Velebit caves
The "Lukina jama" cave is 1431 m deep (2013), the deepest cave in Croatia, 14th deepest cave in the world and the deepest cave in southeast Europe. At its foot, there are ponds and streams including one of the largest known colonies of subterranean leeches ("Erpobdella mestrovi"), which has been ascertained to represent a new species, genus and also family. Other species discovered in the system, include the air-breathing land snails "Zospeum tholussum". Like Lukina jama, Slovacka jama, Velebita and Meduza are also located at the "Hajdučki i Rožanski kukovi" special reserve, a dedicated part of the Sjeverni Velebit national park.
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.