This historic Union Pacific Railroad depot is a stunning symbol of American post-Civil War progress.

Built in 1981 in collaboration with Andy Warhol, this giant piece of pop art stands on the campus of Colorado State University.

For 14 years, this former stray dog greeted every train passenger that arrived in Fort Collins.

The walls of America's oldest masonry fort famously "swallowed" cannonballs.

A small store filled with animal skulls and other curiosities.

Most of this “research national monument” still lies buried beneath the earth.

The high point on the 16th president's eponymous roadway is marked by a massive version of his head.

A monument to the egos of two railroad magnates that fell into decline along with their careers.

Miraculous tree that seems to sprout out of solid rock.

One of the smallest towns in the United States.

The final resting place of a 19th-century gold prospector and convicted cannibal.

Abandoned silver mine strewn with wrecked and twisted structures.

From a canyon-like gorge, more than 20 million gallons of water emerge every day, while a smaller spring drips down, creating the illusion of an endless, gentle waterfall.

A natural formation that resembles a camel resting in the New Mexico desert.

Crafted in 1911, this steam locomotive spent its entire working life in Flagstaff.

A sign at the end of the Santa Monica Pier marks the traditional end of the legendary American highway.

Final resting place of Geronimo, Quanah Parker, and the first atomic gun ever fired.

Earth's tallest volcano is sacred to Native Hawaiians, and home to a dozen world-class observatories.

These picturesque Hawaiian falls are home to a rare species of fish that uses a special sucker to climb up cliffs.

This World War II-era bunker is now packed with posters and props from movies and TV shows filmed nearby.

No one knows exactly how deep the world’s deepest geothermal hot spring is.

This grand stone tower was dedicated to a famed American humorist almost by coincidence.

This vintage theme park is peak Christmas kitsch—and it's open all summer.

The burial grounds of a secret society includes fences topped with fake Tesla balls.