More new discoveries have been made underwater in the last 25 years than ever before. Unique aeroplanes, submarines and great ships - like the Titanic. What should be done with them? Salvage them for scrap or save them as historic monuments? In Scapa Flow, every year, the Royal Navy conducts a service of remembrance over one of Britain's biggest war graves - HMS Royal Oak. At Pearl Harbor, Hawaii, is a similar emotive war grave - the battleship USS Arizona. The island of Guam in the middle of the Pacific was the scene of some of the fiercest fighting in the Second World War. There, a Japanese cargo ship, sunk in 1944, lies alongside a German auxiliary cruiser, sunk in 1917. A unique memorial to two world wars.