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 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 52,835 acres (21,382 ha) are located primarily in Edmonson County, Kentucky, with small areas extending eastward into Hart County and Barren County. It is centered around the Green River, with a tributary, the Nolin River, feeding into the Green just inside the park. With a confirmed 365 miles of passageways it is by far the world's longest known cave system, being well over twice as long as the second longest cave system, which is South Dakota's Jewel Cave with 145 miles of passageways. Cave animals include five bat species, the Kentucky cave shrimp, cave fish, and cave salamanders. Above ground there are rivers, hiking trails, sinkholes, and springs.