Jesus' footprints and the first catacombs in the world.

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

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

The underground sanctuary of a mysterious ancient cult.

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

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

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

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

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

An exclusive waiting room built for the royal family in the 1920s is hidden in Milan’s central station.

The mark of the Devil's horns on a marble column in Milan.

The secret medieval communication system will whisk your words above the noise of the crowd.

Small on the outside, big on the inside, this church is an astonishing act of visual deception.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

Head underground and take a tour of the city's working sewer system, and see the underground river Senne, which defined Brussels for centuries before being covered in the 19th century.

This recently restored crypt was once plagued by liquefying coffins and exploding caskets.

The Belgian royal family's private depot now stands abandoned and forgotten alongside the tracks.

When you're a great Russian Tsar, even your vomit gets a statue.

This Greek temple, open only an hour a day, was the site of a war of tastes between an architect and an artist.

This modern arts center is built around a gem of late-Baroque architecture.

Since 1874, this macabre collection has developed into a museum devoted to Scotland Yard's mementos of dastardly deeds.

A faded reminder of what was once a common sight in London during the Blitz.

A series of carved figures that scandalized British society.

England's most famous front door has an accidental doppelganger.