Telemark: The Skiers Who Sank Hitler's Heavy Water

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Four men who had waited for months on the snow-covered plateau of Hardangervidda, living on reindeer moss, were waiting for the basement of a single fertilizer plant. Because it was the only place in Europe producing the substance Hitler's reactor needed. And the road to that plant ran along the bottom of an icy gorge two hundred metres deep.

When a factory's name enters history, it is usually not because of what was made there; it is because of the possibility that what was made there planted in other people's minds. In southern Norway, in the region of Telemark, just above the town of Rjukan, a plant called Vemork stood pressed against the steep wall of a narrow valley as though glued to it. It was exactly that kind of place. The work done there was one of the dullest jobs in the world: raw material for fertiliser.

When it opened in 1911, Vemork was the largest hydroelectric power station in the world. This was the whole of Norway's wealth: water falling from mountains. The force of the Rjukan waterfall was harnessed to turbines, that force was used to fix the nitrogen out of the air, and from nitrogen came artificial fertiliser. The company that ran the business, Norsk Hydro, built a hydrogen plant beside the power station in 1929. Hydrogen was obtained by splitting water with electricity, and that splitting had one strange by-product.

Not all the water in the world is identical. About 1 hydrogen atom in 6400 carries an extra neutron in its nucleus, and this heavy hydrogen is called deuterium. When Harold Urey discovered the isotope in 1931, and when a few years later the discovery won him the Nobel Prize, the matter was entirely academic. Water made with deuterium is called heavy water. It cannot be told apart from ordinary water by eye, it is slightly denser, it boils at a slightly higher temperature, and its ice sinks. Its proportion in nature is so low that obtaining it in pure form means putting tonnes of water through the same process again and again.

That was Vemork's secret. When you split water with electricity continuously in order to make hydrogen, the proportion of heavy water in what is left behind rises by itself. The plant had something nobody else had: a ready, free, continuously flowing source of heavy water. In December 1934, Norsk Hydro began the world's first commercial production of heavy water. For several years its customers were universities and laboratories. They sold it at 40 dollars a litre, and found few buyers.

At the end of 1939, once it was understood that the uranium nucleus could be split, this plant in a remote valley suddenly became the most strategic factory in the world. To run a chain reaction in a controlled way, you needed a substance that would slow the neutrons down, a moderator. Heavy water was among the best there was. In Germany, the physicist Walther Bothe measured in 1941 that graphite absorbed too many neutrons. The measurement was wrong, the graphite he used contained impurities, but German work was built on that wrong result. The Americans went forward with graphite; the Germans condemned themselves to a single road. That road ran through a fertiliser plant in Telemark.

The Germans were not the first to notice this. In March 1940, Jacques Allier of French intelligence came to a secret arrangement with Norsk Hydro's management, divided the stock the plant had on hand, 185 kilograms of heavy water, among 26 drums, and got it out of the country. The drums went first to France, and when Germany occupied France, on to Britain. The move was the first sign of how flammable a place Vemork was, and it left the German war machine the plant's empty stores.

On 9 April 1940, Germany invaded Norway. Like all the country's industry, Vemork changed hands. Before long the German chemical giant IG Farben ordered Norsk Hydro to increase output. A plant that before the war could extract around 10 kilograms of heavy water a month was now asked for 100 kilograms. A high-concentration room was built in the factory basement: a row of 18 stainless steel cells in which the water was brought to its final and most valuable state.

The chemical giant issuing the orders did not know why the Norwegian engineers were writing those orders down so carefully. Jomar Brun, the manager of the plant's heavy water section, was secretly leaking production figures, factory plans, even photographs of pipes and doors. The information went to London, to Leif Tronstad, a Norwegian chemist and officer. Tronstad was the right man for it, because he had worked on the building of the heavy water plant himself. He had fled the country in 1941 and in London had taken charge of the intelligence and special operations branch of the Norwegian army. The tables in his hands said one thing: the amount of heavy water going from Vemork to Germany was rising. In the middle of 1942, Brun too was smuggled out of Norway, and in Britain the two of them sat down around the same table. There was a single question on that table. How do you destroy the basement of a seven-storey concrete building carved into a mountainside, standing at the edge of a 200-metre ravine, connected to the world by a single suspension bridge?

Bombing from the air was the first thing that came to mind, and it was eliminated at once. Seen from an aircraft, the target was a small rectangle at the bottom of a narrow cleft, in the shadow of a rock wall, and the real target, the cells, sat on the building's lowest floor. With the bombing accuracy of 1942, reaching that basement was a gamble. There were liquid ammonia tanks beside the plant as well, and a single hit could kill thousands of Norwegians living in the town below.

That left the work to be done on the ground. The Special Operations Executive in London decided to send a reconnaissance and reception party to Vemork first. On 18 October 1942, four Norwegians, Jens-Anton Poulsson, Knut Haugland, Claus Helberg and Arne Kjelstrup, were parachuted onto Hardangervidda, the Hardanger plateau. The operation was called Grouse. All four had grown up in that country; they knew what Europe's largest mountain plateau did in each season. Even so, they landed in the wrong place, dragged heavy radio and supply crates across the snow for weeks, and ran out of food. By the middle of December all four were weak with hunger, boiling moss and lichen on the plateau and drinking it. On 23 December 1942, Poulsson shot a reindeer. That animal was what kept the team alive: they ate its meat, its fat, even the half-digested lichen in its stomach, because the vitamin needed to ward off scurvy was there.

While they were waiting for reindeer on the plateau, an entirely different plan had ripened in Britain. A trained demolition party from the Royal Engineers would be sent to Vemork, the men would be landed on the plateau by glider, and the Grouse team would meet them and take them to the factory. The operation was called Freshman. On the evening of 19 November 1942, two Halifax bombers took off from Scotland, each towing a Horsa glider behind it. Counting the pilots, there were 34 men in the gliders.

Not one of them reached the target. Off the Norwegian coast the weather broke, the cloud base came down, icing began. The homing beacon of the Grouse team waiting on the plateau did not give the range that had been hoped for. One of the aircraft, unable to find its way, flew into a mountain as it turned for home, and its crew of 7 was killed. The glider it was towing came down as well. The second glider's tow rope snapped, and the glider smashed into a snow-covered slope hundreds of kilometres from the target. Some died in the crashes; the survivors, wounded, most of them in uniform, surrendered to German patrols.

A month before this, on 18 October 1942, Hitler had signed the order that went into history as the Commando Order: captured commandos, in uniform or not, were not to be treated as prisoners of war, but killed. That is what happened to the men who lived through the glider crashes. One group was shot outside a German barracks near Egersund within hours of being taken. Another group was interrogated for months, moved to the Grini camp near Oslo, and killed there in January 1943. Of the 41 men who took part in the operation, not one came back.

This was not only a disaster; it was also a tip-off. Inside the wrecked gliders were explosive charges, sketch maps and British engineer tools. The Germans looked at the wreckage and learned exactly what Vemork meant to the Allies. Over the following weeks the ground around the factory turned into a different place: machine guns and searchlights were set up at both ends of the suspension bridge, the roads leading to the bridge and the open slopes overlooking the valley were mined, the number of guards was increased, doors were closed with iron shutters. London had had one attempt, and it had used it.

Getting this news to the four men waiting on the plateau amounted to a few coded sentences by radio. The team's code name was changed to Swallow, and the order they were given could be summed up in a single word: stay. In the middle of winter, without supplies, on a plateau swept by German search parties and ski patrols, wait for a second team that might never come. They waited. In London, meanwhile, one thing had become clear: the men who went into Vemork would not be British. They would be Norwegians who knew that valley, that rock, that snow.

On the night of 16 February 1943, six Norwegians jumped from a bomber flying over the Hardanger plateau. The operation was called Gunnerside. At its head was Joachim Rønneberg, 23 years old at the time, and with him were Knut Haukelid, Fredrik Kayser, Kasper Idland, Hans Storhaug and Birger Strømsheim. All were Norwegians, all had trained in sabotage in Britain, and Rønneberg had chosen his team himself. They came down tens of kilometres from the intended point, and on the night they landed a storm swallowed the plateau. For five days they sat in a hunting cabin, waiting for the howling to die down. On 23 February, following ski tracks, they met the four men who had been waiting and starving for months. The first question that came into Rønneberg's mind when he saw these bearded, worn-out, bony men was whether they had the strength left to walk as far as the factory.

Nine men laid the plan in the cabin. The bridge was out: searchlight, sentry, mines. Going round above the valley was out: snow, open slope, tracks. The only route left was the one nobody would think of trying. Go down into the valley of the Måna, the river that runs beneath the cliff Vemork stands on, cross the half-frozen river, climb the opposite wall, and come up onto the single railway line that led to the factory. Local sources said that line had been left unguarded because nobody believed it could be reached by climbing from below. Helberg went down into the valley and tried the route himself, and when he came back he said one thing: it can be done.

On the evening of 27 February, the six saboteurs and the rest of the party crossed the plateau and came to the edge of the valley. The weather was too mild for that winter; the thermometer stood above zero, the snow had grown heavy, the ice was cracking. They climbed down the cliff, crossed the river between blocks of ice, climbed some 150 metres up, and came out onto the railway line. It was past midnight. They waited for the change of guard and walked quietly along the rails. Haukelid's bolt cutters cut the chain on the plant's side gate. Rønneberg had bought those cutters with his own money from a shop in Cambridge, in Britain, because he had decided that the hacksaw he had been issued would be both slow and loud.

In the factory yard they split in two. Four men stayed outside with their weapons trained on the guard hut. Rønneberg and Kayser were the ones who would get into the high-concentration room. The basement door they had expected to use was locked. There was a detail on the sketches in London: a cable duct that entered the building from outside. The two men crawled on their elbows through that duct, among pipes and cables, and dropped into the room itself, directly above the 18 steel cells.

There was one person in the room, a Norwegian watchman named Johansen. He was unarmed and did not resist. Rønneberg and Kayser wrapped the explosive charges one by one around the bodies of the cells. A sound at the window froze them both: two men from the outside party, unable to find the cable duct, had broken the glass and jumped in. The fuses were fitted. Rønneberg cut the fuses, which had been set for two minutes, down to 30 seconds; he did not want to leave without hearing that the explosion had actually happened.

Just as they were about to go, the watchman said something: his glasses. They had been left somewhere in the room, and in wartime Norway having a new pair made was close to impossible. In that room, with the fuses waiting to be lit and the whole climb still ahead of them, the men looked for a pair of glasses. They found them. They told Johansen to run up the stairs, and lit the fuses.

Between thick concrete walls, the explosion gave no more than a muffled thud. The German sentry outside came out of the hut, looked around, decided a mine had gone off by itself, and went back in. At that moment nine men were going down from the railway line into the valley. Behind them, left deliberately in the middle of the room, was one thing: a British Thompson submachine gun. It was a message that this had been the work of British commandos, and that punishing the local population therefore made no sense. The message held. There were interrogations and arrests in Rjukan, but no mass execution.

More than 500 kilograms of heavy water in the cells, the purest form of the product, ran across the floor into the drains and mixed with the water of the Måna. Not a single shot had been fired, and not one person on either side had died. The Germans pushed thousands of soldiers onto the plateau. Five men set off on skis towards Sweden and crossed the border after about 320 kilometres. Haukelid and Kjelstrup stayed on the plateau. Helberg got away from a ski chase with a German patrol, fell from a rock face in the dark and broke his arm, and while being treated at a mountain hotel fell into a far stranger danger when Josef Terboven, the head of the German civil administration in Norway, arrived at the same hotel; he saved himself by jumping from the bus onto which he had been loaded for deportation. The Special Operations Executive, which had planned the operation, counted it the most successful act of sabotage carried out in the entire war.

But destroying a factory and destroying the machinery in a factory's basement are not the same thing. The cells could be replaced, and they were. The Germans piled engineers and materials into Vemork; the plant began producing heavy water again in April and came close to its old capacity by midsummer. The reports reaching London showed the same curve: production was climbing once more. What the February night had bought was a few months.

This time there was nothing left to do on the ground. The factory's defences had hardened one degree further after Gunnerside, and a second team could not be expected to come by the same road. The decision was handed to the American Eighth Air Force. Norway's exile government in London was asked what that decision would mean, and the Norwegian officials objected because of the risk to the town below. Their objection was recorded, and the operation was approved.

On the morning of 16 November 1943, in cloudless weather, 143 B-17 Flying Fortress bombers came over Vemork and dropped 711 bombs. A wave behind them struck the nitrate plant in Rjukan. The aircraft took off from Britain, flew into the interior of Norway and returned; the crews sat in their cockpits for close to 10 hours.

Only a very small part of the bombs fell inside the target area. The narrowness of the valley, the way the target sat hard against the rock wall, the natural spread of bombing from altitude: the result confirmed everything that had been said against bombing in 1942. The upper floors of the electrolysis building and the power station section were heavily damaged, pipelines and the turbine hall were torn apart, but the basement where the heavy water was concentrated stayed standing, thanks to the metres of concrete above it. Most of the bombs that hit anything at all came down on the slopes around the factory and on the edge of Rjukan.

Two of those bombs found a newly built shelter near the plant directly. The shelter had been put up for exactly such a day, to protect the worker families living there. After the hit there were two craters where it had stood. None of those inside survived; most of the dead were women, and there were children among them. The number of Norwegian civilians killed that day at Vemork and in Rjukan reached 21. Houses in the town were destroyed, and six households were wiped out completely. Almost all of the dead were people who had done nothing except live near a factory in their own country that was being run by the enemy. The Norwegian government conveyed its objection to its allies once again, this time in far harsher language.

In military terms the raid was half a success; heavy water production did not stop, and the cells were still in place. But in strategic terms the raid changed everything, because what convinced the Germans was not the damage but the prospect of its being repeated. The reckoning in Berlin was this: this plant in Norway is no longer a place that can be protected. Commandos had got in once, hundreds of bombs had come down once, a third time could happen at any moment, and the whole hope of the German nuclear programme hung on a single valley.

The decision was to wind up the heavy water business at Vemork. The machinery of the concentration plant would be dismantled, whatever heavy water was on hand, including the half-processed liquid, would be carried to Germany in barrels, and production would be set up again there, somewhere bombers worked less comfortably in those years. The dismantling began in December. The liquids in the basement, at different concentrations, were emptied into steel barrels, and their labels were written out as potash lye.

There was still a radio operator on the plateau, and there were still Norwegian engineers working inside the factory. In February 1944 a single sentence of information reached London: the cargo is on its way. It would come down from Vemork by train, cross a lake by ferry, then go down to the coast by train again and on to Germany by ship. The one fragile point on the route was obvious: the lake.

London's man on the Hardanger plateau was Knut Haukelid, one of the two who had stayed behind instead of going to Sweden after the Gunnerside night. When the order reached him there was almost no time left: destroy the shipment.

What stood in front of Haukelid was not a military calculation but a moral one, and he later described it plainly himself. Blowing the cargo up on the train was all but impossible along a route where German patrols moved up and down the rails, and doing it at the coast would be too late. The only place was Tinnsjø, Lake Tinn, and the vessel that crossed that lake was a passenger ferry the people of the district used every day. The railway ferry Hydro carried the wagons on her deck, and on the same crossing there travelled villagers, officials and children. Sinking the ferry meant killing Norwegians.

Haukelid tried to keep the harm to a minimum together with Alf Larsen, an engineer working at Vemork, the local resistance man Rolf Sørlie, and Knut Lier-Hansen. Sunday was chosen, because the Sunday crossings carried the fewest passengers. The place of the explosion was calculated as well: the ship had to be struck at the deepest part of the lake, beyond swimming distance from the shore, but at a point where she would go down quickly. About 8.4 kilograms of plastic explosive were placed below the waterline, against the inner hull at the bow. The aim was not to tear the hull open but simply to make the bow take water: with the ship going down by the head, her propeller and rudder would come out of the water, and the captain would be able neither to steer nor to run her aground. The firing mechanism was made from two alarm clocks.

On the night of 19 to 20 February 1944, Haukelid and Lier-Hansen went aboard. There was neither a sentry nor a lock on deck; most of the crew were playing cards. A Norwegian watchman they met on the lower deck was told they were running from the Gestapo, and the man personally showed them where to hide. The explosive went into the bilge, the alarm clocks were set, and the two left the ship and parted in the dark.

On the morning of Sunday 20 February 1944, the Hydro set out with the wagons carrying the barrels of heavy water on her deck. At 10.45, in the middle of the lake, there was an explosion forward. Within a few minutes the ship went down by the bow, and the wagons broke loose from their fastenings and slid into the water. 29 of the crew and passengers were saved. 18 people died, 14 of them Norwegian crew and passengers, 4 of them German soldiers. The ship went down to a depth of 430 metres and stayed there.

The barrels went down there too. A few of them stayed on the surface for a while; the liquid inside was a little heavier than water, but the barrel itself floated. The Germans reported recovering some of them, but the shipment as a whole never reached Germany. The fluid the German nuclear programme had worked so hard for settled on the bottom of one of Norway's deepest lakes.

After this, heavy water production was never established in Germany. In March 1945, in the town of Haigerloch in southern Germany, in a rock cellar beneath a church, German physicists ran their last experimental reactor: 664 uranium cubes strung on chains, with about 1.5 tonnes of heavy water around them. The arrangement produced neutrons, but it never reached a self-sustaining reaction. The calculations showed that for the same design to work, roughly one and a half times the uranium they had and far more heavy water would be needed. In April an American intelligence team called Alsos entered the cellar and collected the cubes and the notebooks. Only then was it fully understood how far Germany had been from building a bomb.

That opened the meaning of those nights in Telemark to argument, and the argument is still going on. Postwar documents showed that the German programme had been behind from the beginning in resources, staff and political backing, and had stayed years away from a bomb. Even if the heavy water had never been cut off, the cellar at Haigerloch was not going to produce one. But nobody knew that in 1942; the only information on London's table was that the side running the sole source of heavy water was Germany. The decision was made with the knowledge that existed.

Most of the people who built this story did not see its end, or saw it with difficulty. Leif Tronstad, who was behind all the plans, returned to Norway in the autumn of 1944 and, while leading the resistance in Telemark, was shot and killed in March 1945 during a raid in which he had taken a pro-German local official into custody. Vemork's heavy water building was demolished in 1977, with the traces of its concrete foundations left on the site. The old power station is today the Norwegian Industrial Workers Museum, and the same halls hold exhibitions both of these acts of sabotage and of the labour history of a mountain town. This industrial heritage at Rjukan and Notodden was placed on the World Heritage list in 2015.

In 2004 a diving team went down to the bottom of the lake and brought one of the barrels, number 26, up to the surface. The heavy water concentration of the liquid inside matched the low figure written on the German shipping list, and it became clear that the greater part of the cargo had been half-processed water being carried to Germany to be concentrated again. What lies at the bottom of the lake, in other words, is not the heart of a bomb but half of the road towards one.

Joachim Rønneberg, who commanded Gunnerside, lived a long life after the war and died in 2018 at the age of 99. He never liked being called a hero, and whenever he spoke about that night he said the same two things. The first was that the secret of success had not been courage but weeks of preparation and a team whose members knew one another. The second was that the only reason what they had lived through was worth telling at all was to show young people how meaningless war is. Inside the same story there are also the engineers shot near Egersund, the women and children who died in the shelter at Vemork, and the Norwegian passengers who drowned in Lake Tinn. The story of the skiers who sank Hitler's heavy water is the story of a series of decisions taken in one valley, and most of those who paid for those decisions were never in the room where they were made.

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