Starting in January 1940 (prior to the first bombing raids of WW2 against Britain), significant resources were allocated to the construction of bombing decoy sites. The sole intention of the sites was to divert German bombers from their intended targets and deceive them, so that they would drop their bombs onto unoccupied areas of countryside. Large swathes of land were laid out to resemble genuine targets, to draw away the hostile aircraft.
These decoy sites were used to divert planes from:
602 locations in the UK were used to provide 839 static decoys, but not all were bombed. There were also mobile decoys, bringing the total number to about 1000. The sites varied from 5 to 30 acres in size, although the German bombing was not confined to these specific areas.
It was estimated by the Air Ministry, that over 2200 tons of bombs were diverted from their intended targets using the decoys. It would be reasonable to estimate that this would have resulted in about 3000 unexploded bombs.
These sites may also be contaminated with asbestos and hydrocarbons, as both were used in large quantities to achieve the effect of explosions and fires.