (CO2) buildup? Any CO2 problems? 

This is an interesting question. There are two principal things about CO2. First of all, always choose the best performing soda lime for deep diving. You can use the cheap version of soda lime for shallow dives—it doesn’t matter. But for deep dives, below 200 m/656 ft, you should always choose the best performing product for that. 

The other thing is during the decompression. During very long decompressions, like eight, nine, or ten hours of decompression, the human body produces a very small quantity of CO2. That’s the reason why I am able to do a 10-hour dive on the same rebreather with only 2.5 kilograms of soda lime inside. So, no, I have never had any problem with the toxicity of CO2. I don’t think I am very sensitive to it. 

Have you ever had a caustic cocktail? 

Yes, just once. I had just made a ventral rebreather. During my first dive, a connector on the counterlung came loose, and the rebreather slowly filled with water. At the end of the dive, at the surface in the basin, I put my head down to test breathing comfort. The caustic cocktail went into the mouthpiece and I breathed it in. I had a bad cough and a broken voice for several days.

Last question before we get on to your dive, what kind of support team do you use for deep dives? Do you use support divers? Where are they positioned? 

I typically have a team of six divers, not more, positioned down to 80 m/262 ft. I have a diver wait at 80m/262 ft to be available for help if needed, and I descend from there by myself to make my dive. 

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So you are diving solo at depth. That seems to be the typical practice for über-deep dives with the exception of Richard Harris & Craig Challen and the Wet Mules. Do you ever think about doing these uber-deep dives with a team mate at depth vs doing the bottom portion by yourself with support divers high above you in the cave? Would it be safer? 

In fact, it is safer to do this type of diving as part of a team. In most cases, I have a team around me. A dive director and support divers who intervene at different depths. In general, the maximum depth at which my team members intervene is the one where they can spend a lot of time waiting for me if I’m late. Usually, as I just mentioned in the 80m/262 ft zone. Having used more than 20 support divers on some occasions, I now limit my support team to a maximum of five or six divers. That makes it simpler for the dive director to manage. The support divers can make up to two assistance dives during the operation, the second dive being mainly during my final stops.

When the depth allows it, and other divers can accompany me, I prefer to dive in pairs. Like my explorations of Baume des Anges with Rick Stanton and John Volanthen or Durzon and Marnade with Pédro Balordi.

OK, let’s talk about the dive at Font Estramar. Let me first ask, how long you have been exploring the cave and the dives you’ve made there.

My first dive in this cave was in 2013. I did four successive operations in the same year in order to clear the record.

And the dives went as planned?

Yes. My first dive, the first exploration dive in this cave was to a maximum depth of 180 m/591 ft. My second dive was 248 m/813 ft, the third dive 262 m/859 ft, and fourth dive, just before the record, I spent at the same 262 m/860 ft depth with 40 m/130 ft of horizontal exploration, trying to find the way forward. I was unsuccessful, so I turned the dive and ascended.

I understand that you tested your rig in a hyperbaric chamber down to 320 meters/1050 ft of depth before the fourth dive in order to make sure everything worked well before going deeper. 

Yes, I tested it in Switzerland with no problems. 

Good. Everything checked out. So, you were able to find going passage on your fifth dive at Estramar and go deeper and ultimately ended up setting the record. 

Exactly. So, after my fourth dive, I did an analysis of the videos, images, and everything I could remember about the environment. The result was I chose to think about continuing horizontally without going deeper. So, in planning the dive I prepared gases for a max depth of 280 m/919 ft. I used trimix 5.5/83 (5.5% oxygen, 83% helium, 11.5% nitrogen).

That’s a gas density of 10.76 g/l at max depth with the high oxygen and nitrogen levels, which is considerably above the recommended max of 6.2 g/l. Are you worried about your gas density or feel that it is OK for you?

As far as density is concerned, at the depths I go, it’s difficult to respect the recommendations, but as my JOKI rebreathers are very easy to breathe, compared with commercial equipment, breathing is very fluid and doesn’t bother me at the bottom. The most important thing is not to work hard and to use a diver propulsion vehicle (DPV). What’s more, I cycle regularly—at least 5,000 km a year—to get the basic training I need to control my breathing during a violent physical effort.

Back to the dive: you were able to find the way deeper into the cave?

Once I was on the bottom and into the terminus room of the cave, I looked all around me in order to find a pathway or passage. In fact, I didn’t find anything. But then I looked in the direction of the bottom and found a hole or well. That was the way deeper, and I spent another 30 minutes in the cave exploring. 

You must have been excited! How far did you push it?

It was really a pure exploration at this depth. There was a passage, but it was a very, very small passage into this well. At around 270 m/886 ft, I found another room, a little bigger. At that point I had to make a decision. I knew I was very close to the world record, so I decided to try to push a little bit more to 286 m/938 ft, and then I turned the dive and began my ascent. I had not planned to go that deep, and in fact had a bit of stress.

What happened?

I returned to the room at 262 m/860 ft, which was a bit tricky, and looked around but couldn’t find the line. I searched with my light and didn’t find it. I could feel my adrenaline and stress rise. Imagine losing the line at that depth. 

OMG! That would definitely generate some stress!

One of my ties came completely undone, and the line had floated to the top of the room, where I couldn’t quite see it. The visibility is never good in Font Estamar. Fortunately, I did find the line after some tense minutes and continued my ascent. 

Do you feel that you were at your physiological limit at that max depth? Or did you feel like you could go further in the future? 

As I mentioned, I had some signs of HPNS when I hit 286 meters, but within a minute or so, there was nothing. All signs disappeared. I had a completely clear mind when I was trying to find the line. This is really proof to me that there is not a problem with the gas at this depth. Which means I can continue. 

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As you know, Richard Harris, aka Dr. Harry just completed a 230 meter test dive with a hydreliox mix (oxygen, hydrogen, helium) in order to lower the gas density and work of breathing and possibly ameliorate the HPNS. Have you given thought to using hydrogen?

Of course, I have talked about hydrogen with Jean-Pierre Imbert and Bernard Gardet, who was the scientific director at COMEX. But there is no problem for me right now. Using a rich trimix is not a problem. I only have the slightest tremors which then go away. So no, I don’t think there is a reason to change anything. I think I can continue to 300 m/984 ft with helium.

Have you had any close calls on your dives?

Yes, a Bonex scooter that I was towing between my legs imploded at the bottom of St Sauveur. It had suffered some shocks on previous dives in very narrow cavities. The explosion tore a hole around 3 cm in my drysuit—it was no longer a drysuit, but a wetsuit. It was completely filled with water during my seven hours of decompression. 

OMG! How cold was the water? 

13°C for seven hours! Yeah, it’s a little bit cold. 

Were you using a habitat, deco habitat? 

No, no I wasn’t.

That must have been grueling.

It was.

Have you ever gotten bent? Decompression sickness? 

No, no, no, nothing. Again, I think it’s my physiology, and the rigor and specific training and practice, being careful every time for everything I do.

What are your plans for the future Xavier?
This Fall, I would like to continue my explorations of the Gouls of Tourne and try to find the junction between Goul du Pont and Goul de la Tannerie. And I plan to continue my explorations at Port Miou, which is not deep, and also the Source du Marnade and the Durzon.

I am guessing that you have specific goals and objectives for each.

Yes, many goals, and many objectives, in fact. To simplify, the object is to map and explore the different sites, continue to find new galleries, and in the case of Goul du Pont, find out if there is a connection. 

So many caves, so little time! Will you also be returning to Font Estramar. Do you plan to go deeper there?

It’s not a principal target. I continue to think about it and try to make the best decision about this specific site. But it’s not the first target. I want to continue the exploration of the other sites first, but I do think it’s possible to reach 300 meters at Estramar.

Good luck to you Xavier! We’ll be watching. Thank you!


YouTube: World Record Cave Dive -286.2m (939 feet) 30-déc-2019 VF – Xavier MENISCUS – Font Estramar

YouTube: World Record Cave Dive – 282.6 m (927 feet) – Nuno Gomes (1996)

Facebook: Xavier Méniscus-New World Record : Le Joky mCCR

InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties

InDEPTH: When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving .

InDEPTH: Omne Trium Perfectum


YouTube: Cave Diving in France legendary Font Estramar by Europe Cave Diving

Michael Menduno/M2 is InDepth’s editor-in-chief and an award-winning journalist and technologist who has written about diving and diving technology for more than 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving” (1990-1996) helped usher tech diving into mainstream sports diving, and he produced the first tek.Conferences and Rebreather Forums 1.0 & 2.0. In addition to InDEPTH, Menduno serves as an editor/reporter for DAN Europe’s Alert Diver magazine, a contributing editor for X-Ray mag, and writes for He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council.

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