Maps Hidden in Playing Cards

 In World War II The United States Playing Card Company designed cards for POW's in Germany with maps hidden between the paper layers. When soaked in water the cards could be peeled apart and prisoners could see secret escape routes. These "map decks" were closely guarded secrets and few survive, This one from a private collection may be the only surviving example - via Bicycle Cards.

There is a replica deck available on Amazon. The image gives a better idea of what they would have looked like.