Mammoth Cave National Park is an American national park in central Kentucky, encompassing portions of Mammoth Cave, the longest cave system known in the world. Since the 1972 unification of Mammoth Cave with the even-longer system under Flint Ridge to the north, the official name of the system has been the Mammoth–Flint Ridge Cave System. The park was established as a national park on July 1, 1941, a World Heritage Site on October 27, 1981, and an international Biosphere Reserve on September 26, 1990. The park’s 52,830 acres are located primarily in Edmonson County, with small areas extending eastward into Hart and Barren counties. The Green River runs through the park, with a tributary called the Nolin River feeding into the Green just inside the park. Mammoth Cave is the world’s longest known cave system with more than 400 miles of surveyed passageways,which is nearly twice as long as the second-longest cave system, Mexico’s Sac Actun underwater cave.