World’s Deepest Freshwater Cave, The Hranice Abyss, Is Way Deeper Than We Thought

The Hranice Abyss is located in the forests of Přerov District of the Czech Republic, pictured here. Karel Gallas/Shutterstock

The Hranice Abyss in the Czech Republic is the deepest known freshwater cave on the planet. Back in 2016, a team led by renowned Polish cave diver Krzysztof Starnawski ventured into these murky depths and concluded that it extends for at least 404 meters (1,325 feet) underwater. This estimate, however, was simply as far as the diving equipment would allow them to go and its true depth remained a mystery.

Now, researchers have beamed down a number of geophysical tests above the Hranice Abyss and estimated the cave system is at least 1 kilometer (3,280 feet) deep. However, as an estimate, we still have no solid idea about how deep the underwater cave really is; it could be further.

Their research was recently published in the Journal of Geophysical Research: Earth Surface.


Located in the Přerov District of the Czech Republic, the underwater cave is not simply a waterfilled pit, but a complex system of tunnels like caverns that extend through the Earth. Cavernous underwater tunnels, such as this, are typically carved through the soluble rock by slightly acidic rainwater and meltwater that have slowly dissolved away the surrounding limestone. In the past, it’s been suggested that the Hranice Abyss might have been formed by bottom-up forces. National Geographic reported in 2015 that the cave might have been formed by acidic groundwater that has been cooked deep underground by Earth’s mantle and seeped up like a volcano, dissolving the limestone from below. 

While that’s a pretty remarkable theory, this new survey found evidence of structures that were formed by more convenient top‐down processes.


People have been attempting to understand the scale of the Hranice Abyss for centuries. There are some historical reports that a person attempted to determine the depth of the cave in 1580 by holding their breath and diving to its base. Clearly, that attempt wasn’t successful, but development in technology has allowed divers and scientists to go deeper and deeper over the past century. Nevertheless, diving in the murky abyss remains a tricky feat, not least because its acidic CO2-rich water can be an irritant for divers' skin and equipment.

In this latest project, scientists from the Czech Academy of Science used an array of geophysical techniques to assess the underground cave from above, such as gravity measurements, electrical resistivity tomography, and seismic methods. This built up a rough map of the cave, revealing it extends for at least 1 kilometer (3,280 feet) deep. 

However, this is just the latest estimate. As better scientific processes become available, it’s likely that researchers will be able to hone in on a more accurate — and no doubt deeper — estimate than currently available. 

The murky water doesn't allow you to see all the way down, sadly.  Tomas Indrak/Shutterstock



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