An ethereal glowing blue ocean cave in Croatia.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

An unusual "skull with ears" sits in the crypt of this small church tucked away at the end of a narrow alley.

The remains of the left arm of the levitating philosopher-saint are kept in a 13th-century Neapolitan basilica.

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

This shining multicolored metro station deep under Naples is said to be the most beautiful in Europe.

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

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

Architectureal fantasy quarter of Rome.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

This magical-looking island is notorious for the misfortunes suffered by its owners.

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

This small but lovely sinkhole lake is the last remnant of a great prehistoric water basin.

This one-time health resort was used by aristocrats around the world.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

This abandoned Slovenian military barracks has become a punky-political art squat.

A whimsical bridge adorned with decorative dragons.

This cluster of little wooden huts once ground local farmers’ wheat into flour during the days of the Austro-Hungarian Empire.

A curious cave church carved into the face of a cliff.

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

The now-overgrown beach resort was once frequented by Yugoslav military elite.

A neolithic tomb converted into a tiny chapel.

A futuristic, brutalist masterpiece that is also strangely prison-like.

Two distorted Romanesque columns hold aloft a mine cart in honor of laborers who died during WWII.