Operation Gunnerside: The Most Important WWII Mission You Never Heard Off

heavy water wars

Operation Gunnerside was arguably the most important covert mission of World War II. It’s goal was daunting – to prevent Adolf Hitler from creating the atomic bomb first. Gunnerside would do so by denying the Nazi’s enough heavy water (deuterium oxide) to build a nuclear weapon.

The Vemork hydroelectric plant, located in a remote valley in Norway, produced both electricity and fertilizer. A precious byproduct was heavy water. When the Nazis invaded Norway, they took control of the Vemork plant. Major Lief Tronstad, a former chemist at Vemork, now with SOE (Special Operations Execution) was assigned the task of denying the Nazi’s heavy water.

In late 1942, the Allies launched Operation Freshman to destroy the plant with a full assault. Norwegian and British commandos were to parachuted into Norway.

The mission was a complete disaster!

One airplane and two gliders crashed near the plant in bad weather. Those who survived were captured, interrogated and ultimately executed by the Gestapo, 41 in all. Plus now, the Nazi’s knew of the British interest in destroying Vemork.

The Allies still had to disable the plant, so they attempted a second mission. This time a small covert raid called Operation Gunnerside, the brainstorm of Major Tronstad. Lt Joachim Ronnenberg lead the new mission with 5 hand-picked Norwegian commandos, none over the age of 31. Training for the raid included memorizing a scale model of the Vemork plant meticulously built by Tronstad.

In February 1943, Operation Gunnerside got off to a bad start. A freak snowstorm forced the team to be air dropped 18 miles from the plant! It took the men a week to trek cross country on snow shoes and skies to meet up with 4 Norwegian commandos who had managed to survive Operation Freshman.

The original 4 had carried out extensive reconnaissance on Vermork.

Following the failed assault, the Germans had laid land mines along the steep mountainside above the plant. Plus they doubled the guards on a narrow suspension bridge, the main entry to the facility. The weak point in the defenses was the steep, 660-ft deep ravine the bridge spanned, which the Germans judged too treacherous and impassable. But 1 Norwegian had discovered a way to descend the ravine, cross the frozen river, ascend the other side and reach the plant unseen at night!

So just before midnight, the saboteurs crossed the icy ravine in darkness and below zero temperatures. During a changing of the guards, they managed to creep into the facility undetected. The 10 men split into two teams; five would destroy the equipment while the other five would act as lookouts. Lt Ronnenberg and another man began laying the charges. Their target was a battery of 18 electrolysis ‘cells’ which held the last stage of heavy water production.

They encountered a Norwegian foreman inside the electrolysis chamber, reading the instruments and filling out logbooks. The man seemed more concerned about having his reading glasses before they lit the charges. They ordered the man to run upstairs, lie down and keep his mouth open so as not to burst his eardrums. There were four main charges with short two-minute fuses. Ronnenberg did not trust the foreman and decided to cut the fuses to just 30 seconds!

He ordered his men from the room, lit the fuses and ran for his life.

The blast was deafening inside the chamber, but the guards outside barely noticed. As the lookout team held their machine guns ready, only one guard emerged with a flashlight, and after brief inspection, returned to his station! The guards assumed the sound came from the plant’s machinery which occasionally made loud noises.

Ronnenberg’s team met the lookout party just outside the gates. What astonished them all was that the Germans did not yet realize that their facility had been sabotaged. The guards’ attention was probably more focused on staying warm in the below zero winter night than on any loud noise. The foreman would soon report the explosion however.

They hurried back across the ravine’s river and began climbing a zigzag trail leading to the top of the mountain. That was when the plant’s sirens finally began to sound. It took them three long hours to climb 700 meters to the ridge summit. Only then could they could put on their skis and escape down the other side of the mountain.

Unlike its predecessor, Operation Gunnerside was a complete success.

Though the plant itself was only slightly damaged, more than 1,000 pounds of heavy water had been destroyed, along with the electrolysis equipment needed to create it. The Nazis responded with a heavily armed search party of 3,000 German soldiers that scoured the mountainside. They would be too late. All 10 commandoes escaped to the coast alive.

The raid halted production of heavy water for nearly a year, but the Allies knew the Nazis would soon be making it again. Another raid was out of the question due to heightened security, so a bombing raid was launched in November 1943 that severely damaged the facility. This was followed in 1944 by the SOE sinking the ferry SF Hydro near Telemark, carrying a shipment of heavy water to Germany.

The mission was arguable the most productive act of sabotage of the entire war. Every member of the team received war honors, with Ronnenberg receiving the Distinguished Service Order. Their exploits were even turned into 1965 movie The Heroes of Telemark starring Kirk Douglas and Richard Harris.

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