Mammoth Cave National Park

The world's longest known cave system, with more than 400 miles of passages, and a choice of tours wide enough to suit almost anyone: a half-hour self-guided Discovery Tour, a fully wheelchair-accessible tour, the dripstone of Frozen Niagara with only 64 stairs, or the two-mile Historic Tour with 540 stairs. Visitors who want to get dirty can book off-trail crawling tours. The deep cave sits at 54°F and some trails are uneven. First-timers who want scale should take the Historic Tour; formation fans should add Domes & Dripstones, though its 638 stairs make it the most stair-heavy of the regular tours. Not every tour runs all year: the winter schedule from mid-November drops the Historic and Discovery tours, so check dates first.