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

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

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.

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

The martyr's wax-enhanced skeletal remains lie preserved in a glass case in the Santa Maria della Vittoria in Rome.

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

A Nesting Doll of Churches.

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

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

2300-year-old Roman road connecting the empire.

Stone memorial that's said to predict the death of the pope and the site of the Cadaver Synod.

One of seven Jewish catacombs in Rome and one of only two open to the public.

Jesus' footprints and the first catacombs in the world.

An abandoned Roman salami factory has found new life as an art space home to 200 squatters.

The studio built by fascists in the hopes of "reviving the film industry" with the occasional propaganda film opportunity thrown in.

More than 140 granite sculptures decorate the lawn of this French sculptor.

Nicknamed the "Museum of Horrors," this French anatomical collection has been fascinating visitors for over a century.

Street artists transformed an abandoned air vent into the beloved Star Wars robot.

Eco-village hidden in the foothills of the Alps that created a massive system of psychedelic underground temples.

A fake medieval village in the middle of Turin.

The astonishing collection of an infamous criminologist.

An unsanctioned shrine that sprung up outside the late singer's home is causing a stir in London's Highgate neighborhood.

This museum tucked below a charming cafe gives a brief snapshot of photographic history.