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".
Titan (cave)
Titan is a natural cavern near Castleton in the Derbyshire Peak District, and is the deepest shaft of any known cave in Britain, at . The existence of Titan was revealed in November 2006, following its discovery on 1 January 1999 after cavers discovered connections from the James Hall Over Engine Mine to both Speedwell Cavern and Peak Cavern. Previously, the deepest known underground shaft in Britain had been Gaping Gill on the slopes of Ingleborough in the Yorkshire Dales.