Caves and karst features occur in 120 parks in all regions of the National Park System (81 contain caves and an additional 39 contain karst). Over 3,900 caves are currently known throughout the system. Karst is a region of irregular topography with sinks, underground streams, and caves that were formed by dissolution of soluble rock.

The number of caves ranges from as few as 10 to 15 caves per unit—as in the Chesapeake & Ohio Canal National Historic Park—to more than 450 caves per unit—as in Lava Beds National Monument.