Rhydymwyn Valley Works

Rhydymwyn Valley Tunnels

Useful Information

Location: Alyn Valley at Rhydymwyn.
17 Nant Alyn Road, Rhydymwyn, Flintshire.
(53.1923, -3.1915)
Open: Rhydymwyn Valley Site: All year Mon-Fri 8-18, Sat, Sun 9-17.
Tunnel Tours: 3 days during the summer 10-15.
Fee: Tunnel Tours: Per Person GBP 5.
Classification: SubterraneaFactory
Light: provided
Guided tours:  
Address: Rhydymwyn Valley History Society, Visitor Centre, 17 Nant Alyn Road, Rhydymwyn, Flintshire, CH7 5HQ, Wales, Tel: +44-1352-781129. E-mail:
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
Please check rates and details directly with the companies in question if you need more recent info.


1925 "The Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare" signed by all the major powers except Japan.
1939 government purchases 35 ha of the Alyn Valley for a chemical weapons factory and storage facility.
1943 2,200 people working at the site.
1947-1959 the majority of the country’s stock of mustard gas stored at the facility.
1990s site abandoned by the army.
MAY-2008 Rhydymwyn Valley History Society founded.
2017 beginning of underground tours.


Rhydymwyn Valley Works is a historic site with an extremely interesting history. While the site is now open to the public, the underground tunnels are open only on three days during summer. The dates for the current year are published on the website. Tickets are sold online only. Visitors are equipped with helmets, hi-vis jackets and headlamps. The tours are actually free, but a voluntary contribution of £5 per person covers the cost of insurance and equipment.

This site is base on a weapon which is banned by international treaties today, because it is too cruel. With World War I the use of gas during battles started. In 1917 the use of mustard gas started and its use was soon widespread. In 1918 there were 188,706 British and 419,340 Russian gas casualities. As a result in 1925 the "The Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare" was signed by all the major powers except Japan. In th wake of World War II Britain secretly produced mustard gas, the official justification was as a possible retribution in case the Germans use mustard gas. However, neither the factory nor the storage of the gas was safe from air raids. It was considered that if the Germans did intend to use mustard gas, they would attempt to bomb the British production capability to avoid British retaliation. The official terminology was "retaliating in kind". So plans were made for three mustard gas manufacturing factories. Rhydymwyn Valley Works is one of them, Randle and Springfields are the other two.

In 1939 the Ministry of Supply instructed ICI’s Special Products Division to construct a factory and storage area in the Alyn Valley close to Rhydymwyn. The factory produced mustard gas and other chemical warfare between 1940 and 1945. The tunnel complex held the majority of the country’s stock of mustard gas until 1959.

But there was another important research which happened here. Early development into the atomic bomb codenamed Operation Tube Alloys took place at the site. They researched the method of separation of Uranium isotope 235 and 237 by gaseous diffusion. In 1941 three cells for this process were installed in Building P6 at Rhydymwyn, under the guidance of Rudolf Peierls and his assistant Klaus Fuchs. But in 1943 the Quebec Agreement signed by Roosevelt and Churchil, which committed all future development of the Atomic Bomb to the USA and Canada. 20 of the scientists involved joined the Manhattan Project, which developed the first atomic bomb.

After the war the mustard gas which was already deployed as munitions was returned to the site and stored there. A policy decision was made to destroy the mustard gas stocks. It was not as a result of the Geneva convention, actually they were just considered a costly irrelevance in a nuclear world. They were burned until 1959. The army used the site until the early 1990s mainly as a buffer depot. But the stories and bizarre conspiracy theories about the nation’s art treasures stored here, a fourth tunnel, a secret communications centre powered by steam engines, or food being stored in the tunnels. When the site was closed down by the army in the 1990s after the Cold War, the site was managed by the Rhydymwyn Valley History Society. The have a visitor center and exhibition on site which may be visited, also some of the surface buildings. The underground tours were somewaht difficult wo