The abandoned zoo enclosures offer a perfect place to experience the other side of the bars.

An abandoned Jazz Age dream home and con man's hideout in Death Valley.

A Turkish ghost town abandoned in a population exchange with Greece.

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

Surreal riveted sea forts once protected the Kent shores from German attack.

A small flooded ghost town with a few scattered structures remaining.

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

Cretean quarry suggests real-life site for ancient Greek myth of the man-eating Minotaur in his labyrinth.

An Aztec temple, the largest man-made pyramid in the world, sits buried in earth with a Spanish church set on top.

The world's largest mountain carving could fit all of Mount Rushmore inside it many times over.

This abandoned Navy base was transformed into an off-grid alternative living community for thousands of people.

The largest earthwork effigy in the world.

This notorious Canadian mental institution was home to some of the first LSD experiments and origin of the word "psychedelic."

A walk through the historic city center, under the streets of Seattle.

San Franciscans leave gifts and messages for the magical beings that live behind the tiny, mysterious door installed on a tree in Golden Gate Park.

The remains of a ruined mansion memorialize the 1906 earthquake.

A nearly unconquerable island-fort in the Indian Ocean.

Stone age urban planning.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

Abandoned city hidden from the world by tropical forest and isolation on the coast of Kenya.

This hunter-gatherer architecture is believed to be the oldest religious complex known.

Civil War Fort on an island in the Boston Harbor.

An old abandoned mining town in The Adirondacks where Theodore Roosevelt first learned of the shooting of President William McKinley.

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