This 47-story "M" needed 20 years to be completed

A five-square-mile slice of Spain on the Mediterranean coast of North Africa.

This Arab fortress was once the largest citadel in Western Europe.

"Satisfying" is a good way to describe these beautifully aligned rows of Spanish mills.

A small church ornamented with medieval erotic sculptures.

This mountaintop observatory is home to one of the world's largest reflective telescopes.

This chest hospital with a dark, haunted past serves as a fitting set for horror films and a paradise for ghost hunters.

The streets of a sleepy Spanish town proudly exhibit some of the world's finest graffiti.

The largest medieval water tank on the Iberian Peninsula is preserved beneath the Cáceres Museum.

An arresting arch punched straight through a mountain provides one of the most scenic spots, and challenging climbs, on the planet.

A huge cathedral single-handedly constructed by one eccentric monk.

This huge abandoned fortress hidden inside a mountain holds a world of subterranean secrets.

This 7,000-year-old megalithic wonder covered by a Franco-era reservoir emerges in times of drought.

A modernist church in an ancient village.

A whimsical garden of outsider art in the Spanish countryside.

Hundreds of caves once used to store grains and animal feed.

Beautiful tiles decorate many of the town's surfaces and stand testimony to its glorious artistic history.

This influential Renaissance-era library was once the haunt of kings and monks.

Down this yellow brick road, visitors are treated to toadstool houses and massive mushroom sculptures.

The atmospheric ruins of a historic church can be found on a hill above the city of Oviedo.

Geological storytelling.

Now a pile of ruins and rubble, this was a lovely mountain town as recently as 1960.

An abandoned Indian-inspired mansion.

A magnificent, long-abandoned castle built in the wake of the reconquest of Spain.