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 cemetery features modernist statues as strange as its name.

A beautiful but toxic fountain of mercury.

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

Infamous and now demolished panopticon prison built by its former inhabitants.

Not far from Madrid, a set of perfectly preserved trenches date back to the Spanish Civil War.

These three pillboxes still remain as a witness of the Spanish Civil War in a park of Madrid.

The remains of the second-oldest zoo in Europe can be seen within these library gardens.

Among the oldest structures in Madrid, these Moorish walls once guarded the medieval city of Mayrit.

This funky former tobacco factory is one of the most striking examples of Madrid's cultural diversity.

This amazing abuela became a diehard metal fan late in life and an icon of Madrid's '80s rock scene.

Over two centuries old and counting, Madrid's massive botanical garden contains one of Europe's only desert environments.

A lush indoor jungle fills this bustling Madrid transportation hub.

Abandoned for decades, trains still pass through the now-restored remains of one of Madrid's first metro stations.

A preserved building where the dead were once watched for signs of life.

Hidden within a Netherlands crater is a stone bed that lets those who lie in it see the sky as a curved surface.