On a small Puerto Rican island, banks of secret naval bunkers lie abandoned and filled with piles of office equipment.

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

The best preserved Paleochristian basilica in Rome hides a number of oddities and secrets.

The steel cable lets adventurers zoom between two mountain towns at speeds of up to 75 miles per hour.

The spot where the fascist dictator made some of his most famous speeches including declaring war on Britain and France.

This beautiful park on the outskirts of Rome protects the ruins of two colossal ancient aqueducts.

A cave in southern Italy adorned with thousand-year-old Biblical frescoes existed in obscurity until fairly recently.

Crossing this suspension bridge feels like walking through the sky, a fitting tribute to the head of the Apollo program.

Dug as a secret royal escape route, the tunnel became a wartime bomb shelter and dumping ground for vintage cars.

People have been living in the cave houses on this Italian hillside since the Neolithic era.

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

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

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

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.

This medieval Italian town is a sheer (and beautiful) cliffside defense.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

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

The final resting place of the poets Shelley and Keats.

2300-year-old Roman road connecting the empire.

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

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

Pompeii's less famous neighbor is still revealing her secrets.

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