19. Big Bend National Park, Texas

Annual visitors: 440,091

Fun fact: This area is a goldmine for archeologists, who have found tons of dinosaur bones and sea fossils here.

Cliff-like canyon walls cut a stark backdrop to the Chihuahuan Desert, where the Rio Grande river meanders slowly on its inexorable path. The park, spanning nearly one million acres, is home to dozens of natural wonders, including waterfalls, balanced boulder overhangs, and cool wildlife like cougars and Mexican wolves. Backpackers and hikers love the area for its challenging terrain; another popular way to explore the area is via a river tour up the Rio Grande.