Mammoth Cave National Park, 52,830 acres (21,396 hectares), central Kentucky, authorized 1926, est. 1941. Located in a hilly, forested region, it offers numerous outdoor activities. It is the site of Mammoth Cave, the longest recorded cave system in the world. Composed of a series of subterranean chambers and narrow passages formed by the dissolution of limestone, the cave has five separate levels. The known passages have a combined length of over 345 mi (555 km), disclosing limestone formations (stalactites, stalagmites, and columns), lakes, and rivers. Echo River, c.360 ft (110 m) below the surface, flows through the cave's lowest level and drains into the Green River. Hanson's Lost River, an underground stream, joins Mammoth Cave with the extensive Flint Ridge cave system; this long-sought link was discovered in 1972. The temperature (54°F/12°C) and relative humidity (87%) remain constant during the year throughout the cave. The cave contains the mummified body of a man believed to date from the pre-Columbian period. Bats, eyeless fish, and insects are also found. Mammoth Cave was a long-time Native American habitation before it was explored by Kentucky pioneers in 1799. During the War of 1812, saltpeter was mined in the cave for gunpowder. See National Parks and Monuments (table).