The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

Walk 2,300 feet above the ground on the "Troll's Tongue."

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

Better known as the "Body Farm," this unique research facility has been studying how human bodies break down since the 1980s.

More than five miles of underground passageways.

A series of modernist treehouse hotel rooms grace the outskirts of an Arctic forest.

A Hawaiian stairway to heaven—that you can't climb.

A beautiful geyser under environmental threat.

Every autumn, this hillside is ablaze with red summer cypress.

Centuries-old bridges grown from tangled roots.

Pompeii's less famous neighbor is still revealing her secrets.

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

An ethereal glowing blue ocean cave in Croatia.

Stroll through a wonderland of stupendous moss-covered trees.

A gargantuan crystal kingdom.

Keshwa chaca, among the last handwoven Incan bridges, crosses Apurimac Canyon in Peru.

Once the most densely populated place in the world, this island is now a ghost town.

The atmospheric road was never meant to be so creepy.

This abandoned asylum was once a state of the art facility before devolving into one of the most deadly mental institutions in American history.

Wondrous sandstone hoodoos where Native Americans collected clay for pottery, 9,000 years ago.

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

A spectacular cave and a beguiling lake.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.