The abandoned ruins of a beautiful medieval castle just outside Barcelona.

A Franco-era dam flooded (almost) everyone out of this centuries-old Aragonese town.

Stunning Roman aqueduct used for 1900 years and still standing.

Abandoned Nazi train station turned underground astroparticle laboratory.

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

Considered the birthplace of the Copper Age and Bronze Age, the river is tinted red from 5,000 years of mining.

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

Village situated on a narrow volcanic escarpment.

Mysterious Spanish catacombs of unknown origins.

One of Europe's largest supercomputers is housed inside of a disused chapel.

One of the most photographed buildings in a city known for its Modernista architecture is an abandoned squat.

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

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

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

An architect transformed this abandoned cement factory into his castle-like workshop.

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

A Spanish forest is filled with mysterious stone carvings.

A landscape decorated with enormous skull sculptures.

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

Turntable anonymously received orphans, alms, and groceries.

A stately palace has been converted into the world's largest cannabis museum.

Commonly acknowledged as the only public monument to the Devil himself.

This former Baroque palace on the historic Calle Montcada is a rare architectural gem and was once the meeting place of an aristocratic scholarly society.

A gravity-defying "devil's bridge" that has a surprising architectural feature for its stability.