There had been some small-scale use of underground munitions stores in Britain during World War I, although these were more general purpose than specifically for the RAF. Chislehurst Cave was bought in October 1914 and a small portion of the twenty miles (32 km) of tunnels were prepared for storing up to 1,000 tons of explosives and raw materials for the Royal Arsenal, Woolwich, nine miles (14 km) away. Similarly, at the Chilwell Ordnance Factory, a nearby hillside was bored out with a T-shaped storage area in 1915 as part of a total £2.5 million spent on the site, but this store could hold barely 300 tons of matériel. Other sites were also acquired but with the end of the conflict all fell into disuse.
The Air Ministry produced estimates in 1936 that a war reserve would contain 98,000 tons of bombs, 82,000 as and bombs and the balance as incendiaries; to save money only 48,000 tons was purchased. The RAF decided to store these weapons in a number of underground depots, each holding 10-30,000 tons. The decision to expend the extra money to store the matérial underground was taken because the thin-walled bombs and inflammable incendiaries were extremely vulnerable to blast, much more so than artillery shells.
The planning of the pre-war storage was finalized in 1938, with three underground depots (one each in northern, central, and southern England), each of around 25,000 tons capacity serving eight surface forward depots, termed Air Ammunition Parks (renamed Forward Ammunition Depots in 1941). Each surface depot would have a capacity of around 1,000 tons and would supply armaments to the individual airfields, each being positioned within of their recipients.
In the summer of 1936 the RAF acquired a quarry at Chilmark in Wiltshire for its southern depot and an alabaster mine at Fauld in Staffordshire for its northern depot. Making Fauld suitable for use, including surface buildings, cost around £635,000. For the central depot the RAF was forced to create a store, in 1938 it purchased a quarry at Harpur Hill in Derbyshire, built concrete storage structures within the quarry and then backfilled the hole with waste stone. Difficulties with the construction and the weather prevented Harpur Hill from opening until mid-1940. An additional pre-war site for underground storage was later created, a slate quarry at Llanberis was turned into another artificial cavern system with a large two-storey structure built for around £500,000 and opened in June 1941. In May 1941 Linley Caverns in Staffordshire was purchased, despite spending over £1 million Linley was never made useful and was relegated to the storage of obsolete munitions when it finally opened in January, 1943.
The poor quality of the RAF stores led to the War Office providing them with Ridge Quarry and later part of Eastlays quarry at the Central Ammunition Depot. Both were rather poor quality, 96,000 tons of waste stone had to be moved out of Ridge to make it usable and it took over two years to make Eastlays satisfactory for use. The War Office also allowed 4,000 tons of incendiaries to be stored at the nearby Monkton Farleigh Mine.
The pre-war Air Ammunition Parks were all constructed to a similar pattern. Located close to main railway lines with their own sidings, the sites had one fully-enclosed component store, four enclosed stores for incendiaries and usually two groups of paired open-topped concrete storage magazines, each seventy-two feet square. All the storage buildings were widely dispersed and had additional shielding with earthworks, termed blast barriers or traverses). Each open magazine was designed to hold 56 tons of bombs. There were additional buildings for the site staff and their needs.
After the outbreak of hostilities the demands on the Air Ammunition Parks grew far beyond the planned limits. Total site limits were raised from 1,000 tons to 10,000 tons, with the individual stores designed to hold 56 tons of bombs were each, by 1943, holding some 600 tons of matériel. When the parks were renamed Forward Ammunition Depots in 1941 they became central depots with a number of nearby satellite sites, Advanced Ammunition Parks, to further increase holding capacity. Staple Halt became an Advanced Ammunition Park.
The two wartime Forward Ammunition Depots were markedly different from the pre-war designs, reliant on concealment by woodland (South Witham was actually within Morkery Woods) rather than toughness, the munitions were held within standard Nissen huts dispersed to increase safety, with transport on standard nine-foot metalled roads. Each had a stated capacity of 8,400 tons of bombs, 840 tons of incendiaries, and for small arms ammunition.
The depots were a dangerous example of obsolescence for two main reasons. During the war, RAF bombs became much larger, quickly became the new large bomb and the vast production of the and devices was assigned to 'long term' storage. This meant the depots had little existing space to store new, bigger bombs and neither the equipment, training, or ability to handle larger bombs safely. Secondly, the bases had been located on the assumption the majority of fighting would be in support of France with active units in the south and east with support and training units in the south-west and west. There was no thought that France would collapse in early 1940. The new requirements for RAF fighter and bomber operations from the south-west needed a matching supply structure, which had not been built.
As an example of the pace of change, the 1936 estimate of 98,000 tonnes can be compared to an October 1941 estimated requirement of 632,000 tons within the following year. In 1941 the RAF had a total storage capacity of 158,000 tons. This inadequate capacity was reduced after the depot at Llanberis partially collapsed on 25 January 1942 and the Harpur Hill depot was closed soon afterwards as a precaution against a similar structural failure. A total of 23,906 tons of matériel was removed from these two sites by May 1942. Llanberis was effectively abandoned (decommissioned in 1956 the site would cause problems later when, in 1969, it was discovered that terrorists were exploring the site for explosives) but Harpur Hill was reinforced and re-opened, although the storage of potentially sensitive explosives was prohibited.
There was a hurried examination of even more marginal storage sites, but the situation was overtaken by actual events. By early 1942 German raids were markedly less threatening than anticipated - the need for the additional protection of underground storage was no longer paramount, indeed the sites came to be regarded as expensive white elephants. The supply of ammunition was soon changes, with supplies moving directly from the factories to the Forward Ammunition Depots, cutting the underground stores out of the system and relegating them to stores for obsolete stock and chemical weapons, holding points for very large cargoes, and repair centres. Three large new surface sites were also established at Gisburn and Wortley in Yorkshire, and Longparish/Harewood Forest in Hampshire. Longparish was designed to hold 40,000 tons of ammunition and the other sites 25,000 tons each.
The collapse of Llanberis also lead to the decision to remove chemical weapons from subterranean storage, mainly a large number of bombs containing the unstable and corrosive mustard gas. Harpur Hill had been designated the central store for such devices in April 1940, receiving its first load in June of that year of mustard gas bombs evacuated from France. In June 1942 it was decided to move the bombs to a remote site at Bowes Moor in County Durham. As an aside, the War Office chemical weapons store was at Russell's Enclosure in the Forest of Dean, post-war disclosures of the mismanagement there are a little disquietening. The move of RAF bombs to Bowes Moor began in December 1941 with the bombs initially stored in the open under tarps or in wooden sheds. It was founding that the sheep on the moorland would consume the tarpaulins and disturb the bombs, resulting in the addition of sheep-proof fences and gates for the entire site. Fifty new buildings were late added to store the larger bombs. To ease distribution of mustard gas, five Forward Filling Stations were built at or near existing bomb storage sites.
Another blow to the underground stores came on 27 November 1944 when there was an explosion at Fauld involving approximately 4,000 tons of high explosive bombs killing seventy people. The Court of Enquiry concluded that the explosion had been initiated by a mistake in the handling of a damaged bomb by RAF personnel.
Chemical weapons were either disposed of in situ at Bowes Moor by incineration or transported to Cairnryan for dumping in Beaufort's Dyke in the Irish Sea. Almost 71,000 bombs containing tabun had been seized in Germany, these were stored in the open at RAF Llandwrog, near Caernarfon, until 1954 when, in Operation Sandcastle, they were transported to Cairnryan for disposal aboard scuttling ships at sea north-west of Ireland.