Scotland's biggest Cold War bunker is buried deep beneath an Edinburgh hill - and its existence has remained unknown to most people for decades.
Barnton Quarry Nuclear Bunker was built in 1944 and expanded in 1951, but by 1955 it was defunct.
The secluded entrance to the building, which lies 100ft below Corstorphine Hill, can only be accessed down a rough private track.
Spread over three storeys, it was once Scotland's first line of defence against the threat of nuclear war.
It was equipped with a BBC broadcasting studio and would have housed Queen Elizabeth if the city had been attacked while she was in residence.
At its heart, surrounded by a maze of corridors and rooms, is the main operations floor, where information would have been gathered in the 1950s to locate any aircraft in Scottish air space.