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".
Belum Caves
The Belum Caves is the largest and longest cave system open to the public on the Indian subcontinent, known for its speleothems, such as stalactite and stalagmite formations. The Belum Caves have long passages, galleries, spacious caverns with fresh water and siphons. This cave system was formed over the course of tens of thousands of years by the constant flow of underground water. The cave system reaches its deepest point ( from entrance level) at the point known as "Pataalaganga". In Telugu language, it is called " Belum Guhalu". Belum Caves have a length of , making them the second largest caves on the Indian Subcontinent after the Krem Liat Prah caves in Meghalaya. It is one of the centrally protected Monuments of National Importance.