Velebit
There are hundreds of "holes" on Velebit. It has the largest and deepest caves in Croatia. The three-part "Lukina jama" cave is 1392 m deep, making it one of the deepest caves in the world, and the deepest in southeast Europe, while the "Slovačka jama" is 1320 m. What makes it unique is that it is completely vertical, steepest in the world. At the bottom of the pothole is a water course or siphon with branches that are still unexplored. A kind of leech was discovered in the pothole, which has been ascertained to represent a new species, genus and family; it has been named "Erpobdella mestrovi".
Tsumeb
One of the largest and deepest underground lakes in the world lies a little to the east of Tsumeb, on a farm called Harasib. To reach the water in the cave one has either to abseil or to descend an ancient, hand-forged ladder that hangs free of the vertical dolomite walls of the cave for over 50 m. Here, too, SCUBA divers have descended as deep as they have dared (80 m) in the crystal-clear waters and have reported nothing but deep blue below them from one ledge of dolomite to the next with nothing discernible in the depths.