A church dedicated to the Mexican skeleton saint.

66,000 pieces of art, donated by one of the world's richest men.

Part bookstore, part café, and part forest.

Frida Kahlo's childhood home, now a museum of her life and works.

Superstitious? Discover the witchy side of Mexico through its largest esoteric market.

An island filled with hundreds of hanging, decomposing, decapitated dolls.

This massive subterranean fissure is known as the "underground Grand Canyon."

A miniature train-ride through Slovenia's largest cave system.

A whimsical bridge adorned with decorative dragons.

This Croatian fairytale castle is slowly falling apart thanks to years of neglect.

The underground sanctuary of a mysterious ancient cult.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

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

Jesus' footprints and the first catacombs in the world.

Ruins of a 10th century church tucked away deep in a botanical garden.

This medieval Italian town is a sheer (and beautiful) cliffside defense.

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

The Emperor Tiberius' palace of debauchery.

The glowing blue Nymphaeum of Emperor Tiberius.

The last moments of the ill-fated Pompeiians, frozen forever in plaster.

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

Pompeii's less famous neighbor is still revealing her secrets.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

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