Hranice Abyss

Hranice Abyss (Czech: Hranická propast) is the deepest flooded pit cave in the world. It is a karst sinkhole near the town of Hranice, Czech Republic. In 2020 a scientific expedition revealed that the lowest reaches are sediment-filled and it was speculated that the sinkhole was formed by upwelling hot acidic water. The greatest confirmed depth is 519.5 m (1,704 ft), of which 450 m (1,476 ft) is underwater.