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  ...