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.