For the first time in history, National Geographic and a group of scientists search the ocean floor for a series of shipwrecks many Americans never heard about. Everyone knows that on December 7, 1941, the Japanese attacked Pearl Harbor. But what most don't know is just days later, Hitler sent his own force to devastate the East Coast. On December 19, the German Naval War Staff sent three U-boats to American waters. It was an assault so deadly, it was covered up by the U.S. government.