British codebreakers' vital contribution to winning the Battle of the Atlantic during World War II is being remembered at their former wartime headquarters.
Bletchley Park, near Milton Keynes, was also known as Station X
SHARK in the Atlantic is a special event launched at Bletchley Park on Saturday.
The two-day event marks the role of intelligence work in protecting Allied convoys from prowling German U-boats, which was a strategic turning point in the war.
Codebreakers at Bletchley Park in Buckinghamshire decrypted ciphers to crack the Germans' secret codes - and in doing so helped reduce the number of ships carrying essential food and supplies being sunk by German submarines.
SHARK was the name given to a new cipher created in February 1942, when the U-boats introduced a fourth rotor in the Enigma machine, an apparatus they used to communicate in code.
The Royal Navy recovered Enigma, which was used for Nazi codes
The SHARK cipher was very damaging to the Allies as it coincided with a time when the number of German submarines in the Atlantic had risen to 40 and the
Germans were also intercepting and decoding messages about British convoys.
During August and September 1942 alone the Germans found a third of all convoys and had sunk 43 ships.
SHARK was not broken until December 1942, when Bletchley Park received codebooks recovered from the U-559, which had been scuttled by destroyer HMS Petard.
Included in the weekend event are displays on The Battle of the Atlantic and other naval successes, a collection of naval artefacts, lectures, a model boat display, and re-enactors in authentic 1940s uniforms.