The second-largest ceremonial mound in the United States is an artificial hill that is loosely shaped like a pentagon.

Series of underground houses inhabited continuously for centuries.

A natural arch carved by a river gave its name to the so-called "Cave of Forgotten Dreams."

The remote outlaw hideout of Jesse James, Butch Cassidy, and the Sundance Kid.

Crystal clear lake with a shoreline of travertine located in a Colorado canyon.

This "zoo" is a cross between a junkyard and a sculpture garden, featuring animals you won't see anywhere else.

This tea house was a hand-built gift from Boulder's sister city in Tajikstan.

An annual festival celebrating Colorado's most famous frozen corpse.

Hotel designed by indie graphics artists.

One of the most remarkable caves in all of Iceland was created by a once raging river of lava.

These faux-cliff dwellings allow visitors to climb around on a replica archaeological site.

Even in death, he's still your huckleberry.

This naturally-sourced hot spring is now home to a clothing-optional resort.

The largest source of fossilized insects anywhere.

Largest collection of mounted dinosaur specimens in the world.

This prehistoric sinkhole lured exclusively male mammoths to their death, much to the delight of science.

One of America's first dinosaur parks gives a window into Depression-era paleontology.

A waterfall 600 ft. inside a mountain, once the bane of gold miners, is now a spectacle of nature.

After Havre burned down, the best place to build was under the ashes.

Resonant stone, that has been played since prehistoric times.

Home to the deepest rift anywhere on Earth.

One man's ambition created this psychedelic log home.

The rocks of a certain area of Butte, Montana give off a musical ring when hit.

The fossilized remains of ancient California redwoods.