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".
Ogdens Cave Natural Area Preserve
With nearly of the cave mapped, Ogdens Cave is the longest known cave in Frederick County. Several rare invertebrate species are found within the cave, including the thin-neck cave beetle ("Pseudanophthalmus parvicollis"), two amphipods, an isopod, and the Appalachian springsnail ("Fontigens bottimeri"). These species are sustained by nutrients delivered by the cave's stream and by various animals, such as bats and crickets, that come and go from time to time.