A Nesting Doll of Churches.

Jesus' footprints and the first catacombs in the world.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

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

The yawning maw of this pagan visage is said to bite off the hands of liars.

The final resting place of the poets Shelley and Keats.

The underground sanctuary of a mysterious ancient cult.

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

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