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".
Gap Cave
Gap Cave is located just underneath Pinnacle Overlook in the Cumberland Gap National Historical Park, Lee County, Virginia. Currently, the cave has a surveyed length of 18.5 miles and is the 42nd longest cave in the United States and 154th in the world. There are six known entrances: the Gap Cave Entrance (also known as the King Solomons Cave Entrance), an artificial entrance, known as the Cudjo's Cave Entrance, the Stream Entrance, the Soldiers Cave Entrance, the Big Saltpeter Cave Entrance, and the Wellhole. All lead into the same cave, Gap Cave.