A mysterious structure left behind on the Polish mountainside, sometimes called "Hitler's Stonehenge".

This man-made mound monument was built out of soil from all across the Polish empire.

Warsaw's fantastically preserved gasworks...almost.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

An underground city of salt.

Vibrant street art created with mixed media.

This 47-story "M" needed 20 years to be completed

This century-old ruined temple was reborn as "The Church of Skate."

Abandoned waterpark turned counter-culture mecca.

This quaint Spanish village agreed to be painted blue as a marketing stunt but found that they preferred it that way.

A completely underwater art museum sends a stark message about humans' impact on the environment.

"Alleys of Stone" form natural mazes in Spanish natural park.

The world's largest wooden structure.

A Romanesque tower appears when water levels drop.

Bacteria and algae give the salty water its bubblegum hue.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Cliffside houses provide a spectacular view from their perch above the valley.

At the foot of a waterfall lies a peaceful swimming lagoon and evidence of over 60,000 years of human habitation.

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

Wonderland replication in progress by one persistent man.

An enchanting monastery in the Spanish countryside appears lifted from a fantasy tale.

A beautiful but toxic fountain of mercury.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.