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".
List of longest caves
Caves are found around the world. The largest form in areas of karst landforms whose rocks erode easily. Preferable conditions for cave formation are adequate precipitation, enough plants and animals to produce ample carbon dioxide, and a landscape of gentle hills which drains slowly. The highest concentrations of long caves in the world are found in the Pennyrile of southern Kentucky, United States, in the Black Hills of South Dakota, United States, and in the Yucatán Peninsula, Mexico.