An underground river starts from the sinkholes of Fort Drama in the K. Nevrokopi basin and ends in the river Angitis in the Maara cave. Recently, only 6 or 7 sinkholes have been left in the area out of a total of 18 which have been closed by natural phenomena due to subsidence.

Of these sinkholes, all of which are inaccessible due to their surface or very narrow (fracture-like) nature, only one, the so-called Megali Katavothra, is accessible and, after a 300-metre walk inside the mountain, leads to an underground lake.