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".
Neffs Cave
In 1952 the Salt Lake Grotto of the National Speleological Society was established, and several weeks later the grotto's members obtained permission from the National Forest Service to explore the cave. They spent nearly eleven hours in the cave but had not reached the end of the main passage before they had to turn back. Teams from the grotto returned in 1953 and 1956, and in the last visit they succeeded in reaching the bottom of the main passage. They calculated a vertical depth of 1186 feet (361 meters), which was later revised to the present figure of 1163 feet (354 meters). Neffs Cave is the 13th deepest cave discovered in the United States.