Mammoth Cave National Park is the home of the world's longest known cave system, with over 400 miles of mapped caves. Tours of the caves have been offered since the mid-19th century, it became a national park in 1941. Park rangers lead a dozen different cave tours for varying levels of adventure, physical comfort and claustrophobia. Fees apply, but they're very reasonable for what the tours offer. Reservations are suggested. Designated a UNESCO World Heritage Site, Mammoth Caves is located in south-central Kentucky, about 20 miles from Bowling Green.