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On 19 November 1942, a pair of Royal Air Force Halifax bombers shouldered their way through thick winter clouds over Norway with troop-carrying assault gliders in tow. Inside each glider a payload of professional saboteurs from the 1st British Airborne Division weathered a rough ride as the planes approached their intended landing site on frozen lake Møsvatn. Somewhere in the snow-encased hills below, a team of Norwegian commandos vigilantly awaited their arrival.

The ultimate objective of the joint mission was to penetrate and incapacitate the Vemork hydroelectric plant, a fortified Nazi facility nestled high in the mountains of Norway. Though the plant’s original purpose had been the production of electricity and fertilizer, the German occupiers were capitalizing on the facility’s ability to collect large amounts of heavy-water⁠— a key ingredient in the Nazi effort to develop an atomic bomb.

Scientists at Vemork first observed the curious heavy-water in 1934 when it appeared as a by-product of their revised ammonia production process. Physically and chemically the substance is similar to ordinary water, but while the hydrogen atoms in normal H 2 O consist of one proton and one electron, many of the hydrogen atoms in heavy-water have the added weight of a neutron⁠— an isotope known as deuterium. This deuterium oxide (D 2 O) does exist in water naturally, though its ratio is normally only about one part in 41 million, so it had not been previously observed in significant quantities. For eight years Vemork’s scientist had been collecting the exotic liquid for scientific scrutiny, supplying samples to the world’s researchers for basic experiments. The Nazis’ interest, however, was more considerably more sinister.

In the late 1930s a group of German physicists discovered that certain rare isotopes of uranium are fissile, meaning that their nuclei become unstable and split when they absorb an extra neutron. The nucleus shatters into two smaller nuclei⁠— which repel one another with great energy due to their mutually repulsive electric charges⁠— and shrapnel consisting of fast-moving free neutrons. Soon scientists realized that a chain-reaction would be possible inside a clump of fissionable material since the neutrons spawned during one fission could trigger subsequent fissions, and those would trigger more fissions, and so on. Depending on the conditions, this could produce a long-lived source of heat and neutrons, or a short-lived source of exploding and death. They also speculated that a self-sustaining chain reaction would be easier to maintain if they could identify a substance able slow down the loose neutrons to increase their chances of being absorbed.

Frozen heavy-water in ordinary H20

The nuclear Nazis identified Norway’s heavy-water as one of the best candidates to act as this neutron moderator, so when German forces invaded in 1940 the Vemork plant was an asset they were quick to snatch. Under tightened security, the German scientists doubled the heavy-water production capacity and began shipping barrels of the material back to the weapons laboratories in Berlin. The Norwegian civilian workers knew nothing of nuclear bombs or neutron moderators, but the Nazis’ conspicuous interest in the substance prompted members of the resistance to report the activity to British intelligence.

By 1942 the Allied leaders were certain that the heavy water was a critical component in Hitler’s effort to produce an atomic weapon. Such neutron moderators were not necessary in atomic bombs, but the German physicists hoped to use heavy-water to moderate a sustained reaction within their stash of rare uranium-235. They could then expose nuggets of the most common uranium isotope (uranium-238) to the slow neutrons spewing out of the reactor, allowing some of the uranium nuclei to slurp up an extra neutron to become uranium-239. U-239 atoms tend to undergo beta decay a couple times over the course of a few days, finally resulting in weapons-grade plutonium-239.

The Allies could not sit idly by as Hitler’s henchmen made progress in nuclear weaponry, otherwise the war was sure to come to an abrupt and disagreeable end. The British Royal Air Force considered a nighttime bombing raid on the Vemork to be “unrealistic,” so a covert ground assault was mounted. On 19 November 1942, thirty Royal Engineers crowded into a pair of troop gliders and rode to the frozen landscape of Norway towed behind Halifax bombers. In the mountains near the power plant, an advance team of Norwegian commandos waited near the landing zone while the planes struggled through the soupy skies.

As the drone of aircraft engines crept over the horizon towards Jens Anton Paulsson and his three men, there was a dull explosion in the distance. Once its echoes faded only one aircraft could be heard. One of the Halifax bombers had struck a cloud-obscured mountain. The glider pilot⁠— who had managed to cast off from his ill-fated tug at the last moment⁠— executed the most graceful crash he could given the mountainous terrain. The remaining airplane circled the area with its own glider in tow as the crew struggled fruitlessly to contact the landing beacon. Eventually they were forced to give up due to low fuel, but as the bomber set off towards home its tow line broke and sent the second glider diving into the snowy hills.

A Halifax bomber towing a Horsa glider

The Germans wasted no time dispatching Gestapo troops to investigate the commotion. Paulsson and his Norwegian resistance fighters knew they could not reach the distant crash sites ahead of the Germans, so they retreated to their mountain hideaway to await instructions. For three long months the men subsisted on whatever moss and lichen they were able to scrounge in the sub-zero temperatures, their diets punctuated by the occasional bit of edible wildlife. Meanwhile the survivors from the crashed gliders were captured, questioned, tortured, and executed under Hitler’s top-secret Commando Order which stipulated that all enemy commandos were to be put to death without exception.

On 19 February 1943, six of the Norwegians’ countrymen finally arrived by parachute with a fresh supply of food, weapons, and explosives from their British supporters. Following an exchange of greetings, Joachim Ronneberg took command of the group and laid out their attack plan. Once everyone had recuperated, the ten Norwegian men strapped on their skis and set out armed with rifles, submachine guns, chloroform rags, and cyanide suicide pills. Though they had been given no specific details regarding the power plant’s purpose, the men had been assured that its destruction would prevent Hitler from gaining the ability to smash entire cities with a single strike.

At three o’clock in the morning on 28 February, the gang of intrepid Norwegians approached their target. The Vemork hydroelectric plant was perched on the edge of a six hundred foot cliff like a fairytale fortress, and accessible via a 240-foot-long bridge which spanned a deep ravine. The area was peppered with mines, and the bridge itself was well-guarded and brightly lit. Rather than tangle with sentries and landmines, the force elected to descend into the gorge and clamber up the cliff on the other side. The resistance fighters soberly exchanged wishes of good luck then skied down to the ravine floor.

The heavy water collectors in Vemork

After completing the long and treacherous climb up the icy cliff, Knut Haukelid took command of five of the men and broke off to assume covering positions outside the German barracks. The other four split into two demolition teams, each with a full set of explosives in case one of the teams was unable to reach the target. The four men headed to a basement door which they had been told would be left unlocked, but the undercover operative in charge of the task had fallen ill and missed work that day. The two teams separated to seek alternate points of ingress.

Joachim Ronneberg and his partner Fredrik Kayser soon located a hatch which allowed access to a narrow shaft full of wires and pipes, but the men discovered that there was sufficient room to squeeze through. As the factory’s machinery softly grumbled, the pair slowly crawled through the long duct while pushing their explosives ahead of them. At the end of the tunnel the men climbed down a ladder and surveyed their target: a long row of metallic cylinders lining the wall of the heavy-water concentration room. The two raiders sprang into the compartment and caught the lone night watchman completely by surprise. He eagerly complied with their orders to raise his hands, then stood trembling as the armed intruders locked all doors leading into the room. Ronneberg dashed over to the heavy-water tanks and immediately began to place his eighteen explosive charges.

As Ronneberg worked, the factory’s low, steady hum was punctured by the sound of shattering glass from the far side of the room. He and Kayser spun around with weapons at the ready. Through the window emerged the two men of the other demolition team, having been unable to find a more suitable entrance. Together the men set and checked the series of charges, and laid fuses which had been cut to provide a delay of only thirty seconds. A Norwegian civilian wandered into the room and was astonished to see a clutch of commandos putting the finishing touches on their demolition charges. He obediently thrust his arms into the air and joined his captive colleague.

The Vemork plant following an Allied raid

Ronneberg lit the bombs’ fuses and quietly counted to ten. He then ordered the anxiety-stricken prisoners to run upstairs as fast as they could. Hoping to prevent reprisals against the local populace, the raiders dropped a British machine gun on the floor to disguise the attack as the work of British agents. The demolition teams rejoined their comrades outside and the together they dashed away at full speed. After several long moments, a muffled thud was heard from the Vemork building behind them. Three thousands pounds of D 2 O sloshed out of the damaged tanks and into the factory’s drains, destroying four months’ worth of production and severely crippling the heavy-water-gathering apparatus. By the time the Germans realized they were under attack, the ten Norwegian men had donned their skis and slipped away to the safety of the mountains.

The saboteurs had successfully silenced the water plant, but German engineers began repairs immediately and within five months their heavy-water collectors were back in action. By the following winter the Allies had the means to attack the target by air, and during one long day in November 1943, one hundred and forty three American B-17s ambled over the horizon and pounded the Vemork complex area with over seven hundred bombs. Due to the terrain many of the bombs missed and most of the structure managed to remain intact, but the forceful series of attacks persuaded the Germans to abandon the plant.

In a last ditch effort to salvage the remains of the operation, the Nazi scientists loaded their massive bounty of heavy-water into a railcar. Under the care of a large guard detail the precious deuterium oxide began its journey to Berlin. The armed procession boarded a railcar ferry to carry it across lake Tinnsjø, and as the boat crossed the deepest portion of the lake there was a sharp bang below decks. The ferry foundered and sank, dragging the bulk of Germany’s atomic bomb program into a deep and watery grave. The Norwegian saboteur Knut Haukelid⁠— the man who led the covering team on the raid against Vemork⁠— had learned of the plans to move the cargo, and smuggled a makeshift time bomb aboard the ferry before the Germans arrived. Unfortunately fourteen civilians were killed when the boat sank, but resistance leaders reasoned that these losses were acceptable considering the thousands of lives that would have been forfeit if Hitler’s nuclear program had come to fruition.

Most of the Vemork raiders

Though the Norwegians’ handiwork did not manage to completely halt the progress of the Nazi’s atomic bomb project, it created significant stumbling blocks. According to some controversial reports, the Nazis did manage to build and test a small nuclear device just before the war ended, but it was reportedly a crude design far inferior to the bombs dropped on Japan some months later by the US. In any case, Nazi Germany certainly possessed the knowledge and skills necessary to construct a bomb; they merely lacked the resources.

In modern history there are few examples of such small works of sabotage leading to such dramatic effect. By some estimations, the raids at Vemork were all that prevented Hitler from gaining control over Europe and ruling with a plutonium fist. Indeed, had the Nazis worked unhindered, the world’s first atomic mushroom cloud may have loomed over London by the mid-1940s. In that respect, these stalwart saboteurs and their daring mission in the mountains of Norway may have spared the world from a far worse fate.