Room 40
The British Admiralty's codebreaking section in the First World War, named after the room it worked in.
It was set up in late 1914 under Alfred Ewing and grew under the director of naval intelligence, William "Blinker" Hall. It got a flying start from captured codebooks: the Russians handed over the German navy's signal book recovered from the cruiser Magdeburg, which had run aground in the Baltic, and two more German codebooks reached British hands that same autumn. With them Room 40 could track the German fleet, including in the run-up to Jutland in 1916.
Its best-known coup was the Zimmermann Telegram. Dilly Knox was among its staff, and in 1919 it merged with the army's equivalent to form the Government Code and Cypher School — the organisation that later moved to Bletchley Park.