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. Throughout the operation, hundreds of ships sank running the gauntlet of mines and bunkers, creating one of the world's largest underwater archaeological sites. Now, NOVA has exclusive access to a unique collaboration between military historians, archaeologists, and specialist divers to carry out the most extensive survey ever done of the seabed bordering the legendary beachheads. Dive teams, submersibles, and underwater robots will discover and identify key examples of Allied craft that fell victim to German shellfire, mines, and torpedoes, using the latest 3D-mapping tools to plot the relics on the sea floor. Highlighting the ingenious technology that helped the Allies overcome the German defenses, and featuring first-hand accounts from Allied veterans who have returned to the site of this epic battle to share their harrowing stories, "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 assured the success of the most ambitious and risky military operation ever launched.