MI5 spent years trying to decipher crop circles amid fears they were being used to guide Nazi bombers, secret documents from the Second World War have revealed. The files, held in the National Archives, show how British spies suspected Nazi sympathisers were sending secret messages to the enemy using ground markings. Among the suspected codes was a mysterious marking in a cornfield that was shaped like the letter 'G', with its tail appearing to point to a munitions factory in Glascoed, Monmouthshire. Other suspicious marks which showed up when British spies investigated aerial photographs were strange marks in a field near Newquay, Cornwall, and a white circle with the word 'Marden' inscribed inside it in a field near Staplehurst, Kent, while agents were also tasked with probing reports of lights in the sky. The British security service had started looking into the marks after learning that in Poland, Holland, France and Belgium, cornfields had been cut into 'guiding marks for aircraft', with spies beating out marks to assist Luftwaffe pilots find their targets. More via Daily Mail Online.