Hidden at the bottom of a rocky ravine in the Hůrka u Hranic National Nature Reserve in eastern Czech Republic is a small lake of murky green water that can be spotted from a viewing platform above.
At first glance, this might appear fairly unremarkable, but what lies below the surface is the Hranice Abyss – the world’s deepest known freshwater cave.
Located between the Bohemian Massif and the Western Carpathians, the abyss was formed as surface water percolated through the soluble limestone below, creating a cavity that has slowly formed and deepened over time.
The earliest recorded mention of the abyss dates back to 1580, when Tomáš Jordán, a physician and balneologist (a medical specialist who studies the treatment of diseases using natural mineral waters), travelled from Klausenburg in Romania to explore the abyss. In his book detailing mineral water springs and thermal spas in Moravia, he described seeing “bubbles” in the water that “gave off a foul odour”.
Since then, many efforts to explore and map the abyss have tried, and failed, to establish exactly how deep it goes. This is due to the abyss’s extreme depth, limited visibility, acidic water and unbreathable surface layer of CO2 gas – all of which combine to make it a challenge for even the most experienced divers to explore.
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Then there are the numerous rotten tree trunks, logs and foliage that fall into the water, blocking access points or impeding the diver’s equipment and guidelines.
In 2016, divers used a Remotely Operated Vehicle (ROV) attached to a fibre-optic cable to venture even further. This reached a depth of 404 metres (1,552 feet) below water, to confirm Hranice Abyss’s status as the world’s deepest known freshwater cave.
The next-deepest is Italy’s Pozzo del Merro, which is 392 metres (1,286 feet) deep. However, the ROV’s journey was cut short by the length of its fibre-optic cable, which ran out before it could reach the bottom of the abyss, leaving questions about how much further it still had to go.
The answer came in 2020, when scientists used geophysical imaging to map the cave in greater detail and reached the remarkable conclusion that the abyss could be up to 1 km deep. This extended mapping also revealed that the bottom of the cave is connected to a former sinkhole nearby, called the Carpathian Foredeep. Located around 1.2 miles (2 km) from the cave's entrance, this is thought to have opened roughly 19 million years ago before gradually filling with sediment, meaning it is no longer visible at the surface.
In 2022, a team of researchers set a new record when they sent an autonomous underwater vehicle to a depth of 450 metres (1,476 feet) below water, while creating a 3D map of the cave system.
These diving expeditions have revealed that the abyss is an irregular, vertical cylinder ranging between 10–30 metres (30–100 feet) in diameter, with subterranean dry caves off the central flooded cavern. Depending on the time of year, the water temperatures vary between 14.5 and 18.8 ºC (58–66 ºF).
Due to the acidity of the water and high concentration of CO2 in the Hranice Abyss, the deepest parts of the cave are mostly barren of life, apart from an array of bacterial slimes. However, a significant colony of greater mouse-eared bats have been found roosting in a dry section of the cave known as the Dry Rotunda between May and September.
This is around 48 meters below the surface, with the bats accessing it through narrow fissures in the rock face. Common bent-wing bats, also known as Schreibers's long-fingered bat or Schreibers's bat, have also been found roosting in the mouth of the abyss above water level.
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Top image: Radim Holiš, Wikimedia Commons, CC BY-SA 3.0 CZ https://creativecommons.org/licenses/by-sa/3.0/cz/deed.en, via Wikimedia Commons
