This mysterious and once-grand memorial still embodies a remarkable architectural vision.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

Bohemian paradise covered in majestic forests.

The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

The world's largest reflecting pool is just two centimeters deep.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

Archaeological site of Iberian people ruins.

Devastated landscape used for hundreds of years by the Romans as their primary mining site.

A museum dedicated to the Spanish occult in a town that was terrorized by the Inquisition.

Remote, beautiful, and stigmatized, Las Hurdes is Spain's version of Appalachia.

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

A site where “the ravages of the war are more striking…than any other place in contemporary Spain.”

A somewhat mysterious biomedical museum housed inside a Spanish university.

Geological storytelling.

Mysterious Spanish catacombs of unknown origins.

The site of a tragic massacre that spawned a multimedia franchise.

This modern French ossuary contains the jumbled bones of 130,000 WWI soldiers.

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

Artfully arranged bones of 5,800 Austrians.

An Austrian house of bones filled with hundreds of intricately painted skulls.

This Italian pyramid is said to hold the mingled remains of the dead from both sides of the Battle of Novara.

A house of bones — but nobody is quite sure to whom they belong.

The personal charnel house of Anton Bruckner.

The remains of 40,000 humans, meticulously stacked in a tiny churchyard shed.