Almost 70 years after its sinking, the powerful headlights of two high-tech research submersibles illuminate the silhouette of the biggest German battleship ever. The pride of the German Kriegsmarine: Bismarck. In 1941 the Bismarck was sent out to raid and destroy allied supply convoys in the North Atlantic. With resources vital to their nation under threat, the British fleet went after this supposedly unsinkable ship. But who actually sank the steel giant?