Gems from the Crown Jewels were hidden away from the Nazis inside a biscuit tin during the Second World War, as revealed in new BBC documentary The Coronation.
Following orders from King George VI, the tin itself was buried underneath a secret passage at Windsor Castle, in case of enemy invasion.
The Queen did not know the full details of the story until she was told by royal commentator Alastair Bruce, who presents the documentary due to be broadcast this Sunday.
“What was so lovely was that the Queen had no knowledge of it,”Bruce told The Times. “Telling her seemed strangely odd.”
He also revealed how “an electric set of letters” from Sir Owen Morshead, former royal librarian, to Queen Mary, the mother of George VI, helped to unearth the mystery.
The documents describe how a hole was dug in chalk earth, which was covered to hide it from the enemy, and two chambers with steel doors were created.
A trapdoor used to access the secret area where the biscuit tin was kept still exists today.
The Coronation is on Sunday 14th January at 8pm on BBC1