Imagine splitting a card and finding this! I don’t think that happened to any of us magicians, but during World War II, the USPCC teamed up with British and American intelligence agencies to create playing cards with top secret maps hidden between the layers. They were designed with the intent of helping prisoners of war escape German POW camps. The prisoners would soak the cards to make them easy to peel and then open them to reveal maps with escape routes directing them to safety.
It’s unknown how many remain or even how many were created.
Story source—A Map Inside the Cards