Occupying 35 hectares of the Alyn Valley to the south of the village of Rhydymwyn is one of the UK’s most mysterious and fascinating historical sites.

At one point more than 2,000 men and women worked at the Rhydymwyn Valley Works where over 100 buildings and an extensive rail network were built about 80 years ago.

But despite its size, the purpose of the site remained shrouded in mystery for many years with its workers sworn to secrecy and its scientific experiments hidden by a network of underground tunnels and caverns.

Video and images: Rick Matthews 

Now, almost 25 years since it was closed, the site, known officially as the MS factory, is ready once more to reveal its secrets to the public with a limited number of tunnel tours being operated this spring and summer by the dedicated volunteers of Rhydymwyn Valley History Society.

“In the Second World War this was the place where most of Great Britain’s mustard gas was stored in case it needed to be used against the Germans, if, of course, they used it against us,” says Colin Barber, society chairman, as we approach one of the huge entrances to the tunnels.

The Treasury approved the sum of £546,000 for initial work on August 27 1939, and work began in October 1939 on the storage tunnels in the limestone hillside, in the Alyn valley close to Rhydymwyn before the factory opened in 1941.

The factory produced 5.2 million munitions in the war years, many of them smoke generators which were heavily utilised from D-Day onwards. After the war chemical weapons continued to be stored, with the tunnel complex holding the majority of the country’s stock of mustard gas.

Adds Colin: “The gas was stored here up until 1958 when prime minister Eden decided that the use of chemical weapons was an irrelevance in the nuclear age.”

During the Cold War, the site accepted a new role when the government decided to set up a system of food and raw material stockpiles in case of nuclear war.

“It became part of the recovery plan as part of Project Python (a Cold War contingency plan of the British Government for the continuity of government in the event of nuclear war) where people could go into the tunnels for a limited amount of time and where the Bank if England’s gold could also be stored. There are roughly 94,000 square feet of tunnels but they’ve not been used since 1960.

“They’ve been waiting for a disaster to befall us but it hasn’t happened yet!”

If this wasn’t enough, the factory was also once at the forefront of Britain’s nuclear weapon programme with one of the buildings, known as P6, used for the testing of apparatus for uranium isotope separation in 1942 in an early phase of the ‘Tube Alloys’ project before this was moved to America where it was developed later into The Manhattan Project.

With such historical significance, it is perhaps unsurprising that Colin and his colleagues are keen to open the site up to the public and after last year’s official opening, 2018 will see the tunnels open for just five days from April to September.

“After fighting for eight years, I think a lot of people at the Department for Environment, Food and Rural Affairs (Defra) thought I was a little bit like Lord Byron – mad, bad and dangerous to know.

“But I wasn’t going away and last year we had our big opening attended by Ken Skates, Cabinet Secretary for Economy and Infrastructure, who officially opened the tunnels for public managed access.

“We had six open days and took almost 1,000 people through the tunnels and starting in April we’ll be doing something similar.”

Walking around its many acres, it’s clear that the site of the Valley Works has now been returned to nature. It attracts a wide variety of wildlife and is designated as a nature reserve as outlined on a plaque at the entrance gates with a number of its buildings now Grade II-listed.

“After a long time there is finally some work being done on the buildings because potentially the site is going to be taken over by The Land Trust and will be opened up to the public in a more significant way,” says Colin.

“We’ve got the green shoots of something special here. Last week I passed a group of children visiting and it was smashing to see.

“It’s a beautiful place and it should be used by the public and be for their benefit.”

The Valley Works’ surviving buildings are a reminder of a huge building programme that changed the face of Britain forever while its underground tunnels serve as a chilling warning of just how close the world came to nuclear war.

“This place would only have been a sanctuary for the chosen few and not the likes of you or me,” grins Colin.

“The legacy of this site really is incredible. It has three wonderful things: history, beauty and mystery and I think it has such marvellous potential.

“It’s been untouched and fenced in since 1939 and I believe the more files we get from the Ministry of Defence the more we will discover about the plans for this site.”

l If you have any queries about the Tunnel Tours please contact: tunneltours@rhydymwynvalley
history.co.uk or go to www.rhydymwynvalley