Scores of wondrous caves hide under the hills at this unique national park, which covers 73 sq miles. The cavern formations are an ethereal wonderland of stalactites and fantastical geological features. You can ride an elevator from the visitor center or take a 2-mile subterranean walk from the cave mouth to the Big Room, an underground chamber 1800ft long, 255ft high and over 800ft below the surface. For claustrophobics and those prone to panic attacks, the chamber and the elevator ride down to it (which descends the length of the Empire State Building in under a minute) may be a less than enjoyable experience.