Break in the fog helped Allies to capture German naval codes

Early on in the First World War, a group of German warships was carrying out reconnaissance of Russian patrols in the Gulf of Finland when on August 26, 1914, the ships became shrouded in dense fog and were separated from one another. One of the ships, the light cruiser Magdeburg, ran aground on the island of Odensholm off the coast of Estonia, then under Russian rule. Attempts to refloat the ship failed and the crew managed to be evacuated on to a destroyer.
The plan afterwards was to blow up the Magdeburg to avoid its capture but as explosives were being laid for the demolition, the fog began to clear and two Russian cruisers appeared and opened fire. Some of the Magdeburg’s crew were injured, 57 of them were captured, but most damaging was that the Russians discovered copies of the German navy’s codebooks on board, plus charts of the Baltic and North Sea, the ship’s log and war diaries. A copy of the codebooks was passed to the British, and although the German messages were found to have been reciphered, by November the cipher was broken and the naval signals could be read.
There were other early intelligence coups for the Allies. On August 11, a German cargo ship, Hobart, was off the coast of Melbourne when the captain failed to receive a message that Germany was at war. The ship was boarded by the Australian navy and they seized a copy of codes for communications between warships and merchant ships, also used for naval shore bases and later for U-boats and Zeppelins.
The British then obtained a third German naval codebook when a trawler in the North Sea caught a lead-lined chest in its nets on November 30 after it was thrown overboard from a sinking German destroyer. The codebook was used for communications with naval attachés and warships and by admirals at sea.
Most astonishing of all, throughout the war the Germans had no idea that their naval codes had been captured and their messages were being read, giving priceless details of the movements of their ships.