A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Hundreds of hidden passageways weave through the old quarters of Lyon.

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

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

One of the greatest maze designer’s greatest mazes.

The first green space constructed on an elevated viaduct.

A grotto-like lodge decorated with gilded mosaics, rocks and a noisy waterfall was the setting for the most sumptuous pleasure house of 1900s Europe.

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

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

This Parisian fountain is a little bit fantasy and a little bit nightclub.

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

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

Indents in the pavement are all that remain from a grisly death machine in Paris.

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

This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

France's national museum of scientific and industrial instruments.

A surrealist timepiece by the artist of melting clocks.

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

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

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

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

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

Turntable anonymously received orphans, alms, and groceries.