Rhydymwyn Valley Works: Lifting the lid on secret site (Wales)
And speculation has been rife for the last few decades since it was revealed that the works played a role in the research into the first atomic bomb.
[That work included evaluating the atomic bomb research, codenamed Operation Tube Alloys. Many of the scientists who worked on Operation Tube Alloys worked on the Manhattan Project, which developed the first atomic bomb.]
But specialists from Birmingham University, who surveyed the site, released their findings in May 2006 - saying there were no more secrets lying hidden in the underground tunnels as they were empty.
What the experts did uncover, however, were some interesting archaeological finds after being commissioned by site owners - Department for Environment, Food and Rural Affairs (Defra) - to help provide a greater understanding of the site's history and how it came to be used during World War II.
The academics found that the site had been used from the mid 1700s for a range of industries which left behind remains of foundries, waterwheels as well as mine workings. In turn, this led to the building of a main road and a railway in the 1800s which made it a prime site by the time war-time Prime Minister Winston Churchill called on chemicals manufacturer ICI to find a secret location to begin production of chemical weapons.