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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 405 miles (652 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.Mammoth Cave developed in thick Mississippian-aged limestone strata capped by a layer of sandstone, making the system remarkably stable. It is known to include more than 390 miles (630 km) of passageway; new discoveries and connections add several miles to this figure each year. Mammoth Cave National Park was established to preserve the cave system.The upper sandstone member is known as the Big Clifty Sandstone: thin, sparse layers of limestone interspersed within the sandstones give rise to an epikarstic zone, in which tiny conduits (cave passages too small to enter) are dissolved by the natural acidity of groundwater. The epikarstic zone concentrates local flows of runoff into high-elevation springs which emerge at the edges of ridges. The resurgent water from these springs typically flows briefly on the surface before sinking underground again at elevation of the contact between the sandstone caprock and the underlying massive limestones. It is in these underlying massive limestone layers that the human-explorable caves of the region have naturally developed.The limestone layers of the stratigraphic column beneath the Big Clifty, in increasing order of depth below the ridgetops, are the Girkin Formation, the Ste. Genevieve Limestone, and the St. Louis Limestone. For example, the large Main Cave passage seen on the Historic Tour is located at the bottom of the Girkin and the top of the Ste. Genevieve Formation.Each of the primary layers of limestone is divided further into named geological units and subunits. One area of cave research involves correlating the stratigraphy with the cave survey produced by explorers. This makes it possible to produce approximate three-dimensional maps of the contours of the various layer boundaries without the necessity for test wells and extracting core samples.The upper sandstone caprock is relatively hard for water to penetrate: the exceptions are where vertical cracks occur. This protective role means that many of the older, upper passages of the cave system are very dry, with no stalactites, stalagmites, or other formations which require flowing or dripping water to develop.However, the sandstone caprock layer has been dissolved and eroded at many locations within the park, such as the Frozen Niagara room. The "contact" between limestone and sandstone can be found by hiking from the valley bottoms to the ridgetops: typically, as one approaches the top of a ridge, one sees the outcrops of exposed rock change in composition from limestone to sandstone at a well-defined elevation.At one valley bottom in the southern region of the park, a massive sinkhole has developed. Known as "Cedar Sink," the sinkhole features a small river entering one side and disappearing back underground at the other side.Mammoth Cave is home to the endangered Kentucky cave shrimp, a sightless albino shrimp.The National Park Service offers several cave tours to visitors. Some notable features of the cave, such as Grand Avenue, Frozen Niagara, and Fat Man's Misery, can be seen on lighted tours ranging from one to six hours in length. Two tours, lit only by visitor-carried paraffin lamps, are popular alternatives to the electric-lit routes. Several "wild" tours venture away from the developed parts of the cave into muddy crawls and dusty tunnels. The lectures delivered by the National Park Service cave guides are varied by tour, so that in taking several tours the visitor learns about different facets of the cave's formation, or of the cave's human history and prehistory.The Echo River Tour, one of the cave's most famous attractions, used to take visitors on a boat ride along an 。