The largest catacomb system in the world.

Beautiful palace stands tall over the Fergana Valley.

A colorful Peruvian city of the dead.

Iranian tombstones shaped like male and female sexual organs.

Mysterious painted skulls in Moravia.

Reputedly haunted sandstone necropolis with exquisite stone carvings.

This ancient subterranean megalopolis is known for the hot oil traps that were installed above each of its entrances.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

While not Turkey's deepest underground city, this subterranean redoubt is the largest.

The floorboard scrawling of a tragic madman is on public display outside a Paris hospital, much to the dismay of its opponents.

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

Inside a former cabaret, seven floors of erotic art.

Down a cobblestone alleyway is a museum dedicated to Romanticism, with a secret garden.

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

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

The history of radiation is on safe display at the foot of a radiation science laboratory.

Pitié-Salpêtrière was the dumping grounds for women who received the dreaded diagnosis of "hysterical."

Stroll through retro Paris while shopping for old postcards and discarded doll heads.

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

One of the oldest medical collections in Europe focusing on rare surgical instruments.

Weird wax anatomical models reside here, as do other anatomical specimens.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

Behind the pink curtain, Au Bonheur du Jour, an art gallery of erotic archives.