Hitler was just a ferry-ride away from getting hold of a crucial ingredient needed for an atomic weapon to blow up London, a new documentary has discovered.

In the middle of a Norwegian lake, 100 miles from Oslo, naval historians and scientists have located the boat on which the Nazis were transporting barrels of heavy water for use in German nuclear reactors.

The Hydro ferry was sunk on Churchill’s orders in 1944, but until now nobody knew if the craft really was containing vital component that Hitler needed for his nuclear arsenal.

For the new National Geographic series ‘Drain The Oceans’ scientists used multi-beam sonar technology to map the lake bed and virtually raise the ferry to find out what was on board.

They discovered at least 18 barrels which tests showed contained heavy water, and many more are thought to be crushed beneath the sunken boat, enough to put the Nazis closer to becoming an atomic superpower.

Naval historian and author Professor Eric Grove, formerly of the University of Salford, said: “It looked as if Germany might well get a nuclear bomb quickly and the Allies were obviously very concerned about that.

“Heavy water was a vital component of the attempt of the Germans to get their nuclear reactor to work.

“After the war those involved in the German nuclear programme said that the loss of the heavy water was absolutely decisive. It stopped their reactor programme in its tracks.”

By 1934, the Norwegians had succeeded in building the first commercial plant capable of producing heavy water which was perched on a precipice above Lake Tinn at Vemork.

Germany began its atomic program, called Uranverein, or “uranium club,” in April 1939, just months after German scientists Otto Hahn and Fritz Strassmann discovered fission giving them a significant head-start over the Manhattan Project in the US.

Heavy water was critical to slow and control the fission process and when the Nazis invaded Norway in 1940, they took control of the Vemork plant, and by 1942 were producing more than a tonne of heavy water a year.

The Allies were so fearful that Hitler would begin using heavy water to build nuclear weapons that London ordered a series of sabotage missions, culminating in Operation Gunnerside in 1943 in which Norwegian commandos were sent in to blow up the plant, recreated on screen in The Heroes of Telemark.

After the raids the Nazis realised they needed to safeguard their remaining stockpile and on February 20th 1944 began moving a year’s output of heavy water by train and ferry from Vemork, en route to the reactor site in Germany.

Historian Runar Lia, director of the Norwegian Industrial Workers Museum, said: “Hitler’s dream was to develop this bomb that could really devastate and destroy London turning the war in the blink of a second.”

However on Churchill’s orders, Norwegian resistance fighters had attached a timebomb to the ferry to explode when it reached the centre of Lake Tinn, and the vessel sunk 1,500 feet to the lake bottom where it was impossible to recover the barrels.

Dr Fredrik Soreide, of the Norwegian University of Science and Technology said: “After the ferry went down there was a lot of speculation that the heavy water on board the hydro had been replaced by normal water because it was so lightly guarded.

“So we wanted to go down and take up a barrel to prove that this was in fact the heavy water that was being shipped to Germany.”

The new scans revealed the 170 foot ferry still in one piece, tilting downwards on the lake floor. Tests proved that not only was their heavy water in the barrels, but enough to put Germany well on the way to becoming a nuclear power.

In recent years historians have disputed whether Hitler really did have the capacity to build a nuclear weapon, arguing that the Nazis were way behind the Americans. But last year metal detectorists searching near a former research facility in Oranienburg, near Brandenberg, Germany found radioactive material. German test pilots also claimed to have seen unexplained mushroom clouds

Ultimately the US won the race to nuclear weapons, with America’s first atomic bomb test detonation was conducted on May 7, 1945, the day that Germany surrendered.

The new 10-part series “Drain The Oceans” airs weekly on National Geographic from Thursday 6th September at 8pm.