53.185°N 3.189°W / 53.185; -3.189 The M.S. (Ministry of Supply) Factory, Valley was a Second World War site in Rhydymwyn, Flintshire, Wales, that was used for the storage and production of mustard gas. It was later also used in the development of the UK's atomic bomb project. More recently, it became a bulk storage depot for emergency supplies.

The site occupies around 35 hectares of the Alyn Valley, to the south of the village of Rhydymwyn (centred on SJ 205 668). Once part of the extensive Gwysaney Estate, the Parish of Rhydymwyn was established in 1865. Lead mining in the area is known to have been extensive, and a foundry associated with nearby mines is depicted on several early maps for the area. Following the closure of the foundry land use on the site was largely agricultural in character. However, in 1939 the land was purchased by the Ministry of Supply and developed as a purpose built chemical weapons factory and storage facility.

Over 100 specialised buildings were constructed across the site, linked by an extensive rail network established around a spur off the Chester to Denbigh mainline. Other major landscaping undertaken at this time included the canalisation and culverting of the River Alyn, and the excavation of a complex of interlinked subterranean, rock-cut tunnels and caverns. During World War II the plant produced ordnance containing mustard gas, and was associated with the development of the Atom Bomb. In the immediate Post-War period the site was used to store German nerve gas, and it was not until the 1950s when Britain relinquished its chemical weapons (CW) capability that the site as a chemical storage facility was defunct. However, the site remains on the international Chemical Weapons List, and is still monitored as such.

From the mid-1960s the site was used by various governmental departments, its major function being a buffer storage depot to supply emergency rations and foodstuffs, and associated facilities such as mobile bakeries and canteens. In 1994 the site was closed, and a programme of demolition was undertaken. This involved the dropping of buildings onto their footprints, and the rubble being mounded over with topsoil. Several major structures, and many ancillary buildings, still survive across the site.

The surviving buildings are a reminder of a huge building programme that changed the face of Britain forever and the site remains as unique today as it was at its inception.^[1]

• The Treasury originally approved the sum of £546,000 for initial work on the Valley Site.
• By 1943 there were c. 2,200 people working at Valley. The vast majority were directed to work there by the government and billeted with local families.
• There were 5.2 million munitions manufactured in the war years many of them smoke generators which were heavily utilised from D-Day onwards.
• The factory eventually cost £3.2 million and ICI received a £60,000 agency fee for its involvement.
• The site covers 86.8 acres (351,000 m²), has 7 miles (11 km) of secure fencing and has always been “Secret”

M.S. Factory, Valley has not been used since the mid-1990s. In the preceding postwar period many of the buildings were still in use, mainly as a buffer storage depot, but some were demolished because they were considered dangerous. The site has become a Nature Reserve and a Visitor Centre was built on the site of the old gatehouse. The site covers 86 acres (350,000 m²) to the south of the village in a U-shaped valley. The site was once the home to ancient lead mines and a 19th-century metal foundry. The western side of the site is semi-ancient deciduous woodland with an understorey of wild garlic, snowdrops, bluebells and orchids. The River Alyn flows in from the north-west corner of the site and follows the western side of the valley. The river originally meandered through the centre of the valley but it was diverted as part of the early construction works. The river was canalised and given steep concrete sides and a concrete base. The middle section of the river is culverted underground for two sections. The site is now home to 7 herptile species, 8 species of fish, 17 species of butterfly/moth and 8 species of bat. 67 bird species have nested or been observed on the site.

• Chemical warfare