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".
Lazareva Pećina
The Lazareva Pećina, which translates Lazar's Cave, is the longest explored cave in Serbia. Located in Bor municipality, near Zlot, the cave is also sometimes referred to as Zlotska Cave. According to 2012's "Recent Landform Evolution: The Carpatho-Balkan-Dinaric Region", the cave is long. The cave is situated near the entrance of the deep canyon carved into the mountains by the river Zlotska.