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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. 

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.

Additional Resources:

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.


Cave Diving, Du’an China Edition

Finnish tech instructor and explorer Jani Niskanen takes us for a plunge into the underwater world of South China Karst, which was designated a UNESCO World Heritage Site in 2007. Break out the NAV kits.




by Jani Niskanen. Special thanks to Edmund Yiu for additional information on cave diving in China.

Photos by Jani Niskanen unless noted.

Du’an, located in rural southern China, offers majestic hills and clear rivers, is quite the contrast to the bustling megacities of China, and is only a two-hour drive from Nanning, the provincial capital of Guangxi. Du’an is easily as beautiful as the world-famous Yangzhou area, with fewer tourists. Divers are attracted to the magnificent landscape of rivers and caves and underwater cave formations, which are some of the most beautiful in the world. 

Karst landscape and settlements between the mountains.
Author’s equipment ready for the dive at Tun Bang sinkhole.

The area is part of the huge expanse of the South China Karst system which spans from Guizhou, Yunnan, and Guangxi provinces all the way to Vietnam, and was designated a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in 2007. In fact, according to one estimate, the province boasts more than 10,000 kilometers of underground rivers that supply water to the people in the area. As a result, there are many entrances and stairways built by villagers to collect water, some with pumping stations, that were not originally made for diving access. Getting access is not easy as they are located in rural mountainous area with no cellular reception or GPS navigation. Instead, one must rely on guides who know the area.

Leaving the daylight zone.
Cave divers all set and ready to start the dive into “Panda” Cave.

Cave diving in Du’an began in the 1970s when the first Russian divers  visited the area, to observe the underground rivers. According to local knowledge, the next team to visit was a British group that entered the area in the end of the 1980s. The actual official underwater research and mapping were started by a French team of divers in 2011. After that, they visited the area annually until 2019. 

The caves in the area vary in size from the spacious cavities to extreme restrictions that a diver can hardly negotiate through. Inside the caves, seasoned explorers can admire stunning stalactites and stalagmites formed thousands of years ago when many of these caves were dry, though the deeper areas that have been carved out by water flow are not decorated. In fact, in some of the deeper passages the walls are perfectly smooth as a result of water flowing through them over millennia. Diving in these caves feels as if you are diving through time.

Tank carrier. A local farmer helped us to carry tanks to the cave, which he was compensated for. He has perfected his carrying technique and made it look almost easy.

The Du’an region experiences strong seasonal fluctuations in water level, which in some places can vary as much as 40m/130 ft between dry and wet seasons, affecting both the flow and visibility. Diving in Du’an requires a Full Cave rating, and other technical diving training is highly recommended since the caves are rather deep and you will be likely racking up deco during your dives. 

The deepest explored caves in Du’an exceed 230m/750 ft, however average depths for our dives ranged from 30-40 m/100-130 ft. According to the locals, the best time to dive in Du’an Caves is in either April to May or October to November, some say as late as January, when the water situation in the caves is optimal.

Our Underground Journey

Our ten-day dive trip took place at the end of April, when the underground water level had just begun to rise in the springs; consequently, there was almost no flow, and we were forced to carry our tanks far inside the cave to begin the dive. We dived open circuit, and typical gas set up for the dive was to have 32% enriched air in our sidemount tanks and then we left a tank of 100% oxygen for decompression on the line at 6 m/20 ft. We managed to dive ten caves with a total runtime of 576 minutes.

The water temperature in the caves in the area is pleasant 20 degrees Celsius and thus gloves are not needed.
From the patterns on the wall of the cave, one can see how the water has created grooves in the limestone and smoothed the rough edges over the millennia. 

In some of the caves, we encountered extremely narrow passages, making sidemount configuration and single cylinders better for safety, convenience and logistics. Although the visibility ranged from 15 m/49 ft to 2 m/7 ft, the water temperature was a pleasant 20 degrees Celsius/68 degrees Fahrenheit. Drysuits are recommended, but gloves weren’t needed, which made our exploration more enjoyable. At the end of the ten days, we realized we left plenty to explore for our next expedition to this amazing region.

Du’an has one diving center, Blue Flag International, which was originally set up by the Tourism department of the Du’an county government with the help of the French exploration team in 2010/2011, who put in an extensive gas blending system. This modern and fully equipped dive operator is ready to serve even the most demanding of divers. They can support mixed gas divers conducting open circuit trimix and closed circuit  rebreather diving. Oxygen is relatively inexpensive and easy to source locally, and helium is generally less expensive in China than in other parts of Southeast Asia.

Cave diving instructor and local legend, Han Ting, runs the shop. He has explored over a hundred caves in this area and has leads to many more—there are many caves waiting to be explored.

Inside the Veins of Mother Earth. Photo by Vincent Lou

Scuba diving is a growing sport in China. Before COVID-19 restrictions, Chinese divers traveled to Southeast Asian tropical destinations, such as the Philippines and Thailand. Now that the restrictions are in place, divers have sought out domestic dive sites like those in Du’an and diving clubs and shops have begun to organize trips within the country. In addition to the caves of Du’an, there are domestic destinations like Hainan Island, which offers tropical warm waters. An even more exotic location is diving sunken portions of the Great Wall of China.

This story was originally published in Sukeltaja Magazine, the magazine of the Finnish Diver Federation, in May, 2021 

Additional Resources

UNESCO: South China Karst

World Heritage Datasheet: South China Karst

My name is Jani Niskanen. I am an SDI/TDI and PADI instructor with multiple specialties, including TDI cavern. I am TDI Full Cave Diver, and PADI Trimix. I am experienced in sidemount, single and double back mount diving in different environments, warm and cold waters. I am an EFR and FRIT instructor and a registered nurse with over 14 years of experience in the field in ATLS/ACLS emergency care mobile unit. I am a veteran and I have worked several years as a combat medic with the Finnish defense forces in many deployments. I am a scuba technician, compressor operator and trimix blender instructor. I have worked with diving in Thailand, Philippines, Mexico, China and I am dedicated to making good divers and not just “ticking the boxes”. Currently I work at the Finnish Embassy in Beijing, China.

You can find Jani here on Instagram and on Facebook at Expeditiondivers

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