Since the beginning of the Blitz in September 1940, more than 200 of men had lost their lives or had been maimed in dealing with the 10,000 bombs that had fallen on Britain but had failed to explode. Each was a source of menace; all might go off without warning at any time. Meanwhile, their presence blocked vital roads and railways and kept key workers away from factories and laboratories. Families became refugees while bombs slept in their homes. Schools, hospitals and telephone exchanges lay empty and closed. With a German invasion imminent, UXBs were imposing more strain on the nation's resources than those bombs that did explode, and the Prime Minister, Winston Churchill, made countering their threat a top priority. So began an era of remarkable heroism, in which urgency, a lack of knowledge and a spirit of self-sacrifice led to the taking of extreme risks, to extraordinary escapes, and to many, many deaths.