No One Knows What Lurks at the Bottom of This Freakishly Deep Submerged Cave

The entrance to Hranice Abyss. (Image: t: Radim Holiš/Wikimedia Commons)
The entrance to Hranice Abyss. (Image: t: Radim Holiš/Wikimedia Commons)

New research suggests Hranice Abyss — the world’s deepest freshwater cave — is around 1 kilometre deep, which is more than twice the depth of previous estimates.

Back in 2016, scientists measured the depth of Hranice Abyss at 473 metres, but they suspected it was deeper because their remotely operated vehicle had reached the end of its fibre-optic communication cable. Now, using multiple geophysical imaging techniques, a research team led by Radek Klanica from the Czech Academy of Sciences has established a new estimated depth for Hranice Abyss, which is at least 1 kilometre deep. A paper describing this research was recently published in the Journal of Geophysical Research: Earth Surface.

Cross section showing the top ~400 meters of the abyss. (Illustration: R. Klanica et al., 2020)

Located 300 km east of Prague, Hranice Abyss is our planet’s deepest freshwater cave. Other notable examples of deep freshwater caves include Pozzo del Merro in Italy and Zacatón in Mexico, which are 392 metres and 335 metres deep, respectively. As for the deepest cave in the world, that distinction belongs to Veryovkina Cave in Georgia, which extends down for 2.2 km.

Hranice Abyss is situated within a karst — terrain that forms when soluble rocks, like limestone, are slowly broken down by water. The abyss covers 15 square km, the entrance to which measures 104 metres by 34 metres. The width of the cave varies from 10 to 30 metres. Hranice Abyss is a popular destination for daring cave divers, as the upper sections feature several chambers.

To measure the submerged cave, Klanica and his colleagues applied multiple approaches, such as making gravity measurements to spot empty spaces, setting off explosives to measure reflecting seismic waves, and examining the ability of the limestone to conduct electricity in order to spot gaps and other subterranean features.

In addition to characterising the shape and depth of the cave, this approach allowed the researchers to reconstruct the hole’s geological history and acquire a better sense of how it formed. Most caves form in an epigenic (top-down) process, where surface water leaks down through the soluble rock, creating a gap that grows over time. A smaller proportion of caves, however, form in a hypogenic (bottom-up) process, in which acidic groundwater creeps upward, dissolving rocks in the process.

Geological cross section showing the abyss as it was forming with water flowing in (top) and the abyss as it appears today, full of water and with a nearby basin now filled with sediments (bottom). (Illustration: R. Klanica et al., 2020)

Scientists thought that Hranice Abyss was produced by the hypogenic process, but the new research suggests otherwise. The researchers discovered evidence of a former drainage system in the limestone bedrock, pointing to an epigenic origin; water from an adjacent mountain range spilled onto an ancient basin, which formed the abyss, according to the new research. The researchers say their “approach is readily applicable to similar flooded cave systems globally.”

Geologist Francesco Sauro from the University of Bologna, who wasn’t involved in the new study, told Science that the newly derived depth of the abyss is “impressive” and that the new paper is “a good example of how you should do things.” Similar processes could’ve formed other submerged caves, some of which could even be deeper, he said. As for what types of organisms might exist at the bottom of the cave, Sauro said: “We don’t know exactly what could be down there.”

Sounds like a good excuse to explore Hranice Abyss even further. But as the new research shows, scientists will have to bring a longer cable next time.