Mammoth Cave National Park, located in southern Kentucky about 95 miles south of Louisville, lives up to its name. No, woolly mammoths never dwelled in its bowels, but the park's namesake is the longest cave system in the world, with over 390 miles of passageways, more than double that of the runner-up. The park also includes more than 200 additional caves disconnected from the main system. Over 200 animal species live in the cave, including 42 that survive in complete darkness.

Access to the caves is by tour, which starts at $4 for adults. On National Public Lands Day on Sept. 29, the park will offer its self-guided Mammoth Cave Discovery tour free. More info: http://www.nps.gov/maca/index.htm.