Czech Republic:

For decades, spelunkers have flocked to the flooded caverns of the Czech Republic’s Hranice Abyss, which stretches farther below ground than any other freshwater cave system.

Now, a scientific campaign to the cave has revealed it is 1 kilometer deep, more than twice as deep as previously thought. The researchers also say the abyss formed as groundwater seeped down from the surface, not as water percolated up, as previously believed—a finding that could call into question the origin of other deep caves.