Preserve, southeastern New Mexico, U.S. Established as a national monument in 1923 and as a national park in 1930, it covers 73 sq mi (189 sq km).
Beneath the surface winds a maze of underground chambers; one of the largest caverns ever discovered, the Big Room, is about 2,000 ft (600 m) long and 1,100 ft (330 m) wide, and its ceiling arches 255 ft (78 m) above the floor. In the summer a colony of bats inhabits a part of the caverns known as Bat Cave.