The Norsk Hydro plant at Vemork, Norway, was the world's only industrial producer of heavy water (deuterium oxide), a key component Germany needed for its nuclear reactor experiments. After an initial glider-borne assault (Operation Freshman) ended in disaster with all commandos killed or captured, SOE launched Operation Gunnerside in February 1943.
Six Norwegian commandos, trained in Scotland, parachuted onto the Hardangervidda plateau. They endured brutal Arctic conditions, at one point surviving on reindeer moss, before linking up with an advance party that had been living in the wilderness for months. Rather than assault the heavily guarded bridge approach, they descended into a 600-foot gorge thought to be impassable, crossed a frozen river, and scaled the cliff on the other side.
They entered the plant through a cable tunnel, placed explosive charges on the heavy water electrolysis chambers, and escaped without firing a shot. The team then skied 250 miles to neutral Sweden. The sabotage set back Germany's nuclear program by months and is considered one of the most successful commando raids in history.