Connect with us


The Challenges of Exploring Hranice Abyss

Extreme depths, varying visibility, acidic water, and an unbreathable surface layer of CO2 gas, make the Czech Republic’s Hranice Abyss—the deepest freshwater sinkhole in the world—a challenge for would-be explorers. Here freelance writer Petra Pruden details the great lengths—err depths—that the Czech Speleological Society “7-02 Hranický kras” has gone to scoop booty.



by Petra Pruden
Header image courtesy of Michal Guba. Remaining images courtesy of 7-02 Hranický kras.

You don’t have to be a technical cave diver to have heard of the cenotes in Mexico, Indian Springs in Florida, or Ben’s Cave in the Bahamas. These spots are popular not only for their incredible dive opportunities, but also as frequented tourist destinations. Yet, as it turns out, some of the most interesting research and technical cave conditions can be found far from the jungle, sandy beaches, and favored hiking paths. The Hranice Abyss located in the Czech Republic is the deepest known underwater cave in the world, and Czech divers like Michal Guba are just barely scratching the surface of its secrets.

Staring Into The Abyss

Location of the Abyss.

The Hranice Abyss, located in eastern Czech Republic, is the deepest flooded pit cave in the world. The deepest confirmed depth of this karst, recorded in 2016, was 473 m/1543 ft (404 m under the water level), but it’s expected to have a depth well over one kilometre (0.6 miles). The Abyss lies between the junction of two pan-European geological units, namely the Bohemian Massif and the Western Carpathians, and is known for its so-called “gas lakes,” or pockets of carbon dioxide, which have migrated and pooled together in deep underground cavities.

Uncovering an Anomaly

The first recorded mention of the Abyss was in 1580 by a man named Tomáš Jordán who described having seen a water “bubble” which “gave off a foul odour.” (See: Hranice Abyss: The Deepest Flooded Freshwater Abyss in the World) Fast forward several hundred years to the early 1900s, when a number of depth measurements and descriptions of the karst’s chemical phenomena were established. The actual diving survey of the Hranice Abyss began in 1960 and continues to this day. The Czech speleological organization “7-02 Hranický kras” has been continuously working on and monitoring this location for 50 years. One of its members is Michal Guba who has dedicated more than 20 years of his life to technical cave diving, which also includes over five years of cave instruction. Apart from the many years he has spent working on the Czech police force’s special diving unit, Guba is also known for his research at the Hranice Abyss.

Depth-Temperature Graph of the Hranice Abyss.

In his own words, Guba’s objective at the site is “to place and maintain monitors in the Abyss’s so-called ‘warm springs,’ which can be found beneath the surface.” These are pockets of mineralized water which have higher temperatures than the surrounding water springs.

Measuring parameters in the Hranice Abyss.

The monitors measure temperatures, conductivity, and pressure levels via two types of sensors. One sensor is located on a cable that runs down through the water (data transfer takes place over the control panel daily) and the other is a series of Water Level Loggers (pressure sensors), from which data is downloaded every 3 months. Guba hopes these research efforts will help us learn more.

Low Visibility, Acid Water and Unbreathable Surface Air 

Diving in the Hranice Abyss is no simple feat. Limited visibility is one issue. “I remember a time 20 years ago when I could see through 50 meters of water and as much as another 70 m/230 ft out of the water to the people standing on the railing above the abyss,” Guba lamented. Today it’s no longer that way; though visibility varies it has noticeably deteriorated.

Conducitivity-Depth Graph of the Hranice Abyss.

Apart from losing sightlines, divers also have to overcome the water’s conductivity (2000 µS/cm), total mineralization (2.6-2.9 mg/l), and CO2 content (1,500-2,500 mg/l); in other words, divers here are swimming in acid! Guba reported feeling a “stinging” sensation on the parts of his body not covered by his dive suit. And that’s not all! The area one meter above the surface is unbreathable due to the high CO2 content in the air. “It’s something that divers need to remember,” cautions Guba.

As if those weren’t enough variables for divers to take into account at the Hranice Abyss, there are more. Guba cites the challenges presented by numerous rotted tree trunks, logs, and foliage that fall into the water blocking access points or sinking past their sensors, guidelines, and other equipment. 

One final challenge, which is quite unique to the Abyss, has to do with the cave’s considerable vertical length. Guba warns, “If a diver suffers any mechanical or other type of failure that results in loss of buoyancy control, they run the risk of sinking deeper into the cave, which drops some 200 meters, and would greatly complicate rescue efforts.”

Map of Hranice Abyss as known thus far.

Technology to the Rescue

Explorer Michal Guba, surface interval.

Thankfully, divers have managed to overcome, or at least withstand, these challenging conditions thanks to a catalogue of specific equipment. To lessen the visibility issues which are worsened by exhaled OC mixtures, Guba and his team rely on closed circuit rebreathers. “We use the [Divesoft] Liberty, in both back mount and sidemount configurations. Otherwise, we utilize the typical cave diving gear and technique,” he explained. 

Once in the water, divers emerging from deep dives are able to make use of the permanently installed decompression habitat installed at 9 m/30 ft. Note that because of the high dissolved CO2 content in the water, divers must leave on their breathing equipment in the habitat as it quickly becomes unbreathable after it is flushed out with fresh gas. The Abyss also features a fixed permanent line with arrows in several directions for easier travel and positional awareness. Standard mapping techniques and equipment (band and compass) are also used to aid with the poor visibility.

Divers decompressing in the habitat installed at 9m/30 ft. Mind that CO2 build-up!

With this equipment in hand, Michal and his team of divers and researchers are able to monitor the Hranice Abyss and it’s “warm spring” activity. Their time spent at the cave is primarily maintaining the lines, decompression tent, and Level Logger sensors, as well as recording data and taking samples. Their research has additionally been incorporated into various scientific works, including the Neuron Expeditions (a Czech-based diving program dedicated to making new dive-related discoveries around the world).

There is also a plan to acquire a Remotely Operated Vehicle (ROV), with an umbilical cable at least 1500 meters in length in order to explore deeper sections of the Hranice Abyss beyond the Mikado restriction at a depth of 200 m/653 ft. 

  • Buddy Dive Bonaire

More Cave To Be Explored

Even though diving hundreds of meters down a vertical cave of acid and facing visibility challenges and CO2 clouds is a far way from the warm waters of Mexico’s cenotes, Michal is excited about his work at the Abyss. “You can travel and cave dive all over the world, but nothing beats having the world’s deepest underwater cave right in your backyard,” Guba said. 

He also mentioned his hope for the future: that in time, more information and data will be collected about this natural anomaly. With a flooded pit cave of this magnitude, divers truly have no way of discovering everything below. But as technology and equipment improve, who knows what divers like Guba will find. It’s safe to say the Hranice Abyss has yet to show all its cards.

Michal Guba bringing the ROV to 60m depth from where it will explore the deepest part of the cave.

Dive Deeper:

InDepth: Hranice Abyss: The Deepest Flooded Freshwater Abyss in the World

EOS: The Czech Republic’s Hranice Abyss—the world’s deepest freshwater cave—may be twice as deep as thought. Krzysztof Starnawski, where art thou? Err, how deep art thou?

Petra Pruden is a Czech freelance copywriter and editor. She expresses her passion for writing and creative work in the form of articles and interviews with intriguing personalities, who allow her to take a deeper look into the given issue. When not at work, she dedicates her time to marketing, HR, self-development, and the study of the Czech language.


Karen van den Oever Continues to Push the Depth at Bushmansgat: Her New Record—246m

Karen van den Oever recently broke her own world cave diving depth record by a little more than 10m/33 ft at Bushmansgat cave in South Africa. The S.African cave diver conducted the 8 hour 14 min high-altitude dive on open circuit scuba, breathing trimix 4/90 bottom mix, and suffered mild High Pressure Nervous Syndrome (HPNS). Here former world depth record holder, Nuno Gomes who was van den Oever’s cave instructor, offers the details of her record setting dive along with a short history of the women’s depth records.




By Nuno Gomes. Images courtesy of Karen van den Ever.

Karen van den Oever and her husband Francois Bain

Karen van den Oever, from Johannesburg, South Africa, has dived to a depth of 246.65 m/809 ft. This is equivalent to a dive to a depth of 296 m/971 ft when corrected for an altitude of 1550 m/5,085 ft above sea level. The dive was conducted on October 27, 2022, in Bushmansgat cave, South Africa, and is a new women’s world record cave dive. Karen bettered her own previous world record to a depth of 236.04m/770 ft  (283 m/924 ft correcting for altitude), also accomplished at Bushmansgat cave in 2021.

I actually felt really good after the dive, a little tired but overall, quite good. I felt much better after this dive than the previous one. I’m happy that the dive went well, just thinking about what comes next. I have no definite plans going forward, we are looking into diving some of the caves in Namibia and also exploring some of the caves not yet dived in Zambia but no concrete plans yet.”—Karen van den Oever

Karen and Theo van Eeden, with the signed tag.

Women have been making record deep dives for quite some time. Back in 1981, one of the first deep diving records was made by Sheck Exley’s wife, Mary Ellen Eckhoff (USA). She used a dive propulsion vehicle (DPV) to travel into Wakulla Springs cave, as well as staged tanks for decompression purposes. Mary Ellen dived on open circuit, together with Paul DeLoach and John Zumrick, and they reached a distance of 363 m/1192 ft and a depth of 80 m/260 ft, which was a major dive at the time.

In 1996, Dr. Ann Kristovich (USA), a friend of Jim Bowden, considerably extended the record, reaching a depth of 167 m/548 ft on open circuit at Zacaton cave, Mexico. Ann’s world record dive would remain in place for a long time.

It was not until the year 2000 that another woman, Claudia Serpierri (Italy), would beat the previous record, but this time in the sea (Mediterranean Sea). Claudia would reach a depth of 211 m/692 ft on open circuit, diving from a support ship. This dive remains the deepest sea dive by a woman to date.

Toward the end of 2001, Verna van Schaik (South Africa), was ready to challenge the women’s record. First, she did her deepest dive by reaching a depth of  186 m/610 ft  (223 m/732 ft correcting for altitude), on open circuit, at Bushmansgat cave in South Africa. This was not enough for her, and during her next expedition on October 25, 2004, Verna would go back to Bushmansgat cave to become the first South African woman to get her name in the Guinness Book of World Records by reaching a depth of  221 m/725 ft  (265 m/870 ft altitude corrected), on open circuit. Her deep support diver was the late Dave Shaw (Australia), on closed circuit, who died of respiratory insufficiency at a sub-250 m dive at Bushmansgat in 2005.

View of the surface pool of Boesmansgat cave.

Following Verna van Schaik’s dive at Bushmansgat cave, two women divers died trying to break her record, as follows: 

In May 2010, French diver Brigitte Lenoir, died in Dahab, Egypt during a dive in the Red Sea. The accident took place at 147 m/482 ft while ascending from a 200 m/656 ft, on closed circuit. Her body was recovered with an ROV. 

In September 2017, Bulgarian technical diving instructor trainer, Teodora Balabanova, died attempting a dive to 231 m/754 ft, on open circuit, while her husband, Mihail Balabanov, suffered from decompression sickness. 

Karen van den Oever is a science graduate from the University of Witwatersrand in Johannesburg, South Africa, where she currently resides. Like Verna van Schaik, who now resides in New Zealand, she is a CMAS diving instructor, and also a former member of the University of the Witwatersrand Underwater Club. 

Her original cave, trimix and blending training was with me. I also trained her husband Francois Bain. 

Unstoppable Karen van den Oever

Karen had previously dived to 201 m/660 ft (241 m/792 ft altitude corrected) on open circuit in Bushmansgat cave in South Africa’s Northern Cape province on February 27, 2020. That dive’s total dive time was 7 hours and 21 minutes. On March 26, 2021, Karen dove to 236.04 m/770 ft (283 m/924 ft), on open circuit, at Bushmansgat cave, using a bottom gas of trimix 6/85. The total dive time was 7 hours and 18 minutes. That dive is the current deep diving Guinness World Record (women).

  • Buddy Dive Bonaire

Karen’s new world record dive, done on October 27, 2022, was made to a depth of 246.65 m/809 ft (296 m/971 ft), in Bushmansgat cave. The dive was done on open circuit, using a bottom gas of trimix 4/90, and with a total dive time of 8 hours and 14 minutes. The dive would not have been possible without a large team of support divers. 

Karen’s dive computer. Actual depth from rope measurements by independent witnesses was 246.56m/809 ft.

Peter Reid was at 209 m/686 ft (251 m/823 ft); this was his personal deepest dive on closed circuit, and his total dive time was 6 hours and 20 minutes. Don Hauman did deep support at 110 m/361 ft (132 m/433 ft). Her husband Francois provided shallow support and surface support, together with the other team members.

Karen’s support team.

Karen’s Total Narcotic Depth (TND) was 48.06 m/158 ft; the Equivalent Narcotic Depth (END) considering nitrogen only was 9.49 m/31.14 ft, and her maximum Partial Pressure of Oxygen (PO2) was 1.03 Atm. Gradient factors: 40/75.

There were no serious incidents during the dive except that Karen suffered some mild High Pressure Nervous Syndrome (HPNS), which ultimately did not prevent her from going any deeper. Karen had some difficulties recovering the evidence tag from her maximum depth because of the tremors that she was experiencing as a result of the HPNS, but in the end she turned the dive mainly because she ran out of bottom time. 

  • Buddy Dive Bonaire

Dive Deeper

InDEPTH: South African Cave Diver Karen van den Oever Sets New Women’s Deep Cave Diving Record

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

InDEPTH: Extending The Envelope Revisited: The 30 Deepest Tech Shipwreck Dives

InDEPTH: Opinion: Don’t Break That Record

InDEPTH: Fact or Fiction? Revisiting Guinness World Record Deepest Scuba Dive

Nuno Gomes is a professional civil engineer, a CMAS technical diving instructor and a commercial diver. He was born in Lisbon, but his family relocated to South Africa during his youth. He now lives permanently in New York with his family. He has dived all over the world.

He used SCUBA (open circuit) to dive to a depth of 321.81 meters (1,056 feet), inclusive of rope stretch, in the Red Sea off the coast of Egypt near Dahab, in June 2005. The total dive time was 12 hours and 20 minutes. The descent took 14 minutes with two minutes spent at the bottom.

He also used SCUBA (open circuit) to dive to 282.6 meters (927 feet) in the Bushmansgat cave, in South Africa, in 1996. The cave is located at an altitude of 1,550 meters (5,086 feet) above sea level, which resulted in a decompression schedule for an equivalent sea level dive to a depth of 339 meters (1,112 feet) in order to prevent decompression sickness. The total dive time was 12 hours and 15 minutes with four minutes spent at the bottom of the cave.

Subscribe for free
Continue Reading

Thank You to Our Sponsors

  • BuddyDive Bonaire


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