The Portsmouth War Emergency Committee was considering the possibility of providing the civilian air-raid shelters by tunnelling the chalk pits on Portsdown, and at a meeting held on 11 June 1941 the City Engineer reported that although he had not received definite instructions from the Ministry of Home Security to proceed, he had been told verbally to get the Contractors started on the work of excavating the tunnels. The total capacity of the shelters was to be at least 5,000 persons.
Joseph Parkin, the Portsmouth City Engineer, drew-up prototype plans of the shelters, initially for 1,017 people, which was rapidly expanded to 5,100 people in two separate shelters less than a mile apart: the Wymering Tunnel Shelter, and the London Road Tunnel Shelter. Note that this figure refers to the total capacity of both shelters not just to that at Wymering as is often quoted. The combined length of the shelters was 1.8 miles, and the total cost of construction was £73,298 at 1943 prices. Part of the specification stated that a person should only have to pass 60 sleeping people to get access to a clear corridor.
Two vertical shafts were provided in each shelter for ventilation and rescue. They were located at the far corners of the shelters, and consisted of a number of steel ladders each leading to their own platform (or 'interval' as they are called on the plans) within the shaft, so as to prevent a lethal fall. At ground level, above each shaft, was a "detonation cap" of 14 feet diameter by 3 feet thick concrete, the purpose of which was to stop a bomb falling clean down the shaft, and under the cap was a chamber the size of a shed with the exit to the outside offset from the main shaft for the same reason. Above ground was a square ventilation stack 6 feet 6 inches square and built of brick.
It was decided on 5 February 1945 that the Wymering Tunnel Shelter should be closed as soon as possible, and this took place at 9:00 am on the 19 February 1945. The London Road Tunnel Shelter was closed shortly afterwards. Neither shelter was used for any purpose again, despite local rumors of Cold War bunkers for the elite and special weapons repositories.
