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. Excellent ranger-guided tours explore the subterranean expanse. Book ahead if possible (at www.recreation.gov). Tours do sell out, especially in summer and on weekends.
Jaunts range from hour-long strolls to strenuous, day-long spelunking adventures (adults only). The Frozen Niagara Tour is the easiest of the bunch. The magical Violet City Tour takes place by lantern light.
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, though only a few sites have electricity or water hookups (sites from $20); 13 free backcountry campsites; and a lodge and cottages. Reservations for camping (www.recreation.gov) and lodging (www.mammothcavelodge.com) can be made online. Get your backcountry permit at the visitor center.