Hidden in a remote Welsh valley lies an abandoned and secretive chemical weapons facility which played a crucial role in the development of nuclear arms. Although it is long closed Rhydymwyn’s MS Valley Factory remains the most complete chemical weapons site in the UK and it’s still on the international Chemical Weapons List with monitoring ongoing. The 86-acre site is overgrown and surrounded by rusting barbed wire. Deep beneath it a network of sinister and historic wartime tunnels have been largely out of bounds since 1939. But the old MS Valley Works on Nant Alyn Road, near Mold, are of national and international significance. The history goes back to 1939 when the Ministry of Supply (MS) bought the site and developed it as a chemical weapons factory and storage area.You can get more story updates straight to your inbox by subscribing to our newsletters here. More than 100 buildings were erected, linked by a rail network. The River Alyn running through the site was made into canals and culverted and by 1943, 2,200 people worked at the complex. Deep below a vast network of rock-cut tunnels and bombproof caverns was excavated. It was here that more than 3,000 tons of deadly mustard gas was stored, to be used in retaliation if the Nazis used chemical weapons. Thought to be almost impossible to see from the air, the factory was the only site of its type not discovered by Nazi intelligence. And it was work at the remote facility that kindled the nuclear revolution. The survival of the uItra secretive sight owed much to its location tucked away in the elongated site in a wooded valley. Now the public is being offered the rare chance to step inside this eerie facility used first in the fight against Nazi Germany and then for research central to developing nuclear arms. Many may noit be aware of the site’s crucial role in the development of nuclear weapons – codenamed Tube Alloys – that is likely to be its most lasting legacy. It became a centre of excellence for research into gaseous diffusion to provide fissile material with 10 Nobel Laureates were working there at one time. Many of the scientists who worked at the north Wales site went on to work on the Manhattan Project, the top secret US government-led research in World War Two to develop nuclear weapons, ultimately leading to the atomic bombings of Hiroshima and Nagasaki. Physicist J. Robert Oppenheimer, whose story is told in the blockbuster film Oppenheimer, led the laboratory that designed these devastating bombs, yet “Without Rhydymwyn, Oppenheimer would never have been able to develop the Atomic Bomb,” one historian expert believes. After the war the facility was rendered redundant the 1950s when Britain relinquished its chemical weapons capability. It was then used as major stockpile site for emergency rations and raw materials in case of a nuclear war, to be delivered via mobile bakeries and canteens. Top secret plans were even drawn up to relocate 4,700 tons of gold from the Bank of England vault at Mount Pleasant if war broke out again. The MS Valley Factory closed in 1994 and nine years later the site underwent major works with many of the war-time buildings demolished. It’s now a closed off nature reserve that only a handful of people can visit. The UK Government offered the complex to the newly established Welsh Government after devolution with a “dowry” payment, but it baulked at the potential chemical legacy issues. In 2010 the running costs were estimated at around £1m-a-year, for security, maintenance and inspections. The tunnels’ closure fostered rumours about what might have been left behind. Confirmation that all chemicals had been removed came in in 2006 when an independent expert was given access. But the site remained something of a living museum: in 2014, an urban explorer managed to get inside the complex and among lost objects he found a cabinet of old documents detailing the site’s layout, and a worker’s jacket with a pack of cigarettes still inside its pocket. The last known person alive to have worked on the site was Rosina Parry, who was aged 21 in 1941. In 2012 she gave a television interview in which she described her role – she had to inspect mustard gas bombs make sure they weren’t damaged. At the time no one mentioned mustard gas – but rumours flew around. She told S4C: “If ever you got little spots on you, if there was a leak, it used to burn you. We always had our gas-masks, we didn’t go anywhere without our gas-masks! Nobody knew what was going on there really. Nobody knew what I was doing there from where I lived (in Greenfield, Holywell).” Occasional tours are offered for the public to get a glimpse inside this mysterious abandoned facility, and there's always a clamour for spaces.. The next tour date, by the Rhydymwyn Valley History Society, is on Sunday, April 27 with almost 3,000 people applying for the 60 places available. The upshot was an expanded tour roster for 96 lucky people, each paying £10. A second tour date has also been organised, for Sunday, June 22, pending approval by UK ministry Defra. If this happens, an open invite to apply for places will be issued by the society via its website and Facebook page. on a first come first served basis. Society chairman Colin Barber believes it is the site’s crucial role in the development of nuclear weapons – codenamed Tube Alloys – that’s likely to be its most lasting legacy. “It became a centre of excellence for research into gaseous diffusion to provide fissile material. At one point, 10 Nobel Laureates were working there and in the wider area over to Widnes. After the war, a gaseous diffusion factory was established at Capenhurst in Cheshire for uranium enrichment. “At Valley, the work was carried out at the P6 building for the Tube Alloys project before this was moved to the States, later developing into the Manhattan Project. P6 is still standing and, as well as opening the tunnels to the public, we want the building’s importance to be properly recognised.” If places for the June 22 tours become available, you can book via the Rhydymwyn Valley History Society website here. Applicants will be asked to complete a form and pay £10. No under-16s or pregnant women are permitted, nor are dogs or drones allowed. Parking is available on site. More tours may be organised later in the year depending on demand. Get daily breaking news updates on your phone by joining our WhatsApp community here. 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