In March 1942, 135 RAF aircraft bombed Cologne, killing 62 people.
At the end of May, Cologne was the target for the first 1,000 bomber raid on Germany, when over 1,000 planes dropped nearly 1,500 tonnes of bombs on the city. 12,000 buildings were destroyed and over 500 people killed.
Read more about the Cologne 1,000 bomber raid
The German government and high-ranking officers favoured above-ground air raid shelters because of their strength and stability. For the masses however, shelter in basements and cellars would have to do.
The Nazi Party investigated the availability of cellars in each street, as well as their size, depth and access.