On the night of 27 February 1943, in a valley some 170km west of Oslo, nine Norwegian commandos climbed down a steep gorge and across an ice-choked river. On the top of the other side, a railway track led straight to the Vemork hydrogen-electrolysis plant at Rjukan – then the world’s only producer of heavy water. The plant, controlled by Germans in Nazi-occupied Norway, had become an essential element of Hitler’s nuclear programme. The saboteurs cut open the railway gate, entered the plant and mounted two explosive charges. Vital parts of the plant were destroyed, together with weeks of heavy water production.