Britain and Ireland, island nations, have the richest of maritime histories. Thousands of vessels lie wrecked off their shores. Many sank in time of World War and are yet to be discovered. Deep below the surface of the sea, they remain silent witnesses to conflict and carnage. At over fifty metres in depth, they've been too difficult to find; too deep to dive; too dangerous to film. Among those hidden victims are some of the largest and most impressive ships ever to plummet to the bottom - giant battleships, massive cargo carriers, and towering ocean liners. The latest advances in survey and dive technology finally locate and bring these deep-sea wrecks vividly to the screen, exposing a world of deadly intrigue and cover-up. Filmed by an elite underwater camera-team operating at extreme depths; illustrated by state-of-the-art computer graphics; backed up by analysis from top wreck scientists and historians; illuminated by rare eyewitness testimony from survivors.