An underground world of gorgeous, palatial metro stations designed as a nuclear shelter complex.

This former mill tunnel was blasted under the ridge to divert a portion of the Muscatatuck River.

The same water that fills a luxury resort’s pools feeds these free hot springs in the rainforest.

The translucent turquoise hue of these natural swimming pools comes from a high concentration of minerals.

A neglected spomenik overlooking the beautiful Kopaonik Mountains.

Stuck in a bygone era, the oldest subterranean shopping street in Japan hides beneath the popular Asakusa district of Tokyo.

The private train station King Leopold II built for the Belgian royal family now lies abandoned.

Built in 1547, these ruins lie on a rocky promontory in Ballintoy jutting into the wild ocean waters below.

The ruins of this once-grand mansion are linked to one of the country’s most popular clothing brands.

When the tide is high, you can see the Atlantic pushing up through a deep, jagged hole in the rock.

The golden age of flight captured in a downtown airport.

This World War II-era bunker hides between medieval chapels in Nijmegen's city center.

This remarkable sculpture symbolizes the regeneration in this Glasgow suburb.

When the city of Manchester built a new centralized heating network, they did it with style.

This 10th-century seaside abbey holds spectacular views and a window into the past.

The castle that provided the setting for much of 'Monty Python and the Holy Grail' is now a pilgrimage site for fans.

Locals believe this abandoned ruin on the outskirts of Bucharest is haunted.

An eerie cluster of medieval cave-like tombs carved out of a cliff, hidden in a historic mountain pass.

This small, but beautiful waterfall has attracted visits from artists, composers, and two United States presidents.

This eerie sculpture overlooks what was known as the Road of Bones under Joseph Stalin.

A well-preserved example from a 1920s network of airmail navigation arrows sits near Interstate 15 in southern Nevada.

This observation tower is one of two of its kind that still exist in the United Kingdom.

The ruins of a 15th-century castle that has seen its fair share of bitter feuds.

The ruins of a monastery that dates to the 7th century.