Overview: Engineers digging for Crossrail have made a chilling discovery: dozens of skulls under London's Liverpool Street station. It's clear the skulls are from Roman London, but nothing like this has been found before. This documentary unravels the mystery, using ground-breaking archaeological investigation. Who are these people and how did their skulls get there? There are many theories: they could be victims of a massacre, or gladiators; they could have naturally washed out of cemeteries in times of peace or be executed rebels; they could even be the victims of some grisly, long-lost ritual. Now the tools are available to solve the mystery. The investigation covers Boudicca's rebellion, the bloody gladiatorial arena, and head-hunting sites in France. Does the investigation answer the mystery of the Crossrail skulls?