BDSM, 1930s Style.

This banal Parisian entranceway is not what it seems.

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

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

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

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

A lone cannonball remains lodged in the walls of the Hotel de Sens as a sober reminder of a revolution.

This surreal statue brings a famous French novel about a supernatural cad to arresting life.

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

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

A favorite Paris bar for U.S. expats, including Gershwin, Hemingway, and James Bond.

A mechanized clock where a man fights off a dragon, crab, and rooster.

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.

The oldest stone house in Paris was built by its most famous alchemist.

This ancient Roman arch misled treasure hunters for centuries.

A fairy tale castle in modern Rome.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

An alchemist's "magic door" stands in the middle of a Roman park.

A deep mark on the remains of a Roman temple, believed to be left by a legendary warrior.

The ruins of an abbey believed to have been founded by Charlemagne in the 9th century.

Brass cobblestones mark the places where individual Holocaust victims were taken from their homes.

An Italian ghost town that fought obscurity, until a mysterious illness brought it to its knees.