The sinister ruins of one of the worst dam failures of the 20th century lie in this astonishing alpine setting.

A once-luxurious, now abandoned hotel in the Italian Alps.

A dismembered holy head stares out from her beautiful reliquary at the St. Dominic Basilica.

Crossing this suspension bridge feels like walking through the sky, a fitting tribute to the head of the Apollo program.

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

Italian cave with exceptional acoustics and an unsavory history.

The giant skeleton sculpture is surprisingly well-traveled.

Erotic art covers the walls of the largest ancient pleasure palace.

Venice's island of the dead.

Secret refuge and museum of the Duke of Florence.

This museum is rotten with a century of wax medical deformities.

Medieval torture techniques with a modern conscience.

One of the largest caves in the world open to the public.

An ancient artistic garden parlor full of hidden water wonders.

The astonishing collection of an infamous criminologist.

More sculpture than building, this architecturally significant temple was built on the spot of Saint Peter's crucifixion.

Walk through a fairytale in Tuscany.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

This picturesque Umbrian city has a subterranean side that has long remained hidden.

This bronze boar's snout has been rubbed to a golden sheen by visitors seeking good fortune.

This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

This real life asylum of horrors is now an abandoned ruin that hides the unbreakable code to one patient's insanity.

One of seven Jewish catacombs in Rome and one of only two open to the public.