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".
Vikos Gorge
The Vikos Gorge, with a length of , walls that range from deep, and a width ranging from to just a few meters at its narrowest part, is listed by the Guinness Book of Records as the deepest canyon in the world in proportion to its width, though some gorge lobbyists contest that claim. The main part of the gorge stretches from the village of Vikos to Monodendri, and attains a depth of about .