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".
Wyandotte Caves
The main entrance to Wyandotte Cave is 220 feet (67 m) above the level of the Blue River. Wyandotte Cave is known for its long open passageways and large rooms. With 9.2 miles (14.806 km) of passageways on five levels it is the fifth longest cave in Indiana. Included in its formations is Monument Mountain. At 135 feet (41.148 m) tall, Monument Mountain is considered to be the world's largest underground mountain. Wyandotte Cave is also home to a great many helictites, which are considered rare. The cave is also home to the tallest stalagmite in the world, known as the Pillar of the Constitution, but this is only visible on crawling tours.