Mammoth Cave National Park
Lonely Planet review
With the longest cave system on earth, Mammoth Cave National Park has some 400 miles of surveyed passageways. Mammoth is at least three times bigger than any other known cave, with vast interior cathedrals, bottomless pits, and strange, undulating rock formations. The caves have been used for prehistoric mineral gathering, as a source of saltpeter for gunpowder and as a tuberculosis hospital. Tourists started visiting around 1810 and guided tours have been offered since the 1830s. The area became a national park in 1926 and now brings nearly two million visitors each year.