Thankfully Britain never used poison gas during WWII, but we had a huge stockpile on hand in case we needed it. My husband actually made a visit to these secret tunnels some years ago whilst making a radio documentary on Underground Wales, but now they're going to be opened to the public. It is said that when the Allies finally entered Berlin and found all the Nazi's records, they discovered that most of our "secret" establishments were known to the Germans, but not Rhydymwyn. It's a small place and would, at the time, have been Welsh speaking. Not only would a spy have been unable to understand what the people who worked there were talking about, any stranger would have stuck out and everyone would have clammed up.