The world's longest cave (more than 350mi of the five-level labyrinth have been mapped), Mammoth underlies three Kentucky counties. Known to Woodland Indians some 4,000 years ago and rediscovered by frontiersmen in the late 18C, Mammoth Cave was established as a national park in July 1941. Several tours are offered into Mammoth's depths, taking visitors as far as 360ft below ground. Among the sights are the ruins of an 1810-12 saltpeter mining operation; a decorative stone formation called Frozen Niagara; and the underground River Styx, home of eyeless fish. Designated a UNESCO World Heritage site and an International Biosphere Reserve, the cave is home to the world's most diverse cave ecosystem, including some life forms that cannot survive outside its walls. Cave temperatures average 54°F year-round; bring a sweater or light jacket.