The world's largest wooden structure.

This awesome Spanish castle is so fantastical it was actually used on Game of Thrones.

This house's itty bitty size means no humans allowed.

An isolated village literally built into the crevices and rock formations of a mountain.

This Navarrese cave's history of witchcraft and pagan ritual is not just embraced, but recreated each year.

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

Bacteria and algae give the salty water its bubblegum hue.

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

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

The only urban monument in the world designed by the great surrealist painter Salvador Dalí.

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.