This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

A labyrinthine network of chambers with a storied past hides below Edinburgh's South Bridge.

Accessible only at low tide, the deserted island is dotted with World War II ruins.

The fountain marks the area where hundreds of accused witches were burned at the stake.

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

An American ghost town hides along a side street in Scotland's capital.

Long abandoned underground complex to keep royals safe from a nuclear war that never came.

This series of Scottish caves has no clear purpose other than baffling historians.

Underneath this small church are 15,000 bones, arranged by an obsessive academic.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

This spooky statue sits outside a famous Prague theater.

Bullet holes in the church wall are a reminder of the violence that followed the assassination of a high-ranking Nazi officer.

An existential riddle in the world's largest ancient castle.

Monument to a suicide protesting the Soviet invasion.

Increasingly decayed figures stand as a powerful reminder of the human costs of Czech Communism.

One man's personal collection of KGB memorabilia is presented along with his strange viewpoint.

Microscopic scenes; some call it novelty, some call it art.

A saintly relic hidden in a Prague church basement.

An eerie, existential museum dedicated to the author of "The Metamorphosis."

An old asylum graveyard that's been abandoned for half a century attracts those seeking a dark thrill.

This Prague museum collects brutalist relics and memories from the country's former Soviet rule.

A steampunk-styled three-floor factory churning out art, activism, and electronic beats.

The first museum in the world devoted to sexual gadgetry.

Around 100,000 bodies are buried below the medieval headstones, many of which mark graves with multiple bodies stacked up to 12 deep.