This episode follows engineers building London's brand new underground railway, Crossrail, as they race to build ten new train stations across London.A team battles to put together the giant jigsaw that will become a cathedral-sized station at Canary Wharf. It's so big, they have to build it in a dock and underwater. Its architect-designed roof is so complicated it comes in a kit of over 2,500 pieces. As engineers carve out the underground caverns that will become the new stations, they hit on some gruesome discoveries of London's past.