The secrets within a World War II chemical weapons factory are being uncovered for the first time.
The film promotes the role of women in doing wartime work
A 1944 government film about the base in Rhydymwyn near Mold being screened for local people shows what went on in the network of underground tunnels.
The valley works is now a nature reserve owned by the environment department Defra and managed by North East Wales Wildlife.
These tunnels once stored mustard gas and explosives.
The chemicals are all long gone but site manager Dave Williams said visitors to the site still needed to take precautions.
Whenever he goes in he carries a gas meter, which measures levels of methane, oxygen and carbon monoxide within the caverns.
The newly-discovered 50-minute government recruitment film shines a light on a dark time.
The site remained top secret for a long time, and the film's screening on Wednesday evening gave local people their first opportunity to find out exactly what went on underground 60 years ago.
The film includes a voiceover by wartime Prime Minister Winston Churchill and footage of little of the activity that took place at the site.
Although it has been a nature reserve for years, the site is still subject to inspections by scientific experts under the terms of an international agreement because it used to store chemical weapons.
Inspections are still carried out at the site because of its past
During a recent visit, one inspector happened to mention there was a film at the Porton Down research facility which he thought may be of Rhydymwyn.
The site asked to see the film and within minutes it was clear the film was indeed set there.
'Shrouded in secrecy'
Department of Environment, Food and Rural Affairs (Defra) community liaison officer Grant Webberley said the film showed the drill workers had to follow when they started and finished work to avoid contamination, and also promoted the role of women.
"It is a little patronising by today's standards," he admitted.
He went on: "Despite years of research into this site, the activities that took place here during its heyday, and for a considerable period afterwards, were shrouded in secrecy, but in recent years this has changed.
"So there is still a lot this site has to tell us about what was here and what went on here.
"We believe this training and recruitment film which has come into our possession dates from around 1944.
"We're hoping that the public will come along and that it will jog some memories from those who were living and working here when the factory was fully operational.
"We're hoping that some will be able to recognise and identify some of the buildings, and shed even more light on this site's history."