This park on the outskirts of Rome boasts two ancient Roman aqueducts and a medieval tower.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

Ancient ruins hidden underneath a modern luxury hotel.

Brass cobblestones mark the places where individual Holocaust victims were taken from their homes.

This large, circular tomb is the final resting place of the first emperor of Rome.

This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

One of several skulls claiming to belong to the beheaded St. John the Baptist is on display at this Roman church.

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.

2300-year-old Roman road connecting the empire.

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

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

Inside a former cabaret, seven floors of erotic art.

One of Jeff Koons’s largest sculptures and his first commemorative work.

A museum which takes visitors to the côté obscure of Paris.

Where the high court feasted, frolicked, and high courtesans put their royal lovers' diamonds to the test.

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

The narrowest street in Paris.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

Once a 19th-century brasserie, this McDonald's exterior celebrates Alsatian design and the "King of Beer."

A modern-day Lascaux cave underneath Paris.

An off-the-beaten-path boutique stuffed with an eclectic collection of upcycled goods.

A grotto-like lodge decorated with gilded mosaics, rocks and a noisy waterfall was the setting for the most sumptuous pleasure house of 1900s Europe.

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