Prior to the start of World War II, Britain began to anticipate the impending conflict.. So, in 1938, construction began on the Cabinet War Rooms beneath the Treasury building in Whitehall. By August of 1939, the War Rooms became operational and stayed that way until August of 1945. The initial plans included reinforcements, ventilation, sound-proofing, communications and broadcasting equipment. Two key rooms were of utmost importance; the Map Room (operational 24/7 for daily intelligence for the King, the Prime Minister, and military Chiefs of Staff), and the Cabinet Room (in which Churchill held 115 meetings throughout the war). When the Blitz bombing of London began, the government decided that additional reinforcement in the form of a five foot thick slab of concrete was needed. More concrete was added and additional expansions occurred until by 1946 the Cabinet War Rooms had tripled in size. There were dorms for staff, private bedrooms for military officers and senior ministers, as well as rooms for typists and switchboard operators. A transatlantic telephone room for Churchill's private war conversations with President Roosevelt and an office/bedroom (including BBC broadcasting equipment)for the Prime Minister were also added. Once the war ended, the rooms fell into disuse but remained mostly intact. In 1984, the whole complex was opened to the public as a museum as part of the five branch Imperial War Museum.