This hidden Art Nouveau courtyard is an opulent feast for the eyes.

Stalactites and stalagmites in a beautifully lit cave.

This community on a small Venetian island has taken to painting their houses in bright neon hues.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

This Parisian café sticks almost exclusively to an ethos of reuse.

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

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

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

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

The very center of Paris is surrounded by a bevy of strange good luck rituals.

A surrealist timepiece by the artist of melting clocks.

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

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

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

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

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

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

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

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

135 bronze medallions mark a French meridian line which once rivaled the hotshot Greenwich timeline.

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

This mysterious runestone embedded in a church wall is linked to the medieval cult of Saint Erik.

A spiritual sanctuary in a natural cave.