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".
István Cave
The cave is still being explored, but only a part can be visited. Its deepest known area is named "Hell". The most beautiful dripstone structures are the "Mammut's Mouth", "Fairyland", "Column Hall" and "Concert Hall". Due to water filling the cave in 1958 and 1974, the inner caverns are presumably connected with water drains.