Mammoth Cave, Kentucky

Said to be the longest continuous cave system in the world, with more than 400 miles of mapped passageways, Mammoth Cave is popular tourist attraction, and National Park. Several different tours are offered, of different parts of the cave. There are said to be over 130 different species of plants and animals living within the cave system, some of which can only be found there. You can also eat lunch underground, in one of paved dining rooms. The cave is also accessible via elevator.