1941. America had just entered World War II, but terrifying unseen weapons were stalking US targets, sinking over 400 ships just miles from American shores in the worst naval defeat in United States history. The stealthy predators were German U-boats, Hitler's deadly warships of the deep. It was cataclysmic—with bodies and wreckage washed ashore on the eastern seaboard. But the Allies ultimately triumphed, sinking over 700 enemy submarines.

Fifty years later, the personal drama behind this wartime state of siege resurfaced when a team of divers discovered an unidentified German U-boat deep in the waters off the New Jersey coast. What boat was it? And what secret mission brought it there? The divers embarked on a dangerous, six-year quest to uncover the secret of the mysterious wreck. Three divers lost their lives in the search, but the clues uncovered have shed remarkable new light on the struggles and triumphs from this undersea war.

There's more in this spectacular two-hour video journey. Trace the history of submarines and undersea warfare from the American Revolution through World War II, and tour the sophisticated—and cramped—interior of a captured U-boat. Dive in dangerous waters with deep sea detectives as they attempt to retrieve clues that will reveal the decaying U-boat's identity. See how the breaking of secret Nazi codes by the Allies altered the outcome of the war—and helped unravel the mystery behind this German U-boat. And finally, travel to Germany with the divers as they rewrite a part of World War II history, bring closure to families of the lost crew and discover how one man's life was spared.