The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

Installed in an act of one-upsmanship, the annual sundial in this church is also a symbolic victory for the Gregorian calendar.

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

Architectureal fantasy quarter of Rome.

An alchemist's "magic door" stands in the middle of a Roman park.

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

A huge enchanting public park just outside the ancient walls of Rome.

The Pope’s secret escape route.

This beautiful park on the outskirts of Rome protects the ruins of two colossal ancient aqueducts.

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

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

The final resting place of the poets Shelley and Keats.

2300-year-old Roman road connecting the empire.

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

The underground sanctuary of a mysterious ancient cult.

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

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

This simple apartment may have inspired some of the most important theories in the history of science.

The Swiss artist's bulbous and overgrown shape-shifting sculpture controversially graces the center of Bern.

A nearly 500-year-old sculpture depicts a man eating a sack of babies, and no one is sure why.

This cathedral hides a sculpted dog that determines your fate and an unexploded shell from World War II.

Once home to one of the oldest banks in the world, this building is also where Marco Polo's famous travelogue was written.