By 1942, Allied intelligence had concluded something alarming: Germany was actively pursuing an atomic bomb, and the key ingredient they needed was heavy water — deuterium oxide, a special form of water with an extra neutron — which was being produced in vast quantities at the Vemork hydroelectric plant in Telemark, Norway.
If Germany acquired enough heavy water, the physics suggested they could build a working nuclear reactor, and from there, potentially a bomb. The problem was that Vemork was essentially impregnable. It sat on a cliff face, accessible by a single suspension bridge, in the middle of a freezing Norwegian winter.
The British SOE trained a group of nine Norwegian resistance fighters — young men who had fled Norway after the German occupation — in Britain. In late February 1943, they skied across the Hardangervidda, one of Europe's highest and most brutal mountain plateaus, in winter conditions that had already killed two previous British commando teams sent on the same mission.
Rather than crossing the obvious bridge, the Norwegians descended into the near-vertical gorge below the plant, crossed the icy river at the bottom, and climbed up the cliff face on the other side. A handful of men penetrated the most heavily guarded facility in occupied Norway and placed explosive charges on the heavy water production equipment.
The explosion was so contained that German guards in the building heard only a small bang. By the time they investigated, the Norwegians had vanished back into the night, leaving only a British tommy gun behind — deliberately, to prevent German reprisals against Norwegian civilians.
The damage to the heavy water production set the German nuclear program back by months. American bombing raids later that year further disrupted production. When the Germans attempted to ship their remaining heavy water stock to Germany in early 1944, Norwegian resistance agent Knut Haukelid planted a bomb on the ferry carrying it, sinking the shipment to the bottom of Lake Tinnsjø, where it remains to this day.
Germany never built an atomic bomb. Historians continue to debate how close they actually were, but the destruction of the heavy water supply ensured they would not have the chance to find out.