Carlsbad Caverns National Park, 46,766 acres (18,940 hectares), SE N.Mex., in the Guadalupe Mts.; designated a national park in 1930. These connecting limestone caves, with remarkable stalactite and stalagmite formations and huge chambers, began forming 60 million years ago as groundwater started dissolving the rock. The caverns, among the largest in the world, were discovered c.1900 and still have not been completely explored. The temperature of the caves remains constant at 56°F (13.3°C). Seven miles (11.3 km) of trail are electrically lighted. The Big Room, 754 ft (230 m) below the surface, is the most majestic of the many chambers; its perimeter is c.11/4 mi (2 km) long. Each evening during the spring, summer, and fall, the countless bats that inhabit the cave swarm out to feed on insects. The park also contains 76 other separate caves. See National Parks and Monuments (table).