During World War II the German countryside was peppered with unexploded bombs, many of which were never recovered. These unexploded bombs are becoming more dangerous, especially to the construction industry.
At the time, British and American bombers used aerial photography during their bomb strikes, while further aerial photographs assessed the damage done. Nowadays these same photos are being used to help discover bombs which still remain under many city streets and fields. Images are used from wartime archives in Britain and the US, and private companies and the German authorities have to locate the unexploded bombs before building projects can safely begin. According to Spiegel, the German online newspaper, one of the top companies is a small firm called Luftbilddatenbank which translates to mean aerial photo databank, and there's a real need for their services.
It's estimated that during the war around half of all the bombs dropped failed to explode, and over the last few decades these bombs have killed dozens of people, and have caused widespread damage to the economy as services need to be shut down whilst the bombs are located and made safe. Around 2,000 bombs have been located in Berlin since the end of the war, but it's estimated as many as 15,000 bombs could still be undiscovered. Nowadays archive photos have been digitized and used to create a database showing the site before and after a bombing raid. Experienced technicians use digital software to identify craters where a bomb has exploded, and also dark spots which could indicate an unexploded bomb lurking just below the surface.
German law holds property owners partly responsible for any accidents, and firms are encouraged to do as much research as possible before beginning construction work. The need for the services of companies such as Luftbilddatenbank is likely to grow in the future, as eyewitnesses are becoming few and far between, making photographic records even more critical. There's also the problem that the bombs become more unstable overtime, and are more likely to explode. Part of the reason for this is that the timers are made out of plastic which is gradually decaying, and the bombs can be set off by the very slightest vibration.
Luckily there are millions of aerial photographs from this time, and it's amazing how much detail they reveal.