The Pope’s secret escape route.

The remains of this 2nd-century Roman temple are incorporated into a modern building.

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

The only remaining arch of the oldest stone bridge in Rome stands in the middle of the Tiber.

In the early Roman Empire, people deemed traitors and criminals were tossed to their deaths from this rock.

An ancient Roman temple first dedicated to an emperor's wife is now a church.

This curious, round temple is the oldest still-standing marble building in Rome.

This small, ancient Roman temple was once a church dedicated to the patron saint of penitents.

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

A rare example of a medieval house in Rome adorned with a melancholic inscription.

This is the oldest Roman-made bridge in the city still existing in its original state.

This medieval city gate is now a blend of the past and the modern world.

This seaside Italian village doesn't just look Escher-esque, it actually inspired the mind-bending artist.

A narrow opening between the mountains of the Amalfi Coast creates a scenic hidden beach.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

This clifftop arch is a natural stunner.

The Emperor Tiberius' palace of debauchery.

This medieval church in Rome was almost destroyed following a terrorist attack in 1993.

This one-of-a-kind Roman arch is not actually dedicated to the god Janus.

This ancient Roman arch misled treasure hunters for centuries.

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

These hut foundations are believed to be the residence of Romulus, founder of Rome.

Remnants of a 4th-century church with vivid medieval frescoes and ancient sarcophagi hide beneath this old basilica.

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