Historic medieval fortified city in southwest France.

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

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

A haunting collection of the vanished and disappearing natural world.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

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

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

France's most famous cemetery, with some of its most curious tombs.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

Countless stories attempt to explain Poland's mysterious J-shaped trees.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

Beneath Kraków Old Town's main square lies a hologram-filled medieval market vampire graveyard wonderland.

Martin Bricelj Baraga's monolithic sculpture measures the blueness of the sky, and changes color to match it.

A miniature train-ride through Slovenia's largest cave system.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

This one-time health resort was used by aristocrats around the world.

This massive underground salt mine now holds a sci-fi theme park in its depths.

Moss and stone turn this waterfall into an otherworldly liquid veil.

This hauntingly beautiful cemetery outside a medieval citadel is full of winding paths and ivy-covered graves.

This enormous abandoned prison was once dubbed the "Bastille of Romania."