Harlequin costumes, signed scripts, and armor-wearing puppets.

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

This former Roman Catholic Church still contains the embalmed hearts and organs of 22 popes.

The underground sanctuary of a mysterious ancient cult.

Jesus' footprints and the first catacombs in the world.

Architectural monsters are devouring this palace's every door and window.

A collection of objects supposedly singed by the hands of souls in purgatory.

The final resting place of the poets Shelley and Keats.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

Architectureal fantasy quarter of Rome.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

France's national museum of scientific and industrial instruments.

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

Notorious artist squat renovated into legal studios.

Paris' linguistic guide to the international language of, yes, love.

Noirish green neon marks the location of one of the oldest private detective agencies in France.

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

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

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

The former home of France's controversial pop maestro is almost completely covered in ever-changing tributes and graffiti.

Former hunting lodge of a duke who was an avid collector of impressionist works.

This statue was given to France by the United States to commemorate the centennial of the French Revolution.