Watch a Diver Explore What Might Be the World’s Deepest Underwater Cave

Polish diver Krzysztof Starnawski may have discovered the deepest underwater cave yet: Hranická Propast in the eastern Czech Republic.

Starnawski has been probing Hranická Propast since 1999, but only last year did he find a tiny passage in what was previously thought to be the bottom of the cave. He pushed through and took a depth measurement of 1,260 feet before running out of cord.

This depth is only 26 feet less than that of the current acknowledged record-holder, Pozzo del Merro in Italy. But when Starnawski returned with a longer cord, he found that his squeeze-through passage had collapsed. Now the cord touched down at only 1,214 feet, probably having landed on debris from the collapse.

Soon Starnawski hopes to find a clear passage from which to take a new measurement, proving definitively that Hranická Propast is the world's deepest underwater cave.

Most caves are formed when rainwater erodes a limestone-heavy karst landscape, like what underlies a lot of Europe and particularly the Balkans. But Hranická Propast is unique because it seems to have been formed from the bottom up, with hot mineral water bubbling to the surface. The water inside the cave has an unusual acidity and quantity of carbon dioxide, indicating its mineral spring origins.

In this National Geographic video, watch diver and filmmaker Marcin Jamkowski explore the dark depths of Hranická Propast, and try not to feel claustrophobic.