Hranice Abyss is the deepest flooded pit cave in the world. It is a karst sinkhole near the town of Hranice, Czech Republic. The greatest confirmed depth is 519.5 m (1,704 ft), of which 450 m (1,476 ft) is underwater. In 2020, a scientific expedition to the cave revealed that part of the system apparently reaches 1 kilometre deep, albeit with the lowest reaches sediment-filled. Analysis of the water found carbon and helium isotopes which implied that the cave has been formed by acidic waters, heated by the mantle, welling up from below.

A pit cave, shaft cave or vertical cave—or often simply called a pit and pothole or pot ; jama in Slavic languages scientific and colloquial vocabulary —is a type of cave which contains one or more significant vertical shafts rather than being predominantly a conventional horizontal cave passage. Pits typically form in limestone as a result of long-term erosion by water. They can be open to the surface or found deep within horizontal caves. Among cavers, a pit is a vertical drop of any depth that cannot be negotiated safely without the use of ropes or ladders.