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

In the best of all possible worlds, everyone will get a chance to see this statue in which Voltaire's heart is interred.

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

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

19th century pendulum and a clock restored by a rogue group of guerilla artists.

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

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

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

Once a 19th-century brasserie, this McDonald's exterior celebrates Alsatian design and the "King of Beer."

This frog-themed restaurant was frequented by Picasso, Saint-Exupéry, and more.

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

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

Abandoned railway line circling the city of Paris.

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

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