Silent killers of the Third Reich, U-boats scoured the oceans, hunting enemy vessels and the supply ships from around the globe that kept the allied war effort afloat.

The job was not without risk – three of every four German submariners were killed in action. The fate of most is known, but in 1991 American diver John Chatterton discovered a wrecked U-boat, 60 miles off the coast of New Jersey, that presented a true mystery. John, with his diving partner Richard Kohler, set out to examine an unidentified wreck, 230 feet (70m) down, expecting to find an old barge.

Instead they discovered the sleek and deadly lines of a U-boat, now a remote steel coffin for the 50-odd crewmen still on board. But when the men reported the discovery they found no one in America, Britain or Germany knew why a U-boat was there. Over the next six years, Chatterton and his crew explored the wreck during summer and continued their dogged research during winter. Then the team made a breakthrough, or so they thought. A knife was found, inscribed with the name Horenburg.

The German navy records listed only one sailor named Horenburg, who served on U-869, but records show U-869 was sunk off Gibraltar. Had Horenburg simply traded his knife? Or were the records somehow wrong and the U-869 either survived at Gibraltar, or was it never there? In an intrepid investigation that cost three lives, Chatterton and Kohler were determined to get to the bottom of the puzzle, but even they were not prepared for what they finally learned on a visit to Germany.

Hitler's Lost Sub is now available to buy on DVD here