the below is a summary of a Nova program.
In the 1990s, the wreck of an unidentified U-Boat was discovered on the ocean floor off New Jersey in approximately 230 feet of water. It had suffered a massive explosion in the center of the pressure hull, and the conning tower was blown completely off the boat. The wreck was upright on the bottom. The divers who discovered it decided to try to identify it, a task they thought would take them a few days. However, this was not to be the case.
The first problem they had was that the aft section of the boat, which contained the Diesel motor room and the electric motor room, were not only reasonably intact but were sealed by debris at the forward end of the Diesel room. This was problematic because the electric motor room was where U-Boats stored spare parts, which were labelled with the boat's number using metal and laminate tags that would have survived the years of immersion.
During their searches of the accessible crew compartments, the divers turned up a knife with the name Horenberg carved into its handle. They took this knife to Germany and searched the archives of the U-Boat service. There, they found that there was only one man named Horenberg in the Kriegsmarine during the Second World War; he had been last posted to U-Boat U-869. However, the archives also showed that U-869 had met its end near Gibraltar, on the Atlantic side of the Straits, in early 1945.
The divers continued to work the wreck for six years, despite three fatalities. One diver was simply swept away by currents, his body not found for six months; it was never determined what had killed him. A father and son team were diving the wreck when the son became trapped; the father extricated him, but they were low on air and unable to find their reserve tanks. They surfaced, without decompressing. The father died on the boat, and the son died in a compression chamber in hospital.
Another artifact was found; the remnants of an aluminum engineering diagram. In the lower right corner, the symbols IX-C 40 and the word Bremen could be made out. The searchers deduced that the boat was a Type IX-C U-Boat, built in the Bremen yards in 1940. This narrowed it down to perhaps twenty or twenty-five boats out of the nearly 1,000 U-boats built in Germany during the war.
Finally, in a daring (some felt, foolish) maneuver, the team leader penetrated the aft electric motor room. He squeezed past the obstructions in the Diesel room (a fallen oil tank) by removing all but one tank of tri-mix (breathing gas), and holding that one loose in front of him. This gave him twenty minutes of air. The first attempt nearly ended in another death; he was trapped by a piece of steel, and his backup diver was forced to surface due to low gas levels; luckily, he freed himself and made it back to the surface. The second attempt went more smoothly, and he recovered a boxed spare part which was examined on the surface as he removed his gear.
On the box was a tag, reading U-869. The archives were incorrect. They had found the grave of the U-869, a Type IX-C, built in Bremen in 1940. During the final days of 1944, it sank off the coast of New Jersey while patrolling U.S. coastal waters for unprotected shipping.
Due to confusion about her final orders, however, the German Navy (and the Allied auditors after the war) all were convinced that U-869 had met her end near Gibraltar.