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Hranice Abyss: The Deepest Flooded Freshwater Abyss in the World

The efforts to explore and map Hranice Abyss, located in Hranice (Přerov District) in the Czech Republic span more a century. Currently, the monstrous chasm is known to reach 384 m/1260 ft deep.

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By Michal Guba

The cave diving organization “7-02 Hranický kras,” which is a part of the Czech Speleological Society, is responsible for and has overseen the exploration of the Hranice Abyss, the deepest known flooded abyss in the world. The timeline below details the continuing exploration of the abyss. Please note that it is not easy to find divers who are technically and professionally prepared for exploration at a depth of about 200 meters/656 feet in a cave environment. 

In addition to depth, the composition of the water in Hranice Abyss can cause problems for divers. It is a mineral water (‘kyselka’ in Czech) with a high content of CO2 (carbon dioxide), which irritates the exposed parts of a diver’s body. In addition, the water’s composition has influenced the choice of diving equipment. When using open-circuit scuba, the exhalation bubbles cause a chemical reaction in the surrounding water, resulting in a rapid deterioration in visibility—it drops to zero! For that reason, open-circuit dives to depths below 50 m/164 ft were “banned” in 2001. Since then, members of 7-02 Hranice Karst have used closed-circuit rebreathers, which don’t emit bubbles, for exploration beyond 50 m/164 ft. Currently, the dive team has standardized on Divesoft’s Liberty rebreather.

1580: The first unsuccessful attempt to determine the depth of the lake at the mouth of Hranice was conducted by a breath-hold diver in 1580 and was described by Tomáš Jordán of Klauznburk. 

1900-1902: It was not until the turn of the 20th century that a professional teacher, J. V. Šindel of Hranice, repeatedly launched a weighted probe from a boat and reached a depth of 36 meters/118 feet. His findings were not challenged following exploration by geographer J. Dosedla in 1951, but they were quickly debunked by the arrival of divers and modern technologies.

1961: Bohumír Kopecký of Hranice made the very first dive in the Abyss with his handmade diving apparatus, reaching a depth of 6 meters/20 feet. 

1963: RNDr. Jiří Pogoda conducted a systematic dive survey. He found that the bottom forms a slope obliquely pointing deep under the rock massif.

1977: Miroslav Lukáš discovered the first dry space behind Zubatice. The location is called Heaven for its decoration. 

1978: Miroslav Lukáš and Jaromír Andrés discovered another dry space, named Dry Rotunda. There is a greater mouse-eared bat colony during the period from May to September.

1980:  A special glider probe by RNDr. Jiří Pogoda reached an unbelievable 260 m/853 ft.

This was followed by several dives with helium breathing mixture. 

1981: Fraňo Travěnec and Lubomír Benýšek descended for the first time with a trimix mixture to a depth of 110 meters/361 feet. After the borders opened in 1989, foreign divers also began to dive into the Abyss. 

1993: Belgian Michel Pauwels reached a depth of 155 meters/509 feet with a trimix mixture. Depth probes continue to measure areas too deep for cave divers to reach. 

1995: A remote-controlled underwater robot was first used in the Abyss. It was the ROV HYBALL, which at Lift I reached a maximum depth of 203 meters/666 feet; unfortunately, its supply cable got stuck in the fallen wood logs. The operator managed to maneuver it out but its Belgian owner, Carl von Basel, no longer wanted to continue the survey.

2000: Krzysztof Starnawski, who made the “last” deep dive with open-circuit scuba, reached a depth of 181 meters/594 feet and saw the bottom of the core of the Abyss, called Lift I. After the year 2000, divers began using closed-circuit rebreathers, which enabled them to stay longer at deeper depths. 

2003: The underwater robot ROV COLOMBO of the Main Mining Rescue Station a.s. (OKD Ostrava of the Czech Republic) was used to survey Lift I and the “New York” area. It reached a depth of 140 meters/459 feet but was limited by the length of its communication cable (150 meters/492 feet).

For a long time, it was certain that Krzysztof Starnawski of Poland and Pavel Říha saw the bottom at a depth of about 200 meters/656 feet, and that the possible continuation of the vertical direction did not lead directly to the current survey. Therefore, it was decided to provide an underwater robot rather than a diver to investigate the terrain and suggest further action. It was agreed to revive earlier collaboration with the Polish cave diver Krzysztof Starnawski.

2002-2010: Pavel Říha conducted an in-depth survey and mapped Lift I at a depth of 170 meters/558 feet.

2011:  Krzysztof Starnawski had just finished testing a unique, double closed-circuit instrument with which he dove in the Red Sea to the depth of 283 meters/928 feet. 

January 2012: Krzysztof Starnawski settled his 2000 record at 181 meters/594 feet at the Abyss. Two days later he descended to a depth of 197 meters/646 feet, creating a new depth record and discovering a narrow passage (restriction) on the north side that could perhaps be explored.

June 2012: Krzysztof Starnawski achieved an extraordinary discovery in another dive when with much difficulty, he overcame the restriction and descended to a depth of 223 meters/732 feet. By doing so, he confirmed that the Hranice Abyss continued to a greater depth than previous calculations. 

October 1, 2012: On his next dive, Starnawski descended to a depth of 223 meters/732 feet and launched a new probe, which reached 384 meters/1260 feet, setting a new Hranice depth record.

Since 2014: Members of ČSS ZO-7-02 Hranický kras have been using the Divesoft technology for exploration and research into the abyss. Specifically, the Liberty rebreather (back- and sidemount versions) enables members of ZO-7-02 Hranický kras to perform complex work activities up to 100 meters/328 feet (drilling, enlarging holes, positioning sensors, etc.), which fully utilizes the properties of the rebreather, such as low work of breathing and maintaining an optimal PO2. Members conducting dive surveys were also equipped with Freedom dive computers to ensure their safety in the complex depths and so-called “yo-yo profiles” in the Hranice Abyss, and providing for compatibility among the dive team.

June 2015: David Čani made a dive to a depth of 181 meters/594 feet (a new Czech depth record), in which he checked the status of the Lift I axis and, along with other dive participants, practiced procedures to ensure the safety of deep divers performing dives down to 200 meters/656 feet and below. 

July 2015: Krzysztof Starnawski made a dive to a depth of 220 meters/722 feet, and then launched a probe with an electronic pressure sensor. This time he measured the depth of just 365 meters/1198 feet. However, at the ascent, he made a very promising discovery when he examined a new restriction at a depth of 204 meters/669 feet and found that it opened into a passage big enough to drive a Tatra (a truck) through. This discovery was of paramount importance for the safety of divers making dives across the strait at the bottom of a massive well that was previously named as Lift I, because it might give them an alternate exit path.

August 21, 2015: Krzysztof Starnawski made a dive into another well (called Lift II), which was accessible after crossing the strait at a depth of 204 meters/669 feet and has approximately the same slope as Lift I. In this dive, Krzysztof discovered a new opening at 240 meters/787 feet (rock window) into unknown spaces, which he named “Macejko.” In doing so, Starnawski reached a maximum depth of 265 meters/869 feet, setting a new world depth cave record.

2016: The members of the Hranice Karst joined with National Geographic for the “Hranická Propast Step Beyond 400 meters” project with the help of Bartolomiej Grynda, owner of Gralmarine, to test an underwater ROV. On September 27, a new depth of 404 meters/1325 feet was reached during the Gralmarine ROV test dive, making the Hranice Abyss the deepest flooded cave in the world. The robot was again limited by the length of the communication cable of 500 meters/1640 feet. 

The ROV descended to the bottom of Lift I to the “Mikado” restriction and entered the Lift II. In Lift II, Grynda maneuvered the ROV along the cord of the measuring probe to a depth of 384 meters/1260 feet. After reaching the end, he proceeded along the wall to a depth of 404 meters/1325 feet. The robot remained “tangled” near the “Mikado” restriction in Lift I at close to 200 meters/656 feet. The robot was eventually rescued in 2017 by 7-02 Hranický kras and members of the Department of Special Diving Activities and Training from the Police Presidium of the Czech Republic.

2018: Working with the town of Hranice, the ČSS ZO 7-02 Hranický kras opened an information center at the Teplice nad Bečvou railway station detailing the current state of the Abyss exploration. There are 3D glasses available to help the tourists dive into the waters of the Abyss.

“During the dives, we discovered new irregular spaces, which will require further exploration. They are mostly deep and relatively narrow. However, none of the Hranice Karst speleologists doubt that there are still interesting discoveries to be made at Hranice Abyss,” explained 7-02 Hranický kras chairman Michel Guba.

At the moment, speleologists are working to produce maps of both the dry and flooded parts of the Abyss from top to bottom in 3 meter/10 foot increments. At the same time, photographic and video documentation is being conducted to help refine individual measurements.

To create greater awareness of the entire flooded and dry underground labyrinth to the public, all the measurements have been input into mapping programs to create a 3D model. Currently a profile of the cave is displayed on an information board showing the known spaces of the Abyss.

Currently there is an information board with a profile showing the current state of the known spaces on the observation ring near the Abyss.

Hranice Abyss Facts:

  • The deepest abyss of the Czech Republic.
  • The deepest flooded freshwater abyss of the world.
  • First written reference: 1580
  • Recorded on map: 1627
  • Cadastral area: Hranice.
  • Edge elevation of Abyss: 315 meters/1033.46 feet above sea level.
  • Entrance dry esophagus of the Abyss: length 104 meters/341 feet, width 34 meters/111 feet, depth 69.5 meters/228 feet.
  • The depth of the flooded part: 404 meters/1325.46 feet (2016).
  • The total depth of the Abyss: 473.5 meters/1553.47 feet.
  • Depth reached by divers: 265 meters/869 feet (2015).

Additional Resources:

Video of cave diver Jakub Šimánek diving in Hranice Abyss in 2017 recorded by Petr Chmel, a member of a diving club Pragoaquanaut.


Michal Guba worked as a policeman from 1992 to 2017. From 2008 to 2017, he worked as a lecturer, instructor, and deep diver. During a rescue mission (there was a huge flood in the Czech Republic in 1997), Michal decided to take a diving course at CMAS and began diving in 1998. Since 2000, he has been a part of the Hranice Abyss speleo diving team and is currently the chairman.

Michal participated as an expert (de-mine-pyrotechnic and training) on foreign missions in Switzerland, Austria, Bosnia and Herzegovina, Serbia, Croatia, and Montenegro from 2011 to 2016. Michal was awarded the Golden Rescue Cross in 2007 by the President of the Czech Republic Klaus and has received additional awards for his police work. Michael is currently employed by Czech company TRESPRESIDENTES s.r.o.

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My Journey Into Sidemount Rebreathers

With a growing number of users making the transition, sidemount rebreathers are the “New New Thing” in diving technology, especially in cave country. Here, award-winning underwater cameraman/photographer Becky Schott, who is also a rebreather and tech diving instructor, explains her motivation and experience finding the right tool for the right job!

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By Becky Kagan Schott

When I began diving twenty-five years ago I heard the phrase, “A good diver is always learning,” and in many ways I’ve lived by that motto throughout my entire diving career. I’ve been humbled underwater, and I’ve had days when all of my training and years of experience have come together, allowing me to create inspiring imagery with my camera. I wouldn’t be able to do that without experience and the thirst for knowledge. Leave your ego out of it. I mean, we all have one, but the environment doesn’t care if we are male, female, black, white, young, or old, so your ego won’t help you much, but being humble will. I practice skills frequently. I teach others to stay sharp and pass along my knowledge and most of all, even as an accomplished tech instructor, I am always finding new techniques and trying new things. I still take classes and seek mentorship from more experienced divers. I am always learning.

A few years ago while shooting a cave exploration documentary in the Abaco Islands in the Bahamas with Brian Kakuk and Brett Hemphill, I found myself back on open-circuit sidemount, although I prefer to be on closed-circuit rebreather (CCR). We had several stages, and I pushed a large Red Epic Camera along with me. I find open-circuit sidemount to be the most difficult configuration for me to film in. Constantly checking gauges, changing regulators, and moving tanks around, while trying to keep the camera steady and focus on the environment around me, as well as simultaneously directing a team of divers, is challenging. I’ve always enjoyed diving sidemount, and after that shoot I had a handful of other projects that required yet more sidemount diving.

I’ve always felt there is no reason to move on until you have a need to, and after twelve years on backmount CCR, I finally found I needed another tool, a sidemount rebreather. I began a yearlong process of looking at different units, demo’ing some, and talking to explorers and people way more experienced than I in sidemount CCR. For over a decade, one of my best dive buddies, Evan Kovacs, dived a Prism Topaz sidemount unit, and several other dive buddies use them, so I wasn’t that unfamiliar with them. I had a list of options that were important to me, and I began to talk to manufacturers and to watch as more divers started making the transition.  

Becky outside of Ginnie Springs. Photo courtesy of David Schott.

In the end, I chose the Divesoft Liberty sidemount unit because I liked that it is neutrally buoyant and can be clipped off, just like a traditional sidemount bottle with no fuss. It has both onboard diluent and oxygen (O2)  cylinders, a water trap in the counterlung that sits up high against your chest, a 5.5 lb radial scrubber (short), and its clean design with the Manual Addition Valves (MAV) running up the loop to the Diver Supply Value (DSV) (i.e., mouthpiece), which makes adding diluent or O2 manually very easy. This is a clean design, and while I’m shooting I’m not fumbling around looking for any MAVs. There is also an Auto Diluent Valve (ADV) that can be activated by just breathing, if you’re in a head-down position or if you need more loop volume. I found that to be really nice for descents. The unit has sophisticated electronics that some divers may like or dislike.

I thought it might be overwhelming, but a lot of thought has gone into everything on this rebreather. I like the built-in checklist and the ability to make a lot of personal adjustments in the menu system. The calibration is easy, and the unit walks you through predive checks, including positive and negative checks, even showing millibars of pressure. I also like that it’s sold ready to dive with little tinkering. Travel is important to me, and the unit is just 50 lbs/23 kg in a pelican case (minus tanks). Lastly, it has several modes that include CCR, manual CCR (mCCR), and a bailout rebreather mode that I may use down the road.

Putting It to the Test

I was nervous about how I’d feel on a sidemount rebreather after years of diving various backmount units. I put the DSV in my mouth and opened it up—at first a little awkward because I was standing on the steps at Ginnie Springs. I went horizontal in the water and descended into the basin. I immediately felt comfortable and made a few small adjustments, like moving the unit to a D-ring further back and adjusting the loop hoses into a more comfortable place under my arm. I couldn’t believe how well it breathed and the ease of activating the ADV or manually adding gas using the MAVs.


Giant Cave is possibly the largest sea cave on the planet! It’s located just off a dock in Caye Caulker Belize. The cave can spring and siphon and little is known about timing it. This was my 3rd trip to the cave and the first two times it was springing slightly and this trip it was siphoning enough to suck you in. The video is shot with a Paralenz dive camera of Becky Schott entering the fissure crack and first chamber on a Liberty Sidemount rebreather, then buddy Anthony Tedeschi going through the 100ft of restriction on his KISS Sidekick.

After becoming certified on it, I spent another 15 hours practicing skills in open water and then another 20 hours in the first 500 ft/152 m of Devil’s Cave system, just practicing and getting used to the idiosyncrasies of the unit and pulling random drills on myself. Changing set points is easy; I like the vibrating to give you alerts, and the Heads Up Display (HUD) and buddy light are easily seen. I took a camera, and it felt really natural to me. I had fun learning this new tool and gliding through the cave, and it felt really streamlined and clean. It was actually the most comfortable I’ve ever felt in sidemount. I used it with my Hollis Katana harness, and everything trimmed out nicely and felt comfortable in the water.  

In my opinion, a sidemount rebreather is an incredible tool, but it doesn’t replace a backmount unit. It’s a tool for a specific environment or purpose. Like any rebreather, it’s important to put the time on it and practice your skills to become proficient.

I spent more time with it back home in Dutch Springs. That was fun, considering it was March and the water temperatures were a balmy 37 degrees Fahrenheit or 3 degrees Celsius. I had no issues with dry gloves, but everything did take me a little longer to get together in colder water, wearing thicker undergarments. I had added Shearwater transmitters and an offboard gas addition supply, since it didn’t come with any. That’s important to me, and I easily added one to the unit. I added the transmitters to clean it up because I found it difficult to read the gauges or button gauges under my arm.

A Belize cave project suddenly came up at the end of March. This is exactly the reason I wanted a sidemount rebreather and a perfect project with friends. I’d been to Giant Cave and Winter Wonderland on several past trips, so I was also somewhat familiar with the systems. I decided I wouldn’t change my configuration too much and continued to dive drysuit, but instead of a steel tank, I went with an AL 80 in the warm water since the unit is neutral.


Schott liberty giant cave. Photo by Anthony Tedeschi.

Giant Cave is an advanced cave system that’s located off the island of Caye Caulker in Belize, and the entry is in the ocean just off a dock. It’s fascinating in many ways, but one thing that can’t be timed is when it’s siphoning or springing. It siphoned every time I entered the cave that week, making visibility less than 2 ft/0.6 m. It’s a challenge because you drop down a hole that narrows into a small fissure crack that you have to go through head down about 40 ft/12m into another open chamber before squeezing through about 100 ft/30 m of restriction that goes up and down sandhills and twists and turns.

There are also other considerations when diving a sidemount CCR, such as water traps; flooding; work of breathing; positioning; bailout planning; and how many cylinders to take and types (H valve or multiple tanks). There is a lot to consider and, as always, more to learn.

Luckily the jellyfish weren’t there this time so they weren’t siphoned in with us! I’ve done this entry almost a dozen times in the past but never in a siphon when I couldn’t see, and never on the sidemount rebreather. I knew the head-down position was going to be tough, and breathing might be hard during that time. I went for it and before getting to the bottom, in no visibility, I got tangled in the line. Inverted and unable to breathe, I signaled to my dive buddy and safety diver Anthony Tedeschi for some assistance. He was on a KISS Sidekick but he’s been diving it for over four years. He helped me out, and I caught my breath as I descended less than gracefully into the open chamber below. I caught my breath easily since the ADV activated, and I manually added diluent to get a proper loop volume. At this point, a lot of sediment was getting sucked into the cave, but we could see the line, and we slowly followed it in, passing crabs, sponges, and little coral pieces. Once through the restriction, the cave opened up into a huge room, and the visibility was clear. We had a good dive and shot some great video that day.

Exiting the cave was a similarly uncomfortable experience, as now I had to go vertical again, but in a head-up position where all the gas wanted to push its way out of my mouth. It’s a different feeling, trying to hold it in and only let out little bits. If you let out too much, you’ll drain the counterlung. The head-up and head-down positions are the biggest things to get used to. Going slightly head up or down is no problem, but being vertical is more challenging.  

Each day the entry and exit became easier as I committed to the entry and just went without hesitating. Once inside the huge, clear cave system, I could easily stay in horizontal trim, which is where the sidemount rebreather performs best.

Final Thoughts

I’m so happy I took the Liberty with me to Belize for this project. It’s probably the most challenging cave I’ll ever take it to, and it was a great tool for this particular shoot. We dived for a few hours each day, and I’d usually have only a few tablespoons of water in the counterlung and nothing in the canister. After a week in two different sea caves, both sidemount entrances, I was very happy to have gained experience that was totally different than my dives in the Florida caves on the unit. My plan is to continue to gain hours on it, in all types of environments, so that when opportunities arise, I’m ready and more confident. It’s important to me to have the tools that can help me do my job as an underwater image maker and be safe while doing it. I’m looking forward to putting more time on the sidemount CCR and taking it to some awesome locations this year. So far it’s been a fun new journey practicing, and enjoying silent sidemounting.


Becky is a five-time Emmy award-winning underwater cameraman and photographer whose work appears on major networks including National Geographic, Discovery Channel and Red Bull. She specializes in capturing images in extreme underwater environments including caves, under ice, and deep shipwrecks. Her projects have taken her all over the world from the Arctic to the Antarctic and many exciting locations in between, filming new wreck discoveries to cave exploration and even diving cage-less with great white sharks. Her biggest passion is shooting haunting images of deep shipwrecks in the Great Lakes. Becky is a frequent contributor to numerous dive magazines, both US-based and international, and her photography has been used in books, museums, and advertising. She is also a technical diving instructor and leads expeditions all over the planet. www.LiquidProductions.com    www.MegDiver.com

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