The Gibraltar chamber had the innocuous name of the "Stay Behind" Cave. But this was no game. This was a top-secret wartime mission, code-named Operation Tracer, in which six men volunteered to be buried alive in the cave if the Rock were captured by the Germans, so they could continue to monitor enemy movements.

More than 60 years after the end of the Second World War, a retired doctor from Preston has been named as the chamber's last survivor, as researchers struggle to unlock its remaining secrets.

"I had a telephone call one day and they came over," Dr Bruce Cooper said yesterday. Only now has the 92-year-old broken his silence on the mission whose existence was one of the war's best-kept secrets.