One of the most daring clandestine operations of World War II was the 1944 sinking of the Norwegian ferry Hydro with its cargo of "heavy water" destined for the Nazis' secret atomic bomb project. Although the mission was declared a success, no one ever established if the special shipment was actually on board. In this program, NOVA descends 1,300 feet beneath a remote Norwegian lake to find the answer. Exploring the pristine lake bottom with a remotely operated vehicle, the expedition team locates the remarkably well-preserved ship, along with evidence of a mysterious cargo in steel drums. Analysis of the contents of one of those drums will solve a six-decade-long mystery about the role the Allies played in preventing a Nazi nuclear bomb. The program features participants in the Hydro affair, including a member of the Norwegian Resistance who slipped aboard the vessel on the night of February 21, 1944, and helped plant explosives in the bow that were timed to go off the following day when the ferry was over the deepest part of Lake Tinn. Intelligence had indicated that the Hydro would be transporting railroad flatcars loaded with barrels of heavy water produced by the nearby Norsk hydroelectric plant, which at that time was the world's largest power station. The Germans had conquered Norway early in the war and immediately ordered the Norsk plant to double its output of heavy water. Crucial to the Nazi nuclear program, heavy water was extracted from ordinary water by using electricity to break apart ordinary water molecules and concentrating the solution until all that remained was the rare, heavier form of the liquid. With a bigger, "heavier" nucleus than ordinary water, heavy water was an ideal substance for slowing neutrons in a nuclear reactor, a key step in triggering a chain reaction (see Dangerous Water). With a sufficient supply of heavy water and uranium, the Germans could use reactors to produce bomb-grade material for nuclear weapons that would render the Third Reich invincible. Fear of that outcome sparked the Allies to undertake their own crash program. This became the Manhattan Project, which ultimately produced the first atomic bomb. The Norwegian partisans had no inkling of the reason for their mission. All they knew was that it had top priority from their contact in London and that innocent Norwegian civilians were likely to be aboard on the last, fatal voyage of the Hydro. (To read actual telegrams sent between the saboteurs and their chiefs in London, go to See the Spy Messages.) NOVA interviews one of the civilians who survived the sinking and who remembers seeing barrels floating among the debris. These barrels were immediately recovered by the Germans and shipped to Berlin. However, had they been filled with heavy water they should have sunk, not floated. This is just one of the mysteries NOVA solves by snaring a barrel, bringing it to the surface, and seeing just what's inside.