Geology & Geophysics Research Spotlight

World’s Deepest Freshwater Cave May Be a Kilometer Deep

The Czech Republic’s Hranice Abyss is more than twice as deep as researchers thought.

Source: Journal of Geophysical Research: Earth Surface


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Earth’s deepest freshwater cave is more than twice as deep as previously measured, extending up to a kilometer into Earth’s crust, according to new research.

The Hranice Abyss, in the eastern Czech Republic, is the deepest flooded cave in the world and a popular tourist attraction for cave divers.

In 2016, divers used a remotely operated vehicle (ROV) to explore the cave and found it was at least 473 meters (1,552 feet) deep, making it Earth’s deepest known freshwater cave. The next-deepest flooded cave is Italy’s Pozzo del Merro, which is 392 meters (1,286 feet) deep. The ROV’s tether wasn’t long enough for the vehicle to reach the bottom of the Hranice Abyss, so scientists weren’t sure exactly how far down it went.

In a new study in AGU’s Journal of Geophysical Research: Earth Surface, scientists used several geophysical imaging methods to map the cave in greater detail. They found the cave could be up to 1 kilometer (0.6 mile) deep, more than twice as deep as the ROV had gone.

The depth of the Hranice Abyss compares to some of the world’s tallest buildings.
The depth of the Hranice Abyss compares to some of the world’s tallest buildings. Click image for larger version. Credit: AGU

Most caves form by a top-down process in which water on Earth’s surface slowly percolates downward through soluble rock like limestone, dissolving the rock and creating a cavity that deepens over time. But many deep caves form by a reverse bottom-up process, where warm, acidic groundwater slowly migrates upward and eats away at the surrounding rock.

Geologists suspected the Hranice Abyss formed by the bottom-up method like other deep caves, with acidic groundwater carving out a deep column through the surrounding limestone. But in the new study, researchers looked not only at the cave itself but at the geologic history of the surrounding region to figure out how it formed.

Their results suggest the Hranice Abyss formed by the more common top-down method, causing researchers to reconsider their views on how these deep cave systems form. The study authors suspect incorporating the geological development of the surrounding area can help researchers better understand the formation and evolution of similar deep flooded caves elsewhere in the world. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1029/2020JF005663, 2020)

—Lauren Lipuma (@Tenacious_She), Science Writer

Citation: Lipuma, L. (2020), World’s deepest freshwater cave may be a kilometer deep, Eos, 101, https://doi.org/10.1029/2020EO148708. Published on 08 September 2020.
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