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".
Siddha cave
Siddha Cave or Siddha Gufa is a cave located in Bimalnagar, Tanahu District, Nepal. It is the largest cave in Nepal and second largest cave in South Asia. It's around 0.5 kilometer inside and around 2500 people can feet inside. If you would like to go over Siddha Cave then you have to vehicle from Kathmandu on Prithvi Highway which will take around 4 hours to Bimalnagar, then you have to walk up hills on steps. It will take around 1.5 hours from Bimalnagar to reach to Cave.