The world's longest known cave system.

Prehistoric animals decorate this popular outdoor park.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

A house carved into a rock in the 1940s.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

The graffiti-covered ruins of an abandoned trading post in the middle of Navajo Nation.

These elusive, isolated rodents evolved into a distinct, locally iconic subspecies.

One of the most extensive and well-preserved collections of dinosaur footprints in North America.

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

An ode to one of the dirtiest jobs during the construction of the Boulder Dam.

A Norwegian-born sculptor created these soaring guardian angels at the entrance to Hoover Dam.

These markers are used to document elevation changes around the famed dam.

This sign designates the road that leads to the infamous Area 51.

Mysterious meeting place for UFO hunters near Area 51.

A lighthouse built to save lives that took quite a few with its treacherous location.

Two massive rock outcroppings on the Oregon coastline.

The largest wooden building in the world now houses a fascinating collection of aircraft.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

Outside this souvenir shop is a statue of Bigfoot composed of volcanic ash, and plenty more to see inside.

Stroll through a wonderland of stupendous moss-covered trees.

This working class sculpture pounds his hammer all the livelong day.

An apocalyptic seismic event creates a deadly chain reaction that ends with a 6-mile long lake developing within weeks.

One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

The residents at this Yellowstone wildlife park will certify your cooler as "bear-proof."