Mammoth Cave
Mammoth Cave, Kentucky
Mammoth Cave claims to be "the second-oldest tourist attraction in the United States," offering tours since 1816, and it became a National Park in 1941. Mammoth Cave is also the longest cave in the world -- over 400 miles of painstakingly explored and mapped passages thus far -- which is a meaningless statistic to the average visitor, who only sees an hour's worth on the tour, as with most other caves. But if you're going to visit a cave, you'll want to go to the biggest one -- just to say you've been there.
Mammoth Cave pioneered the art of naming its formations with colorful titles such as "Frozen Niagara" and "Fat Man's Misery" -- names that have been copied by countless other subterranean attractions. In fact, Mammoth Cave has pretty much everything -- except the bones of Woolly Mammoths, which is a popular misconception.