Rønneberg, now a First Lieutenant and put in charge of training, selected and led the six-man Operation Gunnerside team, reinforcing the five-man team "Grouse" sent in earlier, during the heavy water sabotage action. After landing at a location from the other team "Gunnerside" spent five days waiting out an intense blizzard in an uninhabited hunting cabin before meeting up with "Grouse". The combined Norwegian team went into action against the Norsk Hydro heavy water production plant in Vemork in 1943, parachuting into the Hardangervidda plateau on 16 February. Rønneberg led the demolition team when the saboteurs, on the night of 27–28 February 1943, entered the Norsk Hydro plant and set explosive charges. The team then escaped from the factory as the explosives went off, without the German guards discovering the saboteurs or indeed noticing that there had been an attack on the plant, probably believing that the heavy snow had set off one of their own land mines. Rønneberg recalled the dawn as they escaped: "It was a mackerel sky, it was a marvellous sunrise. We sat there very tired, very happy. Nobody said anything. That was a very special moment." Although chased by 2,800 German troops, five of the saboteurs, led by Rønneberg, escaped safely to neutral Sweden by way of a 14-day march over a distance of after the successful completion of their mission. The six other members of the sabotage team hid out in various locations in Norway without being caught by the Germans. Eighteen heavy water cells and around of heavy water were destroyed during the attack, as well as a loss of production of of heavy water.