On a freezing Sunday morning in February 1944, an ordinary Norwegian railway ferry exploded without warning and sank to the bottom of Lake Tinn. This episode explains why the SF Hydro, a steam ferry built in 1914 to move fertilizer and passengers between the Rjukan and Tinnoset lines, became the final target in the Allied campaign against German heavy water. We break down what heavy water is, why deuterium makes it an ideal neutron moderator for a reactor using unenriched uranium, and how the Norsk Hydro plant at Vemork went from producing about 20 kilograms a year in 1939 to five kilograms a day under occupation.

After the failed Operation Grouse glider mission and a November 1943 U.S. bombing raid that killed 21 civilians but could not penetrate seven stories of reinforced concrete, the Germans decided to ship the remaining stock back to Germany, creating a single vulnerable bottleneck on the ferry. We follow plant insider Kjell Nielsen’s one-day delay to push the shipment to a low-traffic Sunday, Knut Haukelid and Rolf Sorlie’s two hours in the keel wiring 19 pounds of plastic explosive to alarm-clock fuses, and the deliberate placement of the charge in the bow so the propellers would lift clear of the water and the ship could not be beached. Eighteen people died and 29 survived, pulled from the water by local farmers who chose to rescue German soldiers along with their neighbors.

• Why IG Farben’s 1939 request for five liters of heavy water was refused by the Norwegian government
• How a saboteur bluffed a guard by claiming he just needed a warm place to sleep
• The moral agony of sinking a civilian ferry and the effort to minimize casualties
• The decoy theory that haunted the saboteurs for decades
• The 1990s ROV discovery of the wreck and the sealed barrel that proved the heavy water was real