The world's largest man-made underground caverns offer unparalleled underwater wonders.

Semi-precious Blue John stones have been mined in this cavern system for centuries.

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

This natural Icelandic fissure allows divers to swim right between two volatile tectonic plates.

This massive water spout has been luring visitors for hundreds of years, some to their death.

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

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

Strange red rock formations that have been attributed to a higher power for hundreds of years.

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

The largest source of fossilized insects anywhere.

The remains of the dam that unleashed a 15-foot wall of water on Denver.

Off-limits and full of venomous pit vipers, Brazil’s Snake Island has a frighteningly apt nickname.

Largest sub-glacial lake ever discovered.

Splashes of blue in an otherwise monotonous lunar landscape.

A faux desert landscape in the middle of the jungle that sprouts oases during the rainy season.

The only road through the heart of Glacier National Park.

A serpentine highway twists and turns over the treacherous Norwegian Sea.

An acoustic anomaly with a mysterious cause.

A breathtaking bridge that crosses a vast gorge over a river's torrents, and has a tale of a devil architect attached.

Eons ago, a rushing lava river formed this pristine passage through solid rock.

Thousands of bats stream out of caves every night on the edge on Thailand's second largest park.

Ancient pueblos built into the base of a natural canyon wall.

A fifth-century temple improbably built into the side of a cliff.

Marking the spot where a conquistador gave up his search for the fabled Seven Cities of Gold.