M. S. Factory, Valley – Rhydymwyn, Cymru - Atlas Obscura

M. S. Factory, Valley

Rhydymwyn, Cymru

Following years of secrecy, the hidden history of a World War II site is being brought to light. 


Located in a lovely isolated area of North Wales, Rhydymwyn Valley has been concealing a secret for over 75 years. Part of Britain’s hidden wartime history lies just outside the village.

In 1939, the government purchased a 35 hectares area of the Alyn Valley to be developed as a chemical weapons factory and storage facility, named M.S. Factory, Valley. Over 100 specialized buildings were eventually constructed across the site, which grew to cover 86 hectares and employ 2,000 workers. The facility was cloaked in secrecy, so much so that the early development of the UK’s atomic bomb project took place here and it did not appear on any Luftwaffe bombing maps.

Alongside the building work, an underground tunnel system was dug for the storage of mustard gas. While the use of the gas for attacking purposes was banned, it could still be used for defensive or retaliatory attacks, hence its stockpiling. After the war, the tunnels actually held the majority of the country’s mustard gas stock from 1947 to 1959, when Britain relinquished its chemical weapons capacity. After this, the tunnels had a variety of roles, such as a bulk storage depot for emergency supplies. It was also the planned site for the relocation of Britain’s gold supply during the Cold War.

M.S. Factory, Valley eventually closed in 1994, with almost all of the above ground buildings being destroyed, except for a couple. One of these, the Grade II listed Building P6, was where the atomic research took place, though the interior is inaccessible due to contamination. Today the site is a nature reserve run by the Rhydymwyn Valley Historical Society.

Know Before You Go

Access to the tunnels is strictly limited and can only be arranged as part of a tour run by Rhydymwyn Valley History Society.

In partnership with KAYAK

Plan Your Trip

From Around the Web