There's Only One...Mammoth Cave National Park.
One of the most popular attractions in Kentucky, Mammoth Cave National Park was established to preserve the cave system, including Mammoth Cave, the scenic river valleys of the Green and Nolin rivers, and a portion of the hilly country of south central Kentucky. This is the longest recorded cave system in the world with more than 400 miles explored and mapped. Admission to the national park is free. A fee is charged for camping and cave tours. Activities include cave tours, hiking, horseback riding, biking, picnicking, camping, backcountry camping, canoeing, fishing, and some of the most pleasurable driving in Kentucky. Mammoth Cave National Park's overnight accommodations include campgrounds and hotel facilities.