The eclectic observatory castle of a 19th-century astronomer and explorer.

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

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

A living art gallery painted on a canvas of Monterey pine trees.

An ultra-modern diversity park in Denmark has turned an entire city block electric pink.

The bombed out remains of the old church were kept intact next to the new one, as a reminder of the futility of war.

Madrid's romantically ornate crystal palace has been offering shelter to plants and art for over a century.

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

What started as a tongue-in-cheek shrine is now a full-blown museum celebrating "The Hoff" and his longtime love affair with Berlin.

Banksy's image of a naked man dangling from the window of a sex health clinic.

The Bristol Camera Obscura housed in an ancient windmill.

The world's largest reflecting pool is just two centimeters deep.

The Cathedral of Salamanca has a number of unusual carvings but none so surprising as a modern astronaut.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

The anatomical-pathological collection of a man who changed surgery.

A historic arboretum and non-conformist, non-denominational park cemetery.

A vast overgrown cemetery, once known as one of London's "Magnificent Seven."

A hands-off space for graffiti artists to hone and display their skills.

An abandoned Indian-inspired mansion.

A landscape decorated with enormous skull sculptures.

The remains of Crystal Palace Great Exhibition of 1851.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.