In April 1944, a German submarine prowling the waters off Nantucket torpedoed an American tanker caught straggling behind its convoy. The U-boat took cover beneath the sinking ship to avoid detection, but the flagship USS Joyce closed in and delivered a punishing depth charge attack that forced the damaged vessel to the surface.

Under fierce attack, U-550 sank stern first. There it lay, its final resting place an enduring mystery for nearly 70 years.

But earlier this week, after years of research and days of painstaking searches of the ocean floor, a crew discovered the elusive craft about 70 miles south of Nantucket. Crew members said the submarine was among the last undiscovered German warships along the eastern seaboard, where it once attacked merchant ships and forced blackouts in coastal cities.