 Only once have two submarines dueled, with one submerged submarine sinking another. In February 1945, HMS Venturer hunted down and sank the U-Boat U864 off the coast of Norway. The WWII German submarine was loaded with 65 tons of mercury destined for use in Japanese weapons.  Timewatch  examines the u-boat s final hour from both sides of the conflict and explores the dangers of the toxic wreck today.