 On 26 August 1914 , the German light cruiser Magdeburg had run aground in the Gulf of Finland ; the wreck was captured by the Russian navy , which found code books used by the German navy , along with navigational charts for the North Sea . These documents were then passed on to the Royal Navy . Room 40 began decrypting German signals , and on 14 December , intercepted messages relating to the plan to bombard Scarborough . The exact details of the plan were unknown , and it was assumed that the High Seas Fleet would remain safely in port , as in the previous bombardment . Vice Admiral Beatty 's four battlecruisers , supported by the 3rd Cruiser Squadron and the 1st Light Cruiser Squadron , along with the 2nd Battle Squadron 's six dreadnoughts , were to ambush Hipper 's battlecruisers . 
