The following history is taken from the Portsdown Tunnels webiste which also has some excellent photos of the tunnels whilst in use:
http://www.portsdown-tunnels.org.uk/palmerston_forts/fort_southwick/1_overview_p1.html
Along the top of Portsdown are five Victorian Forts known as the Palmerston Forts. Fort Southwick (pronounced Suth-ik) is one of these. Up until 2002, it formed part of an Admiralty Research Establishment, and was used by the DCSA (Defence Communications Services Agency) as a communications fort for the Royal Navy. The Ministry of Defence National Asset Register of 1998 listed the building as HMS Fort Southwick. All operational use of the Fort ceased in 2002, and during July 2003 it was sold by the MoD to the "Fort Southwick Company Limited".
During WWII an Underground Headquarters - UGHQ (for operation Overlord) was built beneath Fort Southwick. This provided a bombproof, comprehensive Naval, Army and Air Force Operation Control and Communication Centre - call sign 'MIN'. The headquarters’ main role on D-Day was gathering information and coordinating the Allied naval forces. Reports from radar stations were crossed-referenced with messages from shipping to provide an accurate picture of what was happening in the English Channel. This information was then plotted on a large table map at the Fort, and was passed to the Allied commanders nearby at Southwick House.
On the 6 June 1944 there were 700 staff working underground. As well as the UGHQ, over 1000 people were based above ground around the fort, at various headquarters associated with the planning for D-Day. These included the army and navy headquarters for the British/Canadian D-Day beaches, Second British Army and the Naval Commander Eastern Task Force respectively. The underground workings at Fort Southwick were excavated by 172 Tunnelling Company Royal Engineers. Work was started on 16 February 1942, probably as a direct result of the Washington Conference mentioned above, and the complex completed during December 1942. The operation tunnels were about 100 feet beneath the surface. Additional galley facilities were provided in the Central Caponier of Fort Southwick proper. Air conditioning and gas filtration was installed as well as fresh water storage. Access was via 3 notorious staircases from within Fort Southwick - there was no lift. Because of its D-Day connection, its existence was a very closely guarded secret, which still seems to persist even today. Southwick House located in the village of Southwick a mile north from the Fort, was used as the headquarters for SHAEF (Supreme Headquarters Allied Expeditionary Force) although the actual HQ, on and for some time after D-Day, was a tent in nearby woods.
In the late 1990s the underground complex was strengthened and stripped bare. Hampshire County Council expressed interest in purchasing this property from the MOD when it became available, because of its uniqueness and importance during WWII. However because of budget cuts they did not bid for the property when it came up for sale in 2003. Around ten bids for the Fort were received and it was eventually sold to the "Fort Southwick Company Limited".
