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".
Carreg Cadno
Ogof Ffynnon Ddu, Britain's deepest cave lies under the northern slopes of Carreg Cadno. The vertical interval between the sink of the Byfre Fechan stream at Pwll Byfre and its resurgence at Ffynnon Ddu is 300m / 1000 ft. The national nature reserve has been established to protect both the surface features and the vegetation and habitats which they provide and also the extensive cave network beneath.