Between 7 February 1941 and 15 August 1945 the secret command centre for the Battle of the Atlantic was in the basement rooms of Derby House. Known as Western Approaches because its main task was to monitor enemy approaches in the Atlantic west of the British Isles, the labyrinthine nerve centre of Allied operations is pretty much as it was at war's end. Highlights include the all-important map room, where you can imagine playing a real-life, full-scale version of Risk.