Rhydymwyn, Flintshire
Until 1944, this was the most secretive military installation on the British mainland — a major center for the manufacture, assembly, and storage of bulk chemical weapons. At its height, the factory employed more than 2,000 people. Mustard gas and its poorer cousin “pyro” were kept in a sprawling storage facility with units above and below ground. In 1943 atomic research and tests were conducted here by British scientists whose work was later instrumental in the Manhattan Project.