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.
The young British navy doctor was called in to see Surgeon-Commander Murray Levick while on shore leave and told they were looking for a doctor "to do something special". Commander Levick said: "I cannot tell you what it's all about yet but you will need an accomplice." Dr Cooper recommended his friend Arthur Milner, a fellow doctor, and the team was put in place. It included the two medical officers, the executive officer "Windy Gale" and three junior seamen, who would function as radio operators.
The team were warned before they left for Gibraltar that they may have to be sealed inside the operation post for as long as a year, although they were aware that it could be longer. The operation was so secret that not even Whitehall knew about it.
Once in Gibraltar, they lived under cover for two and a half years with the prospect of being moved up to the operation post to be sealed inside. At the end of the war, the team was disbanded and its members resumed civilian life. The Rock was never captured.