During World War II, London lived in fear of a German invasion and subsequent looting. So, King George VI (Queen Elizabeth II’s father) orchestrated a plan to keep the Crown Jewels, usually kept in the Tower of London, safe. And what was his exact strategy? Well, to remove the most precious stones from the Imperial State Crown—Black Prince’s Ruby and St. Edward’s Sapphire—put them into a biscuit tin, and bury them in the grounds of Windsor Castle, The Times in London revealed today.

The container may have been crass, but the measures to protect it weren’t: In that hole, dug beneath a palace entrance, they built two chambers with steel doors. The Jewels were then secured inside; the whole clandestine hiding spot only accessible through a trapdoor. During the construction, they covered it at night with a tarp so German airplanes flying overhead couldn’t discern what was going on.

The whole thing was so secret, in fact, that even Queen Elizabeth didn’t know the full details back then. Royal commentator Alastair Bruce, who interviews the Queen in the BBC program The Coronation (airing in the U.S. on the Smithsonian Channel on Sunday), was the one to break the news to the 91-year-old monarch. “What was so lovely was that the Queen had no knowledge of it,” he told The Times. “Telling her seemed strangely odd.”

While the Jewels have since been returned to the Tower of London, the trapdoor at Windsor Castle still remains—so, perhaps, on her next trip, Queen Elizabeth can witness the little piece of history for herself.