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".
Krubera Cave
Krubera Cave (; Also known as Voronya Cave, sometimes spelled Voronja Cave) is the second deepest known cave on Earth after the Veryovkina Cave. It is located in the Arabika Massif of the Gagra Range of the Western Caucasus, in the Gagra district of Abkhazia, a breakaway region of Georgia, currently under Russian occupation.