It’s fairly easy to accept how Ethan Hunt from Mission Impossible could carry out his missions successfully, regardless of how impossible they all seemed. It’s just a movie anyway, and anything and everything is possible. Hanging by a wall with a single-arm while holding two people dangling in the air with the other? Sure. Jumping without a parachute and perfectly landing in the target window? Easy. Things are kind of different in reality. So when a group of Norwegian commandos successfully descended to the bottom of a valley across a half-frozen river and then climbed a 500-foot-high snowy cliff in the middle of the dark winter night, no one really knew what to expect. But guess what? They successfully nailed a sabotage mission called Operation Gunnerside. Here’s their epic story that could rival Mission Impossible.

The German Atomic Bomb Project

In April 1939, the Nazis started a secret program called Uranverein or “Uranium Club,” led by German nuclear physicist Kurt Diebner. Apart from him, other top scientists in Germany were also recruited, including Nobel Prize winner Werner Heisenberg.

Their idea was to use heavy water as a moderator to slow down the bombardment of neutrons and control the whole fission process, which helps sustain a chain reaction. The United States was using graphite as a moderator at that time.

Now, heavy water is “heavy” because the water molecule was made with two deuterium ions instead of the normal two hydrogen ions. The additional neutron causes the deuterium to have a higher molecular weight when combined with oxygen.

Vemork Hydroelectric Plant at Rjukan, Norway, in 1935. (Anders Beer Wilse, Public domain, via Wikimedia Commons)

When Germany invaded Norway in April 1940, they took control of Norsk Hydro’s Vemork plant located just outside of Rjukan. Originally designed to use mountain water for electrolysis in producing ammonia fertilizer, it recently became the first industrial-scale production site of heavy water in the world. When Leif Tronstad, a chemistry professor at the University of Trondheim, along with the head of Norsk Hydro’s hydrogen electrolysis plant Jomar Brun, collaborated to work on Vermork’s heavy water production facilities. The plant had produced more than 100 grams of heavy water by January 1935. While it didn’t sound much, heavy water and regular water generally exist in about a 1:41 million molecule ratio. After the Germans captured the plant, they forced the workers to increase the heavy water production. By the end of 1941, the production rate was 100 kilograms per month.

Operation Grouse and Operation Freshman

Word got out that the Germans were making heavy water and the idea of Hitler having an atomic bomb was absolutely terrifying to the Allies. The benefit of hindsight tells us now that Germany did not have the resources and money to make an atomic bomb during the war, but Allied leadership had no way of knowing that then.  The Manhattan Project which created 4 atomic bombs for the US, consumed a staggering 25% of the entire war budget.

In the summer of 1941, the increase in heavy water production of the Germans reached the British intelligence. With Leif Tronstad as the source of the intelligence on the Vemork plant, he was appointed as the leader for training the command units that would go and sabotage the operations. The British Special Operations Executive (SOE) Norwegian branch, Company Linge, began recruiting Norwegians who fled to England. They underwent grueling training in Scotland that included night training exercises of climbing mountains, fording rivers, camping, and surviving outdoors for weeks.

The first part of the mission included a small scout team of Norwegians codenamed Grouse. They parachuted from a plane and landed in the plant’s surrounding area, where they could gather intelligence that the attack group would use later on. After a month, Operation Freshman commenced, this time with 39 British air troops that were carried by two military gliders. They were supposed to land near the plant with the guidance of the Grouse group and then attack the plant. However, the bad weather and communication issues caused one of the gliders to crash into a mountain, while the other crash-landed far away from where they should land. All of the men in the glider that hit the mountain sadly died. Those in the second glider were executed by the Gestapo.

The Grouse group was still in the area, surviving the wilderness with whatever supply they had left.

Operation Gunnerside

Three months after they landed, the Grouse group, renamed Swallow, received news that six more Norwegians would be sent to the area for Operation Gunnerside. This small group of special forces was from Company Linge and would parachute to their target zone instead of using a glider. They would then meet with the Swallow group and proceed on their sabotage mission.

A reconstruction of the Operation Gunnerside team planting explosives to destroy the cascade of electrolysis chambers in the Vemork heavy water (deuterium) plant. (Photo: Hallvard Straume, Public domain, via Wikimedia Commons)

On February 16, 1943, the group jumped under cover of snowfall at midnight, led by Joachim Ronneberg. They successfully landed without being detected by the Gestapo. After five days, they reconnected with the Swallow group.

On February 27, the group began their raid on the Vemork plant. As written by Neal Bascomb, author of  “The Winter Fortress: The Epic Mission to Sabotage Hitler’s Atomic Bomb,”

They have three options: they can come down from the mountains above the plant, which is littered with minefields; they can cross the single-lane suspension bridge, which is closely guarded; or they can descend to the bottom of the valley across a half-frozen river and climb a 500-foot-high cliff. They decide to climb the cliff, in the middle of the night, in the middle of winter!

Fortunately, Ronneberg had purchased metal cutters in Cambridge during his day off and brought them. The cutters made it easy to get through the plant’s side gate fence without taking much time and making much noise.

Once in, they split into two groups: a four-man explosives group and a five-man cover squad. The explosives group managed to enter through an access tunnel found by Ronneberg. As he recalled,

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Getting inside I was quite certain that the rest of the party would follow me, but only one chap came. The other ones hadn’t found the entrance to the tunnel. Therefore we decided we would have to do it ourselves and started laying out the charges.

They then placed two strings of explosive charges just right beside the heavy water production cells. To make sure that they would not fail, Ronneberg decided to shorten the fuses so it would only take 30 seconds instead of two minutes before they exploded. In that way, they would still have enough time to escape and, at the same time, hear the explosion.

HM King Haakon VII of Norway at the premiere of the film Kampen om tungtvannet (Operation Swallow: The Battle for Heavy Water) at Klingenberg kino in Oslo. Soldiers in uniform, from the left: Knut Haukelid, Joachim Rønneberg, Jens Anton Poulsson shaking hands with the king, Kasper Idland. (Leif Ørnelund CC BY-SA 4.0, via Wikimedia Commons)

After hearing the explosion that confirmed the success of their mission, they fleed the plant and reconnected with the cover squad before they all skied toward Rjukan, traveling more than 200 miles to Sweden. They then split up and spread out throughout the plateau to make it hard for the German search and pursuit group to find them. As a result, no one was killed or captured.

As General Nikolaus von Falkenhorst described Operation Gunnerside after seeing the damage to the water facilities, it was “the most splendid coup.”