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".
Cave diving
Hranice Abyss, or Hranická Propast, is the world's deepest known underwater cavity. It beat the previous record holder, Pozzo del Merro in Italy, by . Polish explorer Krzysztof Starnawski, who has explored the upper parts of the flooded fissure for decades, has determined the depth to be at least . As of 2017, divers have yet to reach the bottom of the cave.