Magdeburg was three weeks past her second birthday as a commissioned warship. Built at the AG Weser yard in Bremen and launched in May 1911, she was the lead ship of a class designed to answer faster British cruisers: lengthened hull, more powerful turbines, longitudinal framing to save weight, and — a first for German cruisers — belt armour. She made 27.6 knots on trials, carried twelve 10.5 cm guns, and was crewed by 18 officers and 336 enlisted men. For most of her short life she had been a torpedo test ship — about as far from front-line service as a modern cruiser could get.

War changed that in days. On 2 August she shelled Libau, firing the first German shots of the war against Russia at a naval base the Russians had already abandoned. Through August she worked north along the coast, shelling lighthouses and signal stations. On the night of 25 August she was sent to sweep the entrance to the Gulf of Finland and to destroy the signal station on Odensholm.

She never got the chance to do it deliberately. Aground with her double bottom torn open, she could not be shifted. The crew heaved equipment over the side to lighten her; she stayed fast. Since the lighthouse was a target anyway and she was stuck within easy range of it, she shelled it from where she lay — but the signal station survived, and the Russian watchman there had already got word out to the Baltic Fleet.

The torpedo boat V26 reached her around 08:30, tried to tow her off, failed, and began taking crew aboard. Then, shortly after nine, the Russian cruisers Bogatyr and Pallada came out of the clearing weather and opened fire on a stationary ship. The Germans blew up her forward section but could not finish the job. Fifteen of Magdeburg's crew were killed in those minutes — men who had been standing on a deck that could neither move nor fight back. Her captain, Richard Habenicht, stayed aboard with his adjutant and was taken prisoner; he remained in Russian captivity until 1918, when the chaos of the revolution let him get home.

Magdeburg had four copies aboard of the Signalbuch der Kaiserlichen Marine, the Imperial Navy's signal book. One was burned. Two went over the side into shallow water. The fourth stayed in the captain's safe. Russian boarding parties recovered three of the four — the two from the sea and the one from the safe — along with the current encryption key.

Russia kept two copies and offered the third to the Royal Navy on condition the British come and collect it; two Russian couriers handed it over, and the Admiralty took delivery on 13 October 1914. It landed in a newly created department that had almost nothing to work with: Room 40, the Admiralty's cryptographic section, which had been intercepting German wireless traffic without being able to read it. The signal book and key were its first real break.

With the German naval cipher opened, Room 40 could track the movements of most of the Kaiser's warships and pass what it learned to Admiral John Jellicoe. The British ambushed German formations repeatedly on the strength of it — most consequentially at Dogger Bank in January 1915 and at Jutland in May 1916, where the Grand Fleet was at sea and waiting because it already knew the High Seas Fleet had sailed.

None of the men on Magdeburg's deck that morning had any way of understanding what they had lost. A grounding in fog is a mundane accident, the kind that ends careers rather than wars. This one quietly shifted the balance of the entire naval conflict, and it did so because a codebook thrown over the side in shallow water does not sink very far.

The Russian navy cut Magdeburg up where she lay and eventually destroyed what was left. Wreckage still lies in the shallows off the island's lighthouse cliff, close enough to show at low water, and divers work the site today.

Osmussaar has since had a harder history than the cruiser did. The 1934 Estonian census counted 131 people living here, almost all of them Estonian Swedes whose families had farmed and fished the island for centuries. In June 1940 the Soviet military took the island for a base and moved them off. Some returned briefly under German occupation; the last 46 left for Sweden in February 1944. Soviet troops held the island until 1993. Two wrecks lie in these shallows now — Magdeburg, and the German battleship Schleswig-Holstein, grounded here as a gunnery target in 1948 — and the people who might have remembered either of them were gone before the second one arrived.