This imposing concrete structure in the German city of Bremen is 1400-feet long, 300-feet wide, and 90-feet high, and looks every inch the secret Nazi facility that it is. Its purpose was to build and test U-Boats. 🎥WWII By Drone | Smithsonian Channel Aviation Nation

It's so rare that submarines face off underwater that this February 9, 1945, encounter is the first and only such dogfight in recorded naval history. Stream this story and more on the new season of Hell Below: http://bit.ly/2PnVqwW | Smithsonian Channel Aviation Nation