Although the Ruhr had always been a target for the RAF from the start of the war, the organized defences and the large amount of industrial pollutants produced that gave a semi-permanent smog or industrial haze hampered accurate bombing.[1][10][11] The germans also built large-scale night-time decoys like the Krupp decoy site (German: Kruppsche Nachtscheinanlage) which was a German decoy-site of the Krupp steel works in Essen. During World War II, it was designed to divert Allied airstrikes from the actual production site of the arms factory.