The Valley Factory at Rhydymwyn was developed on the site of a former lead mine between 1939 and 1940. It contained purpose built facilities for the wartime production of mustard gas, its storage in underground chambers, the filling of munitions with chemical agent and the assembly of filled munitions with explosive bursting charges and fuses. Later in the war, one of the buildings was used for gaseous diffusion experiments in support of the early atomic bomb project.
Mustard production on site ceased in April 1945 and between 1946 and 1948 the associated plant and facilities were decommissioned. The underground storage chambers continued to house bulk mustard supported by the facilities of a loading bay and the laboratories until 1958 when work started to remove the remaining stockpile. Dismantling and decontamination of all remaining equipment was completed by April 1960.
The Ministry of Public Buildings and Works assumed responsibility for maintaining the site in 1964. From the 1960s the site was used as buffer depot for the storage of a variety of non-perishable foodstuffs such as sugar and flour. Transfer of responsibility to the Ministry of Agriculture, Fisheries and Food occurred in 1992 and the use of the site as a buffer depot ceased in 1994, since when the buildings have remained empty.
(Source: report by Andrew P Smith for Subterranea Britannica).

A selection of the principal features and structures noted on the site when visited on 29 April 2009:
West Process Building (SJ 20509 66538)
North Process Building (SJ 20655 66555)
South Process Building (SJ 20686 66378)
Runcol Building (SJ 20622 66327)
Tunnel Entrance E1 (SJ 20663 66123)
Tunnel Entrance E2 (SJ 20626 66199)
Site of Cleaning Sheds (SJ 20725 66083 and SJ 20673 66196)
Effluent Pump House (SJ 20872 66030)
Shell-filling Buildings: 5.5in shells: (SJ 20865 65919); 25lb shells: (SJ 20888 65885)
Weapons Receipt Buildings: (SJ 20860 65940)
Observation Shelter (SJ 20902 65809)
North Fuse Magazine (SJ 20938 65720)
South Fuse Magazine (SJ 21004 65619)
Fuse loading plinth (SJ 21003 65660)
Staff Changing Rooms (SJ 21018 65811)
Surgery (SJ 20955 65840)
Air Raid Shelters (SJ 20940 65938 and SJ 20954 65916)

B.A.Malaws, RCAHMW, 06 December 2005 and 08 February 2012.