Catacombs of Saint Calixt in Rome

You're standing on the Via Appia, the ancient Roman road running southeast out of the historic centre of Rome. Right beneath your feet lies a whole underground world: the Catacomb of Callixtus. It's a maze of passageways around twenty kilometres long, spread over five levels, under an area of about fifteen hectares. A rough estimate says around half a million people were once buried down there.