A little known pile of scraps and plaques mark the grave of a downed RAF bomber.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

100-windowed castle turned art gallery, built by one dedicated mason.

The post-apocalyptic remnants of a bucolic town ruined by a plane crash.

A subterranean shelter remains intact under a busy Paris railway station.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

An odd museum dedicated to French fakes.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

Statue of death tucked in the University of Medicine courtyard.

A museum of carnival attractions where you are invited to ride the rides.

This professional clown's home is an outsider art wonderland.

Longest canal tunnel in the world, abandoned in the 1960s.

Now a memorial, this factory was once used as a concentration camp for artists by the Vichy government.

Even France's favorite aquatic son Jacques Cousteau, couldn't get to the bottom of it.

One of the largest ochre quarries in the world, hidden in a limestone landscape.

This French "sea of rocks" is packed with stunning rock formations and its own medieval castle.

Sculpture of a legendary French dragon.

A unique underground structure dating back to the 1st century BC.

This massive library holds what was once the largest book collection in the world.

Walk upon the mementos of the dead in this Parisian courtyard paved with medieval tombstones.

Buddha and Jesus tower over this French mountainside headquarters of the Aumism religious cult.

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

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

A beautiful but toxic fountain of mercury.