Wind Cave National Park, 28,295 acres (11,459 hectares), in the Black Hills, SW S.Dak.; est. 1903. Wind Cave, discovered in 1881, was named for the strong air currents that blow alternately in and out of it depending on whether the atmospheric pressure is higher or lower than the air pressure inside the cave. There are 10.5 mi (16.9 km) of explored passageways. The cave's temperature remains constant at 47°F (8°C). Unusual calcite-crystal and boxwork formations make Wind Cave unique. The park's surface is characterized by rolling grasslands where bison, elk, deer, and pronghorn antelope can be found; several prairie dog towns are in the park. See National Parks and Monuments (table).