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".
Hellhole
Hellhole is a large and deep pit cave in Germany Valley, eastern West Virginia. It is the 7th longest cave in the United States and is home to almost half of the world's population of Virginia big-eared bats. At 518 feet (158 meters), Hellhole is the deepest of several caves in the Valley.