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".
Hranice Abyss
The Hranice Abyss (), the English name adopted by the local tourist authorities, is the deepest flooded pit cave in the world. It is a karst sinkhole located near the town of Hranice (Přerov District). The greatest confirmed depth (as of 27 September 2016) is 473 m (404 m under the water level), which makes it the deepest known underwater cave in the world. Moreover, the expected depth is 800–1200 m.