'Gunnerside' was the British parachute delivery of a six-man Norwegian party of the Special Operations Executive to a point near Lake Skryken in German-occupied Norway to link with the 'Grouse' (i) and (ii) teams and then to plan and execute an attack on the heavy water production facilities at the Vermork hydro-electric plant in occupied Norway (16 February/1 March 1943).

During World War II, the Allies made strenuous efforts to prevent, or failing this to slow, the German development of nuclear weapons. A primary factor in these efforts was the removal of heavy water and the destruction of heavy-water production plants, and within this was the sabotage of the Norwegian heavy water production capability at the Vemork power station at the Rjukan waterfall. This hydro-electric powerplant had been constructed in 1934, and was the world’s first site for the quantity production of heavy water (12 tonnes per year). Before the German 'Weserübung' invasion of Norway on 9 April 1940, the French Deuxième Bureau (military intelligence organisation) had removed 408 lb (185 kg) of heavy water from what was currently neutral Norway after the facility’s managing director had reached an agreement to lend France the heavy water for the duration of the war. The French moved the consignment in secret to Oslo, thence to Perth in Scotland and finally to France. The facility was still able to produce heavy water, however, and the Allies were concerned that the occupying Germans would use it to produce more heavy water.

Between 1940 and 1944, a series of sabotage actions by the Norwegian resistance movement and Allied bombing ensured the destruction of the plant and the loss of its heavy water. These 'Grouse', 'Freshman' and 'Gunnerside' operations to terminate the facility’s heavy water production early in 1943.

In 'Grouse' of October 1942, the British Special Operations Executive delivered four Norwegians of an advance party onto the Hardanger plateau above the plant, and in the course of the following month 'Freshman' was attempted by paratroopers of the British airborne arm. These paratroopers were to rendezvous with the 'Grouse' team and move to Vemork, but the effort failed when the troop-carrying gliders and one of their Handley Page Halifax tugs crashed short of their destination. Except for the crew of one Halifax bomber, all the participants were killed in the crashes or captured, interrogated and executed by the Gestapo. In February 1943, a team of SOE-trained Norwegian commandos destroyed the production facility in 'Gunnerside', which was followed by Allied bombing raids. The Germans ceased operations, and attempted to move the remaining heavy water to Germany. Norwegian resistance forces then sank the ferry carrying the heavy water on Lake Tinn.

By the new year of 1943, the British knew that the 'Grouse' team was still operational, and thus decided to attempt another operation with what had become the 'Swallow' team. On the night of 16 February 1943, therefore, 'Gunnerside' began as six more Norwegian commandos were paradropped into the operational area by an adapted Handley Page Halifax bomber of No. 138 Squadron. The six men landed successfully, and found the 'Swallow' team after a few days of cross-country searching. The combined teams then made the final preparations for their assault on the night of 27/28 February. Supplies needed by the Norwegian party had also been dropped.

Meanwhile, following the failure of 'Freshman', the Germans an installed mines, floodlights and additional guards around the plant. The mines and lights were still there at he time of 'Gunnerside', but personnel security at the facility had been weakened over the winter, though the 246-ft (75-m) deep ravine 660 ft (200 m) above the Måna river was still fully guarded.

The combined parties' plan was to climb down into the ravine, ford the icy river and then ascend the steep slope on the far side. It being winter, the depth of the river water was very low, and on the far side, where there was level ground, the Norwegians followed the single-track railway straight into the facility without encountering guards. Even before the landing of the 'Grouse' party, the SOE had a Norwegian agent in the facility, and he had provided detailed plans and schedules. The demolition party used this information to enter the main basement by a cable tunnel and through a window. The only person the men encountered was a Norwegian caretaker, and he was very willing to co-operate with the demolition party. The saboteurs then placed explosive charges on the heavy-water electrolysis chambers, and attached a fuse to provide time for their escape. In an attempt to avoid German reprisals against the local population, a Thompson sub-machine gun was deliberately left behind to suggest that this was the work of British forces and not the local resistance. When the fuses were about to be lit, the caretaker became concerned that his hard-to-replace spectacles were somewhere in the room, and there followed frantic but successful search and the fuses were lit. The explosive charges detonated, destroying the electrolysis chambers.[12]

The raid was considered successful. The entire stock of heavy water produced during the German occupation, totalling more than 1,100 lb (500 kg), was destroyed together with equipment critical to the operation of the electrolysis chambers. Although 3,000 German soldiers were dispatched to search the area for the commandos. All of these escaped, however: five skied 250 miles (400 km) to neutral Sweden, two travelled to Oslo, where they aided the Milorg military resistance organisation, and four remained in the region for further resistance work.

'Gunnerside' halted heavy water production for several months, but did not permanently damage the Vemork facility, which had been repaired by April. The came to the decision that a another commando raid would be almost impossible as the level of German security was considerably heightened.

Almost as soon as production resumed, American bombers of the USAAF began a series of raids on Vemork. The facility was attacked in November in a massive daylight raid by 143 Boeing B-17 Flying Fortress heavy bombers, which dropped 711 bombs. The attack caused considerable damage despite the fact that at least 600 bombs missed the plant. On 16 and 18 November, 35 Consolidated B-24 Liberator heavy bombers attacked the hydro-electric power station at Rjukan. The missions lasted 9.5 to 10.5 hours respectively. These attacks signalled that the Allies had come to the appreciation that there was less need for any ground assault than had been the case a year earlier as night bombing, which had been deemed impractical as a result of the German air defence’s strength was now a realistic possibility. Convinced that air raids would result in further serious damage, the German opted to abandon the facility and remove its remaining stocks and critical components to Germany in 1944.

On being informed of the German plan, Knut Haukelid (the sole commando to have remained in the area), decided to recruit locally and attempt the destruction of the shipment. Haukelid found two people, and the three men decided to sabotage the ferry in which the heavy water was to be shipped across Lake Tinn. One of his recruits recognised a man of the ferry’s crew and spoke with him, taking an opportunity to slip into the lower part of the vessel, plant the bomb and slip away. Some 18.75 lb (8.5 kg) of plastic explosives and two alarm-clock fuses, were fixed to the keel of the Hydro, which was to carry the railway cars with drums of heavy water across Lake Tinn.

The ferry and its cargo sank in deep water shortly after its departure around 24.00 on 20 February 1944, effectively bringing to a halt Germany’s atomic bomb development programme.

Despite the mission’s intention to cause minimum losses, 18 people were killed and 29 survived. The dead were 14 members of the Norwegian crew and passengers, as well as four German soldiers.