A large grotto built in the 16th century amidst Florence's famous Boboli gardens.

Thousands of well-dressed mummies below a monastery.

Stalactites and stalagmites in a beautifully lit cave.

Renaissance villa and gardens complete with a tormented colossus.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

Architectureal fantasy quarter of Rome.

A 16th-century horror show built in a lovely Italian garden.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

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

Infamous and now demolished panopticon prison built by its former inhabitants.

This abandoned mansion has a window for every day of the year, and is a temple for ghost hunters.

Bold black and white motifs of life and death add a dramatic flair to this 17th-century crypt.

These ruins are all that remain of Tortosa’s lost coal gas manufacturing industry.

The walls of these four caverns are filled with art that dates back to the earliest human presence in Europe.

To fit bodies into this Renaissance crypt, they were first left to decompose in the crypt's rotting room.

The last of three massive blast furnaces that were once at the foundation of the Port of Sagunto economy.

Originally slated to become a luxurious resort, this structure is now a graffiti haunt reclaimed by nature.

Tools, pottery, and mummies showcase the history of La Palma's native people.

This art installation at an abandoned lipstick factory has distinct anti-capitalist vibes.

The mysterious remains of a martyred teenage saint are venerated inside a small country chapel.

The quirky - and private - constructions of a secluded artist depict the house of God.