Where is the deepest underwater cave in the world?

Hranice Abyss is located near the town of Hranice in the eastern Czech Republic. It is a water-filled limestone sinkhole whose submerged passages have been explored by divers and robots to great depth. Measurements have suggested depths of more than 450 meters below the surface, though the full extent may still be greater. Unlike a typical cave, it is a flooded vertical shaft that has fascinated speleologists for decades. It is considered the deepest known underwater cave on Earth, and exploration continues.