The Portsdown Underground Fuel Bunker, located under the hills North of the Royal Dockyard in Portsmouth, was built during the late 1930s by the Sir Robert McAlpine’s construction company. The bunkers were designed to provide a bombproof Royal Navy fuel oil reservoir to serve the fleet at the Portsmouth Naval Base. The oil was needed as a guaranteed supply for Royal Navy warships in case oil deliveries to western British seaports were blockaded by the German Navy during World War II. Three such bunkers were built in the UK, with the others being at Inchindown some 4 miles to the north of the then naval base at Invergordon, and Lyness at the Scapa Flow Naval Base. The Fuel Bunker had an official capacity of 137,700 tonnes of furnace fuel oil, which was the main warship fuel of World War Two. Fuel Furnace Oil is a residual fuel left over once all other products, such as petrol and diesel, have been distilled from crude oil. It is a cheap substance, ideally suited to the running of a large Navy. It is no longer used by the Royal Navy.
The whole complex, which is 37 metres underground and consists of 3.2 kms of tunnelling and contains nine 11 metre high concrete storage tanks. There is also a large underground pumping station containing 2 pre-war pumps which were used to pump the fuel to, and from, the Oil Fuel Jetty in Portsmouth Harbour 9.6 km away. The pipeline consisted of two 16 inch FFO pipes, and one 10 inch diesel pipe which ran from Portsdown through Portchester, Fareham Creek, Fleetlands, Frater, Priddy’s Hard, Forton Lake and terminated at the Forton Oil Fuel Depot, Gosport. The walls of the bunkers are made of concrete 30 cm thick, and in places the roof is 6.7 metres thick; no German bomb produced during WWII was capable of this level of penetration.