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.
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.
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 underground river. The tour was discontinued for logistic and environmental reasons in the early 1990s.
Mammoth Cave headquarters and visitor's center is located on Mammoth Cave Parkway. The parkway connects with Kentucky Route 70 from the north and Kentucky Route 255 from the south within the park.
Further connections between Mammoth Cave and smaller caves or cave systems have followed, notably to Proctor/Morrison Cave beneath nearby Joppa Ridge in 1979. Proctor cave was discovered by Jonathan Doyle, a Union army deserter during the Civil war, and was later owned by the Mammoth Cave Railroad, before being explored by the CRF. Morrison cave was discovered by George Morrison in the 1920s. This connection pushed the frontier of Mammoth exploration southeastward.
At the same time, discoveries made outside the Park by an independent group called the Central Kentucky Karst Coalition or CKKC resulted in the survey of tens of miles in Roppel Cave east of the Park. On September 10, 1983, a connection was made between the Proctor/Morrison's section of the Mammoth Cave system and Roppel Cave. The connection was made by two mixed parties of CRF and CKKC explorers. Each party entered through a separate entrance and met in the middle before continuing in the same direction to exit at the opposite entrance. The resulting total surveyed length was near 300 miles (480 km).
On March 19, 2005, a connection into the Roppel Cave portion of the system was surveyed from a small cave under Eudora Ridge, adding approximately three miles to the known length of the Mammoth Cave System. The newly found entrance to the cave, now termed the "Hoover Entrance," had been discovered in September, 2003, by Alan Canon and James Wells. Incremental discoveries since then have pushed the total to 400 miles (640 km).
It is certain that many more miles of cave passages await discovery in the region. Discovery of new natural entrances is a rare event: the primary mode of discovery involves the pursuit of side passages identified during routine systematic exploration of cave passages entered from known entrances.