As various people come forward with
dispensation to reveal what once were state secrets, we're learning all sorts of things about World War II. For instance, hundreds of allied prisoners of war in German concentration camps were helped to escape via smuggled maps and tools that were cleverly hidden inside the boards and playing pieces of Monopoly and other board games. British airmen were briefed before missions with data about the countryside, and were told that should they be captured, it was their duty to attempt escape. They were told to watch for "special" Monopoly sets and other board games that would contain escape kits, and specially printed silk maps, hidden in special compartments beneath the board surface. The Monopoly games were marked by a red dot in the Free Parking square.
In addition to the silk maps, game sets included the parts to assemble a compass, small metal tools like files, and local currency, hidden beneath the fake monopoly money. Hundreds, perhaps thousands, of POWs were assisted to escape by virtue of the "special" games. The effort to create and distribute the board games via the Red Cross was enormous, and completely secret—not even the Red Cross knew. MI9 worked directly with a British printer to manufacture and distribute the games in absolute secrecy. You can read more about it here.