This gaping hole in the middle of a remote mountain lake looks like a portal to another dimension.

Long-abandoned art nouveau baths beneath Milan's Piazza Oberdan.

This impressive rupestrian engraving dates back to the Upper Palaeolithic period.

This oddly shaped church resembles a rocket.

This 13th-century monastery chiseled into the side of a mountain is equipped with a bridge that can be lifted to protect against raids.

A Byzantine-era church hidden in a wooded area below the Acropolis.

This little Greek plateau is home to thousands of rustic, white-sailed windmills.

An abandoned Byzantine fortress and notorious prison.

This 15th-century island fortress later served as home to the executioners no one wanted to live near.

These ruins in Volvi were once at the intersection of the Eastern and Western Worlds.

The oldest cemetery of modern-day Athens blends the modern and ancient worlds.

Thousands of rusting train cars have been slowly falling apart on the edge of Greece's second largest city.

An ancient octagonal weather station named for the eight Greek gods of wind.

The ruins of an abandoned medieval city where the last Byzantine emperor was crowned.

The railway carts travelers to central Athens's highest peak, which offers stunning vistas of the city.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

Monasteries on rock pillars, once accessible only by frayed ropes.

The Cycladic architecture makes this Athens neighborhood feel like an island paradise hidden in the shadow of the Acropolis.

The Roman Emperor's lost summer home and grotto full of enormous sculptures sit along the Italian seashore.

An incredible menagerie of more than 3 million taxidermied animals at the oldest science museum in Europe.

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

Medieval torture techniques with a modern conscience.

Dramatic sea stacks adorn the coast of this small village on the Adriatic coast.

Where Aleister Crowley taught magick in "La Chambre Des Cauchemars," his "Chamber of Nightmares."