Starting in February of 1942, the codebreakers at Bletchley Park faced a serious problem: They couldn't break the German Army's latest "M4" flavor of the Enigma. Although work continued on breaking this new version of the code, the outlook was grim. But on October 30, HMS Petard and three other Royal Navy destroyers attacked the German U-boat U-559, eventually forcing it to surface and the German crew to abandon ship. While the Germans swam away from their ruined U-boat, three British seamen left Petard and swam to the sinking U-559. What they pinched from the wreck saved tens of thousands of lives for the Allies.

Made for an exhibit at Bletchley Park, here's a video explaining and commemorating what became known as "The Petard Pinch," an act of wartime bravery that led to the breaking of the M4 Enigma.