On December 23, 1942, Jens-Anton Poulsson skied alone across the Hardanger plateau, or Hardangervidda, in the Telemark region of Norway, one of the most hostile mountain areas in Europe. Poulsson had grown up in the town of Rjukan—a few days’ ski away—and had even built a cabin on the plateau with his family years earlier. But in 1940, Hitler’s blitzkrieg swarmed Norway. As Poulsson skied onto the dazzling, white plateau, he was hampered by exhaustion and malnourishment, but he was also dogged by something else: Despite being on home turf, he was behind enemy lines. And if he didn’t find food, he and his men would starve.

Poulsson was one of many Norwegians who’d made their way to England during the outset of the war and he was one of a smaller number that had been trained as saboteurs and commandos by a branch of British intelligence called the Special Operations Executive, or SOE. He and three companions, Arne Kjelstrup, Claus Helberg, and a radio operator, Knut Haugland, had parachuted onto the Hardangervidda from Great Britain on October 18, 1942. Though the four young men had no way of knowing, they were embroiled in one of the most important efforts of sabotage in World War II.

By 1942, a team of German scientists—including the Nobel-prize winning physicist Werner Heisenberg—worked on what the Allies feared was an atom bomb. One of the main components in Heisenberg’s experiments was a substance called heavy water, or D20, and the only factory producing it was a hydroelectric plant called Vermork, in the small town of Rjukan, in occupied Norway. Allied spies confirmed the Nazis were shipping large quantities of heavy water out of the plant to Germany. With shipments from Vermork, Heisenberg’s team could theoretically build a weapon to win the war.

Vermork’s natural defenses made destroying the plant a difficult proposition. Fortunately, the Allies had an ace in the hole: a scientist-turned-resistance fighter named Lief Tronstad, who had worked at Vermork before escaping to England and joining the SOE. A small team of men stood a chance of monkey-wrenching the heavy water facility, located deep in the basement. Tronstad recruited members of the Norwegian resistance who, like Poulsson, had escaped to England. The best were sent to an all-Norwegian unit: the Kompani Linge. Though the hand-picked men were well-trained fighters, the four Norwegians selected weren’t chosen for their competency in battle so much as their ability to survive the harsh winter on the plateau. The German threat was dwarfed by the danger of frostbite, starvation, and exposure. The Hardangervidda offered a place to hide, but it could also kill the men in minutes. Above all, the saboteurs had to be extremely talented skiers and survivalists.

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On October 18, Poulsson and his men—code-named “Grouse”—parachuted into Norway. They were instantly swallowed by a raging storm and pinned down for a day before they could move. All that week, they skied toward a cabin that would serve as a base camp. A sudden thaw slowed their progress as they dragged heavy loads through the isothermic snow. Poulsson fell through the ice on one of the Hardangervidda’s many lakes; Kjelstrup saved him with a ski pole. Finally, they reached their destination, a cabin outside of Rjukan.

The Hardangervidda in winter.

Initially, Grouse was not supposed to participate in the actual sabotage. Poulsson’s team would coordinate a glider landing on the Hardangervidda for 35 British commandos, who would raid the plant. But on November 20, the two gliders crashed in a storm, miles away from the landing zone. The Englishmen inside were captured, tortured, and executed by the Gestapo. The SOE scrambled for backup plans. Grouse’s new mission, the SOE radioed, was simply to wait—a task easier said than done for four men hiding in the desolate Scandinavian winter. With the deaths of their British counterparts and the knowledge that the Gestapo likely knew men were hiding on the Hardangervidda, the Norwegians’ vigil took on a new pall of desperation.

By the time Poulsson strapped on his skis and shouldered his hunting rifle on December 23, he and the rest of the Grouse team had been rationing their scant food for two months, living on foraged moss mixed with oatmeal. They were bearded, emaciated shadows of the men who’d parachuted into the bitter air above their homeland in mid-October. But Poulsson’s luck was about to change. The plateau, for once, shone with the sun, a dazzling change to the storms Grouse had suffered. He made out a mass of dark shapes moving across the Hardangervidda. He’d found the reindeer herd. After a desperate chase, he managed to bring one of the beasts down. His men would have real food again.

In Scotland, the SOE weighed their options. A bombing of the plant would kill civilians in Rjukan, too. Tronstad argued vehemently against such an option. In late November 1942, the SOE recruited five more Norwegian fighters to join the strung-out Grouse team. Led by a tall, Clint Eastwood look-alike named Joachim Rønneberg, the new group, code-named Gunnerside, would parachute in and attempt a raid on the plant. If all went well, they’d then ski 280 miles through the wilderness to neutral Sweden. Though Rønneberg was only 23, he possessed that immutable quality of leadership vital to keeping men alive in harsh environments. His men, Knut Haukelid (a Brooklyn-born Norwegian who grew up in Rjukan), Birger Stromsam, Fredrik Kayser, Kaspar Idland, and Hans Storhaug were carefully picked from Kompani Linge. Like Grouse, most of them had lived on skis their entire lives.

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Rønneberg ordered special sleeping bag/bivouac sack hybrids for his men and hired a nutritionist to concoct an early precursor to freeze-dried backpacker meals: weight was paramount. Though the raid would only last a few hours, the ski to Sweden would take weeks.

The Vermork facility. The hydrogen production facility has since been demolished.

On January 23, Gunnerside parachuted into Norway and rendezvoused with the Grouse team. Since the capture and killing of the British glider teams, security had increased around the plant. Floodlights shone everywhere. The only bridge was now heavily guarded. Finally, on February 27, 1943, Rønneberg and the others (with the exception of Haugland, who remained on the Hardangervidda with the radio) skied toward Vermork. The nine men removed their skis and slid down the ravine as quietly as they could manage. Once on the other side, they snuck along an unguarded railway line. Rønneberg removed a pair of bolt-cutters from his rucksack and levered open a chain-link fence. Fredrick Kayser and Ronneberg crept inside through an unlocked vent while the rest of the men took up positions outside. Deep in the bowels of Vermork, Rønneberg and Kayser burst into the heavy water manufacturing room. A bewildered Norwegian worker watched as they laid charges. The clock was ticking. Somehow, Ronneberg still had time to help the poor night-shift worker find his glasses, which he’d misplaced in the fright of seeing the saboteurs. They left behind a Thompson submachine to suggest the raid had been conducted by Brits, rather than local (and still operational) resistance.

As quietly as they’d come, the team retreated. They were halfway to their skis by the time the explosives blew up and the alarm sounded. The entire inventory of heavy water produced during the German occupation, over 1,100 pounds, was destroyed along with equipment critical to make it. Not a shot had been fired, and no one had been killed. Cloaked by a storm, the team skied back into the night. By the time the Nazis realized Norwegian skiers had sabotaged the plant, the men were long gone. Rønneberg led five men toward Sweden. The rest remained in Telemark to continue resistance operations until the war’s end.

In 1947, Knut Haugland, the shy, lanky radio operator, perhaps unable to come to grips with civilian life, signed on with anthropologist Thor Heyerdahl to undertake a similarly wild scheme: the sailing of a balsa-wood raft called Kon-Tiki across the Pacific. Neither knew the raft would become a household name. Most assumed they’d be killed in the crossing. Heyerdahl (an SOE-trained resistance fighter himself) wanted to prove that Polynesia had been populated from South America. Modern-day anthropologists now refute Heyerdahl’s theory, but Kon-Tiki promised more than scientific merit: it offered adventurous solace for men who had known little, apart from uncertainty, for an entire war.

Joachim Rønneberg, the last of the “Heroes of Telemark,” died in October 2018. He was 99 years old, still sharp as a tack. After the war, he headed as often as he could into the of woods and mountains of Norway, finally able to enjoy his native country’s wilderness without fear of death or capture.