On rare days, this abandoned castle's gatekeepers open its doors, revealing the eye-popping splendor hidden inside.

The glowing blue Nymphaeum of Emperor Tiberius.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

A neutrino detector built into the largest man-made underground cavity in the world.

A sulfuric hellscape in southern Italy.

A submerged archeological park still holds the wonders of a Roman Sodom.

This half-finished "city of toys" was almost Italy's own Las Vegas.

An Italian hillside ghost town.

A petite white lighthouse perched on the ancient core of Stromboli Volcano.

Geometry and mystery come together in an epic show of octagonal masonry.

The remains of a town so thoroughly destroyed by French forces that the citizens simply left.

Sitting on an Italian hilltop is this surreal fortress built by the mad inventor of electrohomeopathy.

This one-time health resort was used by aristocrats around the world.

Eccentric concrete mansion and museum designed and built by hand from the inside out.

This crumbling, graffiti-covered mansion sits tantalizingly within reach of the proletariat who frequent a public park.

The abandoned husk of a mental institution that failed to save a Jazz Age icon still sits nearly unchanged.