Mammoth Cave National Park is a U.S. national park in central Kentucky, encompassing portions of Mammoth Cave, the longest cave system known in the world. The official name of the system is the Mammoth-Flint Ridge Cave System for the ridge under which the cave has formed. The park was established as a national park on July 1, 1941. It became a World Heritage Site on October 27, 1981, and an international Biosphere Reserve on September 26, 1990.
The park's 52,830 acres (21,380 ha) are located primarily in Edmonson County, Kentucky, with small areas extending eastward into Hart County and Barren County. It is centered on the Green River, with a tributary, the Nolin River, feeding into the Green just inside the park. With 400 miles (640 km) of surveyed passageways Mammoth Cave is by far the world's longest known cave system, being over twice as long as the second-longest cave system, Mexico's Sac Actun underwater cave.