Drift through underground caves on a tube.

The land is full of geologic eye candy, such as otherworldly spires, mushroom-shaped hoodoos, and prehistoric fossils.

For those with no fear of heights: a skywalk 4,700 feet up on a Chinese mountain.

The latest in a series of long glass bridges suspended at terrifying heights.

A total of 999 stairs lead visitors through a massive hole in a mountain known as the Gateway to Heaven.

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

This everlasting lightning storm may be the world's largest generator of ozone.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

The most alien-looking place on Earth.

This remote Colorado canyon is home to the second-largest collection of natural arches in the world.

Germany's Glienicke Bridge has a shady history of clandestine meetings, espionage, and spycraft.

Home to items never before seen by the public.

The sinister ruins of one of the worst dam failures of the 20th century lie in this astonishing alpine setting.

A swath of France so devastated by war it is still forbidden to go there.

A toxic ghost town sitting atop a massive coal fire.

The only geyser in the state of Pennsylvania is caused by pressure from an abandoned coal mine outside of the ghost town Centralia.

Home to a timed geyser created by complete accident.

A miraculous natural phenomenon known as "the spring that breathes."

One of the largest free-standing structures ever built, this dirigible hangar is now simply a giant skeleton.

This airfield played a key role in both World War II and the Cold War.

Standing in a state of "arrested decay," this ghost town allows for a detailed peek into Gold Rush life in an unusually harsh climate.

Deep vertical fissures like miniature slot canyons cut across the cemented volcanic rock of Black Point above Mono Lake.

Nine limestone rock stacks on Australia's "Shipwreck Coast."

A clear example of the process of erosion in action.