Hranice Abyss, the World's Deepest Freshwater Cave, Is Twice As Deep As Previously Thought

The world's deepest freshwater cave, the Hranice Abyss, is over twice as deep as once thought, scientists have announced. The cave, in the Czech Republic, was found to be around 1 kilometer (3,280 feet) deep after researchers visited the site to perform a survey of the site. The team, led by Radek Klanica, from the Institute of Geophysics, Czech Academy of Sciences, used a range of geophysical techniques to take new measurements of the cave. A survey in 2016 found the cave to be at least 475.5 meters (1,560 feet) deep. However, at this time the optic communication cable that was used to measure the cave prevented them from going any deeper, Science magazine reports. In a study published in JGR: Earth Surface, researchers have now reported the cave goes far deeper than was once thought—around 1km. The exact depth of the cave is not known.

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As well as providing a new depth estimate, the team also performed multiple tests to work out how the system formed. They found it was created by groundwater seeping down, rather than water moving upwards—known as hypogenic formation—as was traditionally thought. Researchers say the idea of hypogenic formation for systems like Hranice is based on the knowledge that present-day groundwater contains acidic elements that come from a deep source. This water is thought to be heated by the mantle below and bubbles up, carving out the rock above. The presence of carbon and helium isotopes from deep underground in Hranice Abyss had given weight to this theory. However, the latest analysis suggests another method of formation.

The team's findings indicate the cave formed by water from the surface running underground, eroding the base. This took place before the mid-Miocene transgression event, when sea levels rose and the cave was flooded. The carbon and helium isotopes can be explained by an upwelling of water at a later point, they said. The team say the formation model they created for the Hranice Abyss could be applied to other flooded cave systems across the world. They said characteristic features of hypogenic formation are often missing in giant shafts. "Detailed multidisciplinary studies of the world's deepest flooded cave systems and their associated formation mechanisms are rare," they wrote, pointing to several cave systems that are currently considered hypogenic in origin. "Geophysical imaging of these systems could reveal important new evidence of the geologic evolution and formation processes of these systems." Francesco Sauro, a geologist at the University of Bologna, who was not involved in the study, told Science magazine the newly discovered depth of Hranice Abyss may mean other caves "could be even deeper."