One of its engravings is believed to be among the first representations of medical and surgical instruments.

An autonomous republic of Orthodox monks.

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

150 or so scrounged-up lamps, chandeliers, and lanterns light up this alley in central Athens.

These caves secretly held national treasures during World War II.

Legend says this sacred tree was a gift from goddess Athena.

Heralded as the world's first theater, these ruins are often overlooked as they reside in the shadows of the Acropolis.

The only surviving choragic monument is a trophy awarded in 334 BC for the winning dramatic chorus.

This small 16th-century church in Athens is almost entirely surrounded by a modern building.

This small museum once served as a royal hunting pavilion for King Otto Friedrich Ludwig.

Where commuters walk through remnants of an ancient Greek necropolis.

The heads and relics of 179 monks slaughtered by pirates are on display within this Ceonobic Monastery.

What once sat in the world’s spotlight is today a largely disused ghost complex.

Go inside to really appreciate this 500-year-old technological masterpiece.

The Pope’s secret escape route.

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

Single runway interrupted by a busy road that cuts across its middle.

It's always summer at the beach inside this former airship hangar.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

A memorial in the Tiergarten is a reminder of the suffering inflicted on homosexuals by the Nazi regime.

Tiny memorials honor individual victims of the Holocaust.

A monument to the three wars that led to German unification towers within a Berlin traffic circle.