A disquieting Aztec sculpture displays hundreds of stone skulls representing the victims of human sacrifice.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

Once described as the "Eagle's Nest," this ruined ancient city rests on a remote mountaintop.

This remarkably well-preserved Egyptian plinth is pretty well traveled for a giant piece of stone.

A gorgeous, Gaudí-esque stairway constructed in the city of Istanbul by a prominent Jewish family.

A deadly ancient portal to the underworld.

This famous city eluded concrete detection until the early 2000's.

A small etching on the white marble parapet was written in runic script by a Viking mercenary.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

The mosaic floor of a 4th-century synagogue features a pagan zodiac wheel alongside traditional images of a Torah ark and menorahs.

This daunting 1st century fortress was purportedly the site of one of the most dramatic revolts of the Jewish-Roman wars.

These mysterious ancient stone spheres were created by a civilization lost to time and are now mostly lawn ornaments.

Send a postcard while vacationing with Satan.

A glowing underwater cliff attracts marine life and divers in equal measure.

Nature took her revenge on the "Wickedest City in the World."

Collection of fossils, plants, and ancient animals encased in amber.

Renowned restaurant tucked in a cave once occupied by buccaneers.

This trio of Dominican cave lakes comprise a colorful biosphere of natural life.

Hundreds of pieces of ancient Taíno art cover the walls of this Caribbean cave system.

This 15-foot red anchor sits on a rise overlooking the sea lanes of eastern Aruba.

Historic fortifications form the boundary of Vatican City, but it’s not entirely surrounded by walls.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Breathtaking and huge 430 year old maps of Italy.