Blue-green pools and smooth stones comprise one of Spain’s most idyllic hidden wonders.

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

This small Spanish village is home to dozens of public paintings meant to trick the eye.

The cathedral of a controversial, conservative, schismatic sect that declared the Catholic Church heretical and founded its own succession of popes.

The tallest tower in Cáceres's Old Town.

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

A neighborhood park in the city of Cáceres hides what some believe are revolutionary examples of Neanderthal cave art.

The medieval castle built as headquarters for the Pope's secretive Knights Templar.

One of Europe's largest groups of dolmens remains largely off the typical tourist track.

This star-shaped military outpost is now simply trying to protect itself from neglect.

The 5,000-year-old megalith is believed to be Scotland's tallest standing stone.

The Stonehenge of Wales is one of the grandest neolithic dolmens still in existence.

This crumbling Portuguese ruin overlooks the populace it once ruled.

"Portugal's Stonehenge" is made up of nut-like stones aligned with the heavens.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

An abandoned mine now resembles a post-apocalyptic landscape of derelict buildings and blood-red pools of water.

Behind the pink curtain, Au Bonheur du Jour, an art gallery of erotic archives.

This massive library holds what was once the largest book collection in the world.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

This Parisian gem is one of the most incredible ethnographic museums in the world.

Dead weights honor the dead of Portugal's fishing industry.

This small chapel is built of human bones and decorated with a golden skeleton.

Perhaps the most spectacular grotto along the Algarve coastline.

This unassuming chapel in Portugal plays host to a ghoulish collection of bones.