The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

These slanted cubic abodes are a creative if odd solution to a zoning pickle.

Peek through a transparent pyramid and drink in the mountain made of books.

This giant middle finger statue sends an unambiguous message to Italy's bankers.

What looks like a suicide in progress is actually a statue of Sigmund Freud.

A once grand hotel now sits eerily abandoned in the Catalan countryside outside Barcelona.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

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

This semi-secret storybook hedge maze is also the oldest garden in the city.

An abandoned lighthouse awaits its destiny: being buried under shifting sand.

Nearly 40 miles and 1,000 years of limestone mines under Denmark.

The old observatory in Copenhagen has a spiral corridor used as a racetrack.

A natural wonder in south Denmark.

A 19th-century subterranean reservoir repurposed as a contemporary art cave.