Luftwaffe navigators flying over 1940s England had few tools to ensure their bombs were striking the right bases and cities. They used maps, compasses, airspeed indicators, and aerial photographs to try and find their assigned targets.
Apparently, the British capitalized on this by building fake cities and airstrips to confuse the bomber crews. The effort was commanded by former British Royal Engineer Col. John Turner who employed set designers from movie studios to create the decoys.
By 1940 the Royal Air Force had already dispersed some of their planes to satellite stations, sparse outposts that hosted a dozen fighters or less with small ammo and fuel dumps. Turner and his men started by creating fake version of these satellite stations. The fake versions were positioned so attacking bombers would reach the decoy station before the real station and hopefully become confused.
Turner's men would build a fake runway and park about 10 fake airplanes at it. A group of men were assigned to move the aircraft around every day and repair any damage done by enemy bombs. To really sell the ruse to any German spies who might be watching, RAF planes or other aircraft were sometimes sent to land at the fake stations.
After success with the satellite stations, orders for simulated aircraft manufacturing plants provided a greater challenge for the team. Full-size decoys were constructed, complete with cars in the lot. During bombing runs the men would set fires in sections of the fake factory to simulate damage, but they were crafted to be easily put out once the bombers left.
In 1940 and 1941, the British government decided to protect full cities using the decoys. The team knew they couldn't construct an entire city, but they also knew the Germans were mostly limited to night missions. So, the team came up with a series of scaffolds and lights that looked at night like a city with poor light discipline. It gave the appearance of open doors and unshaded skylights, glowing furnaces, and train depots.
Like the decoy factories, the "cities" were rigged for simulated fires and explosions. The first wave of a German bombing raid was allowed to pass without any fireworks, but diesel fuel and paraffin wax would be dumped onto burning coals ahead of the second wave. The goal was to convince the second wave that the first had found the target and that this burning "town" was it. The second and follow-on waves would then focus on the decoy.
The exact level of success has been debated, but official estimates are that 730 bombing raids were diverted by the fake targets. Colin Dobinson authored "Fields of Deception," a book about the decoy operations. He estimated about 3,000 lives were saved by the efforts, according to an article in Engineering and Technology Magazine.