When World War II broke out in Norway, Joachim Ronneberg was a surveyor. A few years later, he was a hero, the leader of an almost suicidal mission to bring down the lynchpin of Nazi Germany’s nuclear program. Reuters reports that Ronneberg died on October 21, 2018. He was 99 years old.

The Norwegian’s life changed forever when Germany invaded Norway in 1940. Like many other members of the Norwegian military, Ronneberg managed to flee to Britain, where he became involved with the Special Operations Executive, a World War II organization devoted to spying on Nazi activities and sabotaging them.

The SOE trained 23-year-old Ronneberg to lead a daring, almost suicidal mission: penetrate Norway, break into a nuclear plant, and destroy its supply of heavy water, a dangerous substance that can help create weapons-grade plutonium and fuel nuclear reactions. Britain had been tracking Nazi attempts to create a nuclear bomb throughout the war, and worried that Axis forces would use such a weapon to destroy part of Europe.

The lead-up to the operation was a feat in and of itself: Ronneberg and his team had to wait out a brutal blizzard before they could parachute into their location. They skied to the Norsk Hydro heavy water plant in Vemork, Norway on February 28, 1943.

The operation was swift and carefully calculated. Shortly after midnight, the men broke into the plant and rushed inside, ready to knock out guards with chloroform. But once inside, they found that the door their Norwegian contact was supposed to have left unlocked was impassable. Luckily, they had access to building plans supplied by a Norwegian who had designed the plant.

Ronneberg and a fellow operative made their way through a narrow cable shaft, captured a guard, set explosive devices, got out of the compound, and exploded 3,000 pounds of heavy water, the equivalent of five months of factory production. The pair skied away by cover of night. Not a single shot had been fired.

“It was marvelous,” Ronneberg recalled later. He skied over 300 miles until he reached Sweden. Then, according to plan, he took off his uniform and presented himself as a refugee at the Swedish border wearing just his underwear. “We had come from occupied Norway,” he recalled. “We had now come from war-battered Britain…now we came to neutral Sweden…It was like coming to heaven, more or less.”

Ronneberg continued to lead secret operations against the Nazis during World War II, but his feat at the Vermork plant is his most famous, in part because it has been dramatized in several movies and TV shows, including 1965’s The Heroes of Telemark, starring Kirk Douglas. Ronneberg received a number of international awards for his bravery, and worked in broadcasting after the war.

Though he was a war hero, Ronneberg spent much of the 1970s helping young people understand the true cost of war. And he rarely spoke about his work or the operation. When he did, he was humble. The group succeeded because of “thorough preparation, excellent intelligence and a good portion of luck,” he told the AP in 2003. “Not the least the last.”