An abandoned saucer-shaped monument in the Bulgarian mountains.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

This attraction was abandoned after the Chernobyl disaster.

The remains of diamond fever taken over by the desert.

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

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

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

The abandoned Japanese amusement park once loomed as a dystopian Disneyland.

The largest of London's subterranean rivers and once a mad, bad center of London life.

Odd outsider art installations surround a Gold Rush-era ghost town.

A seemingly endless landscape of pure salt stretches far across Bolivia.

Remnants of a storybook amusement park hidden behind a strip mall.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

An eerie, derelict amusement park in Berlin.

A theme park that suffered massive flooding during Katrina was abandoned, and now proves irresistible to trespassing explorers.

A toxic ghost town sitting atop a massive coal fire.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

A fiery crater has been burning in the Karakum Desert since 1971.

World's first "penitentiary," meant to be humane, drove men insane.

A beautiful, dense forest in the shadow of Mount Fuji, with a tragic reputation.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.