Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

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

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

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

This 16th-century monastery has been turned into a surrealist architecture compound.

As can be expected from a centuries old church built in a canal city, the undercroft of this house of worship is beautifully flooded.

The underground sanctuary of a mysterious ancient cult.

Renaissance villa and gardens complete with a tormented colossus.

Architectureal fantasy quarter of Rome.

Stalactites and stalagmites in a beautifully lit cave.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

An alchemist's "magic door" stands in the middle of a Roman park.

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

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

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

Ancient Greek oracle of the dead.

Hideout of thieves, monks, and goat-footed gods.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

This legendary city has been a tourist attraction for centuries.

The hallucinogenic center of the ancient world.

This ruined Japanese stripclub sits half burnt to the ground.

A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

The self-mummified monks of Japan.