Carlsbad Caverns
Credit: David Parsons / Getty Images

The area near the Guadalupe Mountains in southeastern New Mexico was once home to an ancient, inland sea roughly 250 million years ago. The grounds have since dried out, leaving behind the vast maze of Carlsbad Caverns. Visitors can go spelunking with park rangers through some of the 119 caverns that formed when sulfuric acid dissolved the surrounding limestone, including the Big Room, which stretches more than six football fields. 

Don't Miss: Let the Big Room swell with tourists and have a ranger take you to Slaughter Canyon Cave, an underground wilderness that requires flashlights and headlamps. It's a slightly more strenuous adventure that begins with a half-mile trek to the cave, but you’ll see a large crystal-lined column called the Christmas Tree and the Chinese Wall, a shallow dam of rimstone.