Caves of Diros

Caves of Diros form part of an underground river. About 5,000 meters have been exposed and are accessible by small boats and through narrow passageways. One is surrounded by formations of stalagmites and stalactites. Archaeological research has shown that the caves served as places of worship in Paleolithic and Neolithic times and their inhabitant believed that the caves were the entrance to the underworld.

Source: wikipedia.org