The Hranice Abyss, the English name adopted by the local tourist authorities, is the deepest flooded pit cave in the world. It is a karst sinkhole located near the town of Hranice. 

The greatest confirmed depth (as of 27 September 2016) is 473 m (404 m under the water level), which makes it the deepest known underwater cave in the world. 

Moreover, the expected depth is 800–1200 m. In August 2020, a scientific campaign to the cave revealed that it is 1 kilometer deep.

The Hranice Abyss was created by a collapse of the cave roof, which was eaten away by warm carbonic acidulous spring water in limestone.

According to en.wikipedia