The Monopoly sets that came with an escape map

In 1941, Britain's secret service MI9 had a problem: how to get escape kit to the thousands of Allied prisoners held in German camps. The answer was in Leeds, at the John Waddington factory, which printed two very useful things: Monopoly games and maps on silk, a material that doesn't rustle, doesn't fall apart in water and folds down to the size of a thimble.

The firm set up a secret room and produced special editions: the board concealed a silk map of the region around the camp, a tiny compass and metal files travelled hidden among the pieces, and real banknotes — marks, francs — were slipped in under the toy money. The parcels reached the camps through fictitious charities, never the Red Cross, so as not to compromise it. A discreet red dot on the Free Parking square told prisoners that this particular set was one of the good ones.

Escapers were under orders to destroy the game after use, and the operation stayed secret for decades. That's why almost none survive: the most valuable Monopoly sets in history ended up, nearly all of them, in a stove.