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With the longest cave system on earth, this national park has some 400 miles of surveyed passageways. Mammoth is at least three times longer 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. Guided tours have been offered since 1816.
The area became a national park in 1941 and now attracts 600,000 visitors each year.
The only way to see the caves is on the excellent ranger-guided tours and it's wise to book ahead, especially in summer. Tours range from subterranean strolls to strenuous, day-long spelunking adventures (adults only). The Historic tour is especially interesting.
In addition to the caves, the park contains 85 miles of trails – all for hiking, 60 miles for horseback riding and 25 miles for mountain biking. There are also three campgrounds with restrooms, though only a few sites have electricity or water hookups (sites from $20); 13 free backcountry campsites; and a hotel and cottages. Reservations for camping (www.recreation.gov) and lodging (www.mammothcavelodge.com) can be made online. Get your backcountry permit at the park visitor center.