The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

Countless grotesque faces stare out from this eerie Prague art wall.

A Jenga-style cylindrical tower of books that appears to go on forever.

Thirty creepy ghosts now inhabit this decaying 14th-century church.

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

A collection of anatomical samples from one of the first anthropologists to suggest that humans reached North America by crossing the Bering Strait.

The risqué mixed-media sculptures are supposedly a metaphor for Czech politics.

The hidden lair of an alchemist rediscovered in a 2002 flood.

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

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

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

A burial vault located under Tiber Island in the middle of the Italian capital.

The ruins of an abbey believed to have been founded by Charlemagne in the 9th century.

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

Ancient ruins hidden underneath a modern luxury hotel.

One of the most important yet well-hidden museums dedicated to the history of medicine.

Jesus' footprints and the first catacombs in the world.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

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

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

Architectureal fantasy quarter of Rome.

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

At first glance, this area seems like just another city park, but the lush landscape hides a dark past.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.