On the surface, London is a buzzing, modern metropolis – but underneath lays a secret, hidden world, all but forgotten by the millions of people above. This program uncovers 2,000 years of subterranean history: a world of ancient caves and perfectly preserved Roman remains; mysterious rivers and gruesome plague pits; impenetrable vaults and top-secret bunkers. Digging deep, the program unearths some of the most extraordinary stories of the darkest part of the city.