The Allies dropped 44,923.2 tons of bombs on the city during World War II, destroying 61% of its built up area. During the Bombing of Cologne in World War II, Cologne endured 262 air raids by the Western Allies, which caused approximately 20,000 civilian casualties and almost completely wiped out the central part of the city. During the night of 31 May 1942, Cologne was the target of "Operation Millennium", the first 1,000 bomber raid by the Royal Air Force in World War II. 1,046 heavy bombers attacked their target with 1,455 tons of explosives, approximately two-thirds of which were incendiary. This raid lasted about 75 minutes, destroyed of built-up area (61%), killed 486 civilians and made 59,000 people homeless. The devastation was recorded by Hermann Claasen from 1942 until the end of the war, and presented in his exhibition and book of 1947 "Singing in the furnace. Cologne – Remains of an old city"