In 1938 work began at the quarry site of Harpur Hill to build an RAF bomb store which was to be RAF 28 MU. The high explosive bombs would be stored in a series of reinforced concrete galleries, covered by 60 feet of overburden, to give a similar level of protection against a bombing raid as would be provided by subterranean tunnels. The proximity to the quarry site not only gave a ready supply of overfill, but also meant the increased activity of the construction work would easily go un-noticed, being mistaken for further quarrying work.
The concept was thought to be successful and in 1939 it was used again in the design of RAF 31 MU at Llanberis, North Wales. But following a collapse there in January 1942, which buried 14,000 tons of bombs, some 14% of the RAF’s total supply at the time, Harpur Hill was inspected, and when cracks were found in the structure, it was temporarily closed down, with most of the bombs in storage there transferred to the ill fated Fauld quarry bomb store, RAF 21MU.
The RAF left Harpur Hill in 1960 and the tunnels have since been used as a mushroom farm, for cheese storage, and now belong to transport company Norbert Dentressangle. Many of the smaller bunkers, which can still be seen today, are currently used by Sheffield University who have 14 laboratories and a comprehensive workshop on the site, they claim it is one of only a handful of establishments capable of carrying out hazardous testing with explosives and the like,
The adjacent site is home to the UK Health and Safety Laboratories, which following the 7/7 bombings in London received ex Jubilee line trains to be used for explosives testing inside tunnels. The trains could be seen on the lab site as recently as 2017, but the pictures from 2018 below only show the rusty steel tunnel in which some of the experiments on the trains were conducted.