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".
Deer Cave
Deer Cave was known as the largest cave passage in the world until the discovery of Sơn Đoòng cave in Phong Nha-Kẻ Bàng National Park in Vietnam. It is part of the Gunung Mulu National Park, a UNESCO World Heritage Site characterised by amazing caves and karst formations in a mountainous equatorial rainforest setting. 
The Clearwater cave, with approximately of known passages, is considered the longest cave system in Southeast Asia. It is recognised for its clear underground stream, popular with tourists. The Wind cave is located on the same trail leading to Clearwater cave, and is known for its blowing wind. It contains a chamber dubbed King’s Room, a cavern containing huge columns of stone on both the ceiling and the floor. The Lang cave is the smallest of the caves open to the public. However, its size allows up-close opportunities to see some of the common cave inhabitants such as bats and swiftlets.