The plan is undated, but probably originates from around 1938, when proposals for the re-use of the tunnels (called 'caves' on the item) were drawn up as the threat of war mounted. The plan shows tunnels occupied by the Admiralty and the GPO. The original was marked 'secret' and the copy is stamped 'secret'.
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Hanoverian Tunnel, Early 20th Century Military Headquarters, Hanoverian Barracks