It was a few minutes before midnight and the moon was glancing brightly off the deep snow. In the shadows of a ravine, 10 saboteurs clambered up the sides of a near-vertical cliff, clutching at rocky outcrops and dangling spruce branches. In the distance was their goal, the Norwegian Norsk Hydro heavy water plant. This factory was of vital importance to the Nazi war machine - the only place capable of producing the heavy water (deuterium oxide) necessary for Adolf Hitler to build an atomic bomb. Its destruction was so crucial to the Allied war effort that Winston Churchill had ordered it to be given "the highest possible priority".The stakes could not have been higher: if Hitler's scientists managed to build an atomic bomb, they would win the war. But if the factory could be destroyed, then Hitler's atomic ambitions would be at an end.The story of the attack on Norsk Hydro has been told in numerous books and films, notably the 1965 movie The Heroes of Telemark. But what has never been revealed is that the entire operation was orchestrated by an elite secret inner circle working directly for Churchill.They were the members of the Special Operations Executive: six maverick gentlemen with a passion for destruction who planned - and carried out - the most audacious guerrilla attacks of World War 2.They operated under a veil of total secrecy: even cabinet ministers were not aware of their work.As a result, their extraordinary story has remained hidden for more than seven decades.The Special Operations Executive was led by Colin Gubbins, a dapper Scottish Highlander. A self-taught master of sabotage, Gubbins had overseen countless clandestine operations, including the assassination of Hitler's favourite commander, Reinhard Heydrich.Gubbins had brought together five like-minded experts who believed, like him, that the Nazis would only be defeated by tearing up the rule-book. "This was total war," he said, "and total war is a very cruel business indeed." Churchill agreed and provided unlimited funds to Gubbins and his team, referring to them as his "Ministry of Ungentlemanly Warfare".In 1942, Churchill put Gubbins in charge of planning his most important mission to date, an attack on Norsk Hydro. Gubbins was quick to see the difficulties. The heavy water plant was perched atop a 215m shaft of vertical rock. Three of its sides were sheer, while the fourth - which joined it to the adjacent mountain - was covered in mines. There was but one point of access: a narrow suspension bridge under 24-hour guard.Even if the saboteurs managed to force an entry, they'd still have to plant their explosives unseen by the Gestapo guards before making their escape. Gubbins thought it a suicide mission.But Churchill was insistent and so Gubbins set to work. He felt that the only hope of success was to parachute a team of Norwegian saboteurs onto the lonely Hardanger plateau. They would then have to cross the snow-bound plateau, scale the gorge and break into the plant.Gubbins selected his Norwegian saboteurs from men who had fled to England following the Nazi invasion of their country. Their leader was a bold 23-year-old named Joachim Ronneberg. He and his comrades were trained by two key members of Gubbins's inner circle, Eric "Bill" Sykes and William "Shanghai Buster" Fairbairn, who ran a secret killing school at Arisaig House in the Scottish Highlands.Once the Norwegians had completed their training in Scotland, they were sent to Brickendonbury Manor, a Hertfordshire mansion run by another of Gubbins's specialists, George Rheam. A dour northerner with a passion for destruction, Rheam was the country's leading expert in industrial sabotage. He built an exact replica of Norsk Hydro in the grounds of Brickendonbury Manor and then put Ronneberg and his men through an intensive training programme.Finally, the big day arrived. On the evening of February 16 1943, Ronneberg's team were parachuted onto the Hardanger plateau, landing in the teeth of an Arctic blizzard within striking distance of Norsk Hydro.Their sabotage mission got under way 10 days later, under the cover of darkness. The saboteurs clambered down into the vertiginous gorge below Norsk Hydro and then began their treacherous ascent. Unseen by the guards, they reached the plant's perimeter fence. After using bolt-cutters to gain access, they split into two pre-arranged groups. One, led by Ronneberg, was to break into the plant and blow up the equipment. The other was to provide cover against any Gestapo attack.Ronneberg crept through a ventilation duct and attached the explosives. "The charges that had been made at Brickendonbury Manor fitted like a glove," he later said.The saboteurs were still inside the plant when the explosives detonated. The sausage-shaped charges were fabulously destructive, imploding into the machinery and causing catastrophic damage.By the time the alarm was raised, the entire stock of Hitler's heavy water had drained away. - ©The Daily Telegraph..