The green concrete barrier that provided defense from anything up to a direct hit by a heavy bomb of 1000 kilograms (2200 pounds). It was installed in 1940 when the threat of air attack and the possibllity that the Germans knew of the War Rooms led to its installation. Inside, in the dark, you can see that the concrete goes down almost twenty feet. Right-click and choose "view image" to enlarge.
Welcome to the Imperial War Museum The Imperial War Museum, the multi-branch national museum of war and wartime life from the First World War, through the Second World War, to the present day.