Ancient ruins hidden underneath a modern luxury hotel.

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

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

This ancient market is often referred to as the oldest shopping mall in the world.

Jesus' footprints and the first catacombs in the world.

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

2300-year-old Roman road connecting the empire.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

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

Rome’s legendary 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.

A medieval, mosaic-lined baptistry built by a heretical Christian sect.

Psychoanalyst Carl Jung reported a paranormal experience after visiting this mosaic-lined Roman baptistry.

This 6th-century church has the largest and best-preserved collection of Byzantine mosaics outside Constantinople.

An incredible mosaic-lined monument said to have held the remains of an ancient Roman empress.

Italian poet Dante Alighieri is entombed in this small mausoleum in Ravenna, and his hometown of Florence has never been too happy about it.

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

The middle finger of modernity.

This volcanic cave lake is essentially hard proof that Iceland is the most magical place on Earth.

Three columns of volcanic rock shoot out of the ocean.

A black sand beach where glacial fragments wash ashore and look like sparkling diamonds.

A sulfuric martian landscape that looks far better than it smells.

This replica Viking village was created for a film that was never shot and is now open to visitors.

If a glacial river suddenly diving into a sheer chasm via a dramatic, multi-stage waterfall isn't peak Iceland, it's certainly close.