A former top secret munitions site where researchers worked on the world's first atomic bomb has been turned into a nature reserve.
Bomb-makers used a secret underground tunnel
The once top secret Valley munitions site at Rhydymwyn, near Mold in Flintshire, was formally handed over to North East Wales Wildlife by the UK government on Tuesday.
The site was deliberately left off maps to conceal its identity and location.
It is now home to wildlife including grass snakes, owls, bats and lizards.
At a ceremony on Tuesday, North East Wales Wildlife was formally given long-term responsibility for the ecology of the site after it was handed over by the Department for Environment, Food and Rural Affairs (Defra).
Peter Green, head of Defra's Estate's Division, said: "North East Wales Wildlife have made a contribution to the transformation of this site from a derelict munitions factory to a haven for wildlife.
"It seems entirely appropriate that their invaluable input should be formally recognised in a legal agreement."
The munitions site was once a key part of the Allied war effort.
The work included evaluating the atomic bomb research, codenamed Operation Tube Alloys, which made the site one of Britain's greatest wartime secrets.
Workers used the walls to write down formulas
Many of the scientists who worked on Operation Tube Alloys worked on the Manhattan Project, which developed the first atomic bomb.
They were forbidden from using pieces of paper to prevent their projects being seen elsewhere.
Instead, they used the walls of the buildings to work out their formulae - which can still be seen today.
Staff also produced hundreds of tons of mustard gas in World War Two.
At its peak, 1,756 people worked there, and by November 1942, they were manufacturing 40,000 mustard shells every week.
The site has been open to the public on a limited basis since 2003.
Bird and badger hides have been constructed to allow people to watch the wildlife at the mile-long site.
Pippa Perry, of North East Wales Wildlife, said: "We are delighted to be formally given responsibility for the wildlife at the site and for working with the public and the local community to raise awareness of the hugely diverse wildlife at the site."