The hydrogen plant at Vemork in Norway, some 50 miles west of Oslo, was a mass producer of heavy water (deuterium oxide), an essential component in the creation of plutonium for early atomic weapons. With the German invasion and occupation of Norway, the British authorities were concerned the Nazis would increase production and transport the heavy water to Germany to step up their development of a nuclear bomb. Several bombing raids and commando attacks were carried out, delaying production and destroying much of the water already produced. Eventually the Germans decided to move the remaining stocks of potassium hydroxide, used to produce the heavy water, to Germany and to continue production there. The only way to transport heavy items from this area of Norway was using the local ferry service, which operated three steam-powered ferries: Hydro, which had been built in 1914, Rjukanfos and Ammonia. The Norwegian resistance and the British authorities  decided to wait until the barrels containing the remaining heavy water and potassium hydroxide were aboard the ferry, and sink it in deep water in Lake Tinnsjø.

On Sunday, 20th February 1944 the railway wagons and material were shunted to the ferry station at Mæl and loaded aboard the ferry Hydro. The resistance had been informed by directors from the Norsk Hydro plant of the transport arrangements. Saturday night three agents boarded the ferry and placed charges on the keel, before leaving again. The charge – eighteen pounds of plastic explosive and two fuses made from alarm clocks –was placed near the bow. Although the weather was good on the day, the water temperature was –9°C, so the resistance wanted the sinking to happen as close to shore as possible to give any Norwegian survivors at least some hope of being rescued before freezing.

The bomb exploded at 10.30am: the ferry listed and then sank quickly. At this point the lake was 1,410 feet deep – too much for the Germans to attempt to salvage the barrels. Despite the preparations of the resistance, 14 Norwegian crew and passengers died, as well as a number of German soldiers, although 29 Norwegians were saved. Around 20 years ago using modern salvage techniques, some of the barrels were recovered, and were found to have contained high quality heavy water that would have been crucial to the Nazi atomic bomb project. After the war the action at Vemork was considered to be one of the most successful sabotage acts of World War II: it formed the basis for a very successful film, The Heroes of Telemark, and a later TV series, The Heavy Water War.