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".
Tears of the Turtle Cave
Tears of the Turtle Cave is located in the Bob Marshall Wilderness in western Montana and is currently the deepest limestone cave in the United States since passing New Mexico's Lechuguilla cave in 2014. As of August 2016 the cave is known to be 1659 feet deep and 1.32 miles long. The cave consists mostly of roughly 2 foot wide fissure passage passing over dozens of short rope drops. With a mean temperature of 39 degrees F (4 C), it is muddy and poorly decorated.