Beneath the streets of Rome lies an ancient city of the dead known as the Catacombs -- a labyrinth of tunnels, hundreds of miles long, lined with tombs. Now, NOVA goes inside a previously-unknown complex within the tunnel system: a mysterious mass grave, locked away for nearly 2000 years. NOVA's forensic investigation opens up fascinating new insights into the daily life and health of Roman citizens at the heyday of its mighty empire.