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".
Bill Steele (cave explorer)
Continuing to explore caves throughout the United States, Steele led and participated in expeditions to Sistema Huautla in Mexico almost every year through the 1980s. In 1987, the Huautla expedition connected the deep cave Nita Nanta to Sistema Huautla; which established it as the second deepest cave in the world. Steele went to Kijahe Xontjoa on the plateau to the east of Huautla with an expedition of Swiss explorers in 1993, and explored depths over 1,000 meters.