It's believed that this small church was connected to the Hagia Sophia via a secret tunnel.

This church houses the tomb—but not the remains—of St. Nicholas, the likely progenitor to Santa Claus.

An underground network of massive stone arches and their skylit vaults support ruins of portico structures above.

This historic hotel preserves many treasures from its heyday hosting passengers on the Orient Express.

This former Imperial summer home was the venue for a royal slap that once echoed 'round the world.

The most metal church of all time was built in record time because of an international, pious pissing contest.

The ornate miniature mansions were built for Istanbul's feathered residents.

An abused and forgotten metropolis, abandoned for centuries.

This is not only one of the oldest collections in Europe, but also possibly the most beautiful.

The bizarre visions of H.R. Giger, tucked away in a Swiss medieval city.

2300-year-old Roman road connecting the empire.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

The underground sanctuary of a mysterious ancient cult.

The middle finger of modernity.

The yawning maw of this pagan visage is said to bite off the hands of liars.

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

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

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

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

Architectureal fantasy quarter of Rome.

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

Where commuters walk through remnants of an ancient Greek necropolis.

The Cycladic architecture makes this Athens neighborhood feel like an island paradise hidden in the shadow of the Acropolis.