The U 166 was a Type IX submarine built in Bremerhaven, Germany and commissioned in March 1942. She had a short life as she was sunk in July of that same year.
The U 166 was the only submarine sunk in the Gulf of Mexico. For decades its location remained a mystery. She was finally located in 2001 during a pre-lay survey for an offshore pipeline. However, controversy still remained about her sinking. Robert Ballard was able to solve that mystery. How he did that is described in the video below.