actions in the Black Sea against the Russian Black Sea Fleet and allowed the Germans to station two U-boats at Varna, one of which came under Bulgarian control in 1916 as Podvodnik No. 18. Russian mines sank one Bulgarian torpedo boat and damaged one more during the war. Baltic Sea In the Baltic Sea, Germany and Russia were the main combatants, with a number of British submarines sailing through the Kattegat to assist the Russians. With the German fleet larger and more modern (many High Seas Fleet ships could easily be deployed to the Baltic when the North Sea was quiet), the Russians played a mainly defensive role, at most attacking convoys between Germany and Sweden. A major coup for the Allied forces occurred on August 26, 1914 when as part of a reconnaissance squadron, the light cruiser ran aground in heavy fog in the Gulf of Finland. The other German ships tried to refloat her, but decided to scuttle her instead when they became aware of an approaching Russian intercept force. Russian Navy divers scoured the wreck and successfully recovered the German naval codebook which was later passed on to their British Allies and contributed immeasurably to Allied success in the North Sea. With heavy defensive and offensive mining on both sides, fleets played a limited role in the Eastern Front. The Germans mounted major naval attacks on the Gulf of Riga, unsuccessfully in August 1915 and successfully in October 1917, when they occupied the islands in the Gulf and damaged Russian ships departing from the city of Riga, recently captured by Germany. This second operation culminated in the one major Baltic action, the battle of Moon Sound at which the Russian battleship was sunk. By March 1918, the Russian Revolution and the Treaty of Brest-Litovsk made the Baltic a German lake, and German fleets transferred troops to support the White side in the Finnish Civil War and to occupy much of Russia, halting only when defeated in the west. Other oceans A number of German ships stationed overseas at the start of the war engaged in raiding operations in poorly defended seas, such as , which raided into the Indian Ocean, sinking or capturing thirty Allied merchant ships and warships, bombarding Madras and Penang, and destroying a radio relay on the Cocos Islands before being sunk there by . Better known was the German East Asia Squadron, commanded