MI9, the British directorate responsible for escape and evasion, developed an ingenious method of smuggling escape aids to prisoners of war. Working with the Waddingtons game company (which printed both playing cards and maps under license), they created special decks of playing cards that contained silk maps between the front and back layers of the card.
When soaked in water, the cards would separate to reveal sections of a silk escape map. Specific cards contained specific map sections, and POWs were told which cards to soak through coded messages. The maps showed escape routes from various prison camps to neutral countries, with safe houses and border crossings marked.
Waddingtons also hid maps inside Monopoly boards, along with real banknotes, compasses disguised as buttons, and tiny files. These "escape kits" were sent through fictitious charitable organizations. It is estimated that of the 35,000 Allied servicemen who escaped from POW camps during the war, approximately one-third were aided by MI9's smuggled equipment.