Krubera Cave
The difference in elevation of the cave's entrance and its deepest explored point is . It became the deepest-known cave in the world in 2001 when the expedition of the Ukrainian Speleological Association reached a depth of which exceeded the depth of the previous deepest-known cave, Lamprechtsofen, in the Austrian Alps, by . In 2004, for the first time in the history of speleology, the Ukrainian Speleological Association expedition reached a depth greater than , and explored the cave to . Ukrainian diver Gennadiy Samokhin extended the cave by diving in the terminal sump to 46 metres' depth in 2007 and then to 52 m in 2012, setting successive world records of 2,191 m and 2,197 m, respectively. Krubera is one of the two known caves on Earth deeper than 2,000 metres.
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.