Sunk off Gibraltar?
As the war drew to an end, Admiral Doenitz, commander of the German Navy - the Kriegsmarine - ordered all his remaining U-boats on a desperate mission to disrupt Allied shipping between the United States and Europe. At the end of 1944, three U-boats left their base in occupied Norway and headed towards the North American coast, only to be given new orders to redirect their attack near the Strait of Gibraltar. Two of the U-boats acknowledged the orders. The third, the Type IXc U-869, sailing on her first patrol, was never heard from again.

According to US military records, U-869 was sunk off Gibraltar in 1945. Kohler and Chatterton spent nearly a decade trying to prove otherwise. On their third exploratory dive on the wreck, the divers had entered a breach in the hull and inside discovered swastikas, Nazi eagles and other artefacts that had left them in no doubt that this was a wartime U-boat of the Kriegsmarine. They had also retrieved a knife engraved with the name 'Horenburg'.

Subsequent research at the U-boat archive in Germany revealed that only one man with the name of Horenburg had served on a U-boat - the mysterious U-869. But the German records tallied with those of the US Navy - that the U-869 had met its end off Gibraltar. The knife was not clinching proof of another fate.

Deadly hazards
To confirm the identity of the U-boat, the diving team were left with no option but to retrieve the hull plaque deep within the recesses of the sunken boat, a mission fraught with danger. The U-boat lay at a depth of 70 metres (230 feet), 30 m (100 ft) below the safe depth for recreational diving. The divers had to use a special Trimix blend of nitrogen, oxygen and hydrogen to reach the wreck.

Once inside, they had to negotiate jagged wreckage and tangled debris, an environment posing deadly hazards to even the most experienced. In six years, the wreck claimed the lives of three divers. But the team pressed on, in part because of their sense of responsibility to the families of the lost U-boat crew.

In the winter of 1997, John Chatterton devised a daring but highly dangerous plan to get past the wreckage and into the U-boat's electric motor room to retrieve the plaque that could confirm the evidence provided by Horenburg's knife.

Had the divers found the elusive U-869? If they had, why was its wreck found so many thousands of miles from where it was supposed to be? And what caused the U-boat to meet such a terrible fate?

Using ingenuity backed up by years of expertise in the cold waters of the Atlantic, the divers attempt to answer the first question. By going back to the archives of that most secret of codebreaking centres, Bletchley Park, they find the clues that could answer the second question. And their detailed knowledge of U-boats provides them with a good idea of how U-869 was destroyed.