World’s Deepest Freshwater Cave is Twice as Deep as Previously Thought

The deepest freshwater cave on Earth, the Hranice Abyss, stretches about 3,280 feet deep, Charlotte Hartley reports for Science magazine.

Measurements taken in 2016 with a remotely operated vehicle secured its spot as the deepest freshwater cave when it dove to a depth of 1,300 feet in the abyss. But a combination of geophysical techniques, including the use of electrodes and small explosives, have now revealed a rough map of the caves, featuring sediment-covered trenches that reach more than a half mile deep. The process is detailed in a paper published last month in the Journal of Geophysical Research: Earth Surface. The evidence also points to a new explanation for how the cavern formed.

University of Bologna geologist Francesco Sauro, who wasn’t involved in the new study, tells Science magazine that the researchers’ use of multiple geophysical techniques is one of the study’s strengths.

“It’s a good example of how you should do things,” Sauro tells Science, adding that the new estimate of the cavern’s depth is “impressive.”

The Hranice Abyss took its place as the deepest freshwater cave in 2016, Kat Long reported for National Geographic at the time. Divers explored the limestone caverns and used a remotely-operated vehicle to travel to about 1,300 feet below the surface.

But as Reuters reported in 2017, the vehicle’s cable stopped it short of reaching the bottom. Still, the depth measured in the Hranice Abyss still beat the next-deepest freshwater cave, Italy’s Pozzo del Merro, which is about 1,280 feet deep. (The absolute deepest known cave on Earth is the Veryovkina Cave in Georgia, at more than 6,800 feet deep.)

The Hranice Abyss is located in a limestone formation in the Czech Republic. The stone face is full of holes worn away by acidic snowmelt, leaving it Swiss cheese-like, reports Science magazine. Many caverns were formed by this top-down erosion, but scientists long believed that the Hranice Abyss was formed from the bottom-up because of chemicals found in the water that come from deep underground.

The research team led by geophysicist Radek Klanica of the Czech Academy of Sciences first used an array of electrodes to map the structure of the limestone based on its ability to conduct electricity. Another array of sensors measured slight variations in gravity around the abyss. Finally, a series of small explosions created waves that bounced through the cavern and back to sensors on the surface.

The series of techniques revealed that the deepest parts of the Abyss are filled with sediment, but also suggest that the cave was formed as groundwater drained through the limestone from the mountains above.

Later, it’s possible that water from underneath the cavern surged up and joined with the eroded abyss, bringing the chemicals from deep underground with it.

Klanica tells Science magazine that the results suggest that other scientists may need to reexamine other deep caves that they think formed from the bottom. Sauro agrees.

“It could be that other caves have the same story,” Sauro tells Science magazine, “or that [similarly formed] caves could be even deeper.”

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