On June 6, 1944, the Allies launched the biggest armada in history to invade the Normandy beaches and liberate Europe from the Nazis. In less than 24 hours, more than 5, 000 ships crossed the English Channel, along with thousands of tanks and landing craft and nearly 200, 000 men. Hundreds of ships sank while running the gauntlet of mines and bunkers, creating one of the world's largest underwater archeological sites. Now, NOVA has exclusive access to a unique collaboration between military historians, archeologists, and specialist divers to carry out the first complete survey of the seabed bordering the legendary beachheads. Dive teams, submersibles, and underwater robots will discover and identify key examples of the Allied craft that fell victim to German shellfire, mines, and torpedoes. D-Day's Sunken Secrets unfolds a vivid blow-by-blow account of the tumultuous events of D-Day and reveals how the Allies' intricate planning and advanced technology was vital to assure the success of the most ambitious and risky military operation ever launched.