National Park
Carlsbad Caverns National Park
While a cave might not sound quite as sexy as redwoods, geysers or the Grand Canyon, there’s no question that this one measures up on the national parks’ jaw-droppingly ginormous scale: to simply reach the main chamber, you have to either take an elevator that drops the height of the Empire State Building or, more enjoyably, take a spooky 1.25-mile subterranean walk that goes down and down (and down) from the cave mouth into the yawning darkness.