A half-mile long and three stories high, this bridge over the Des Moines River Valley mimics a descent into a mine shaft.

A natural gas leak provides a flickering flame beneath a waterfall.

Bizarre lunar-like landscape with thousands of large stone "hoodoos."

This maze-like slot canyon is a unique gem within an already fascinating geological area.

A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

The largest earthwork effigy in the world.

A modernist landscape of water and sci-fi concrete.

An arsenic-filled pool in the Grand Canyon takes on the appearance of a giant, overflowing pumpkin.

The mysterious wonder of an underground waterfall is illuminated by multicolored lights.

A meditative space out of a fantasy story.

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

A bizarre roadside attraction and the first instance of miniature golf in the world.

A clear blue swimming hole with hidden caves, still unexplored.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

Breathtakingly blue walls shimmer inside the "Glacier Behind the Town."

The scuba diving training facility was named for the steam that rises from the cave's chimney.

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

This geothermal hot spring hides beneath a slowly growing mineral dome.

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

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

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

Mysterious rocks that ring musically when struck.

Hollowed out cedar log tubs set in an ancient towering forest.

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