This did not, however, mean the end of the Vemork story. Operation Gunnerside had successfully destroyed the electrolysis cells and poured their contents down the drain, but the plant was intact, as was the already-established German supply of heavy water. And by April, intelligence reports showed that Vemork was already repaired and running again, having shipped precious heavy water from Germany to restart it without another year’s loss. But in October, far worse news arrived.

In honor of his 1922 Nobel Prize, the Carlsberg brewing company gave Bohr the lifelong use of this house in Copenhagen, next to the brewery. A pipeline runs between the two, providing it with neverending beer on tap. Apart from his time in exile during the war, Bohr lived here continuously from 1931 until his death in 1962. The Carlsberg Residence has housed numerous scientists and humanists since.

Niels Bohr, the great Danish physicist, was admired and respected by all in the field; as a result, Werner Heisenberg, perhaps the greatest physicist to stay in Germany, came to him for advice in 1941, dropping hints about the Nazi nuclear program. (To this day, there is much question as to Heisenberg’s real role as scientific leader of that program: was he, as he later claimed, trying to sabotage the program the entire time? The open question there, and the famous meeting between the two scientists, has prompted not only endless analysis, but an excellent play) But Bohr had enemies, as well: he had been helping refugees escape the Nazis since the 1930’s, and in 1943, he received warning that he was to be arrested and shot. With the help of the Danish Resistance, he escaped by sea to Sweden.

The next day, September 30th, Bohr met with King Gustav V of Sweden, where he convinced him to publicly commit to protecting Jewish refugees. On October 1st, Hitler ordered all Danish Jews to be rounded up and deported; on the 2nd, King Gustav announced his intention to help them on the radio. Over the next few days, the Danish Resistance managed to smuggle 7,200 of Denmark’s 7,800 Jews, along with nearly 700 of their non-Jewish spouses, over the sea into Sweden.

But for Bohr, this was only a stop: on October 6th, he reached Scotland, and in his pocket he had Heisenberg’s designs for an experimental heavy-water nuclear reactor. Meeting regularly over the next few months with Sir John Anderson (Britain’s “Home Front Prime Minister”), and comparing notes with their American counterparts, they concluded that the Nazis were indeed on their way to a working nuclear design.

On November 16th, a flight of B-17’s made their way to Vemork, timing their arrival to be during the lunch period, to minimize worker casualties. (As Tronstad had warned them, Vemork’s original purpose as a fertilizer factory was still very much a going concern, and when bombs hit the ammonia storage tanks, the resulting explosions would kill large numbers of workers. The Allies had, by now, largely abandoned the notion of precision bombing: a few months earlier, the U.S. Bomber Command’s Operation Gomorrah had had the brutally simple objective “To destroy HAMBURG.” But still, they took effort, and spent extra time circling over enemy airspace, to protect who they could.) 140 aircraft made it through enemy flak, dropping over 700 bombs; while none hit the aiming point directly, the damage to the factory was such that it never again would function during the war.

But the heavy water itself remained undestroyed in the plant, and could be used again if the Germans built another factory – or could simply be used to power reactors directly. On February 9th, 1944, Allied agents in Norway reported over the radio that this remaining water was to be transported back to Germany under heavy guard within a week or two. Only one person was on the ground and in a position to respond: Knut Haukelid.

Knut Haukelid. Credit: Norges Hjemmefrontmuseum.

Haukelid had been living behind enemy lines for the entire past year, organizing military operations against the Nazis. He snuck into Rjukan that night to meet with Vemork’s new chief engineer, Alf Larsen. Larsen agreed to help, recruiting the transport engineer as well, and gave Haukelid the details of the planned transport: 39 drums labelled “potash-lye,” going by train from Rjukan to Lake Tinnsjå, then crossing the lake by ferry, then by train to the port, and by ship to Germany.

Vemork itself was no longer a good target; after the bombing raid, the Germans took security there very seriously, and a one-man raid was out of the question. Haukelid similarly ruled out the trains, being difficult to destroy and certain to create tremendous civilian casualties. So the target had to be the ferry – but that, too, would have civilians aboard. Worse, the shipment’s German guards would certainly have to be killed in the raid, and their deaths would bring heavy reprisals onto the local population.

Haukelid and the engineers discussed their options at length. By this point, they were openly discussing the possibility of nuclear weapons; the engineers doubted that the Germans were anywhere close to a solution, or whether one was even possible, and felt that the risks to the local population were not worth it. Haukelid finally radioed London for permission, emphasizing the engineers’ concerns. London, knowing the state of both the Manhattan Project and having seen the designs Bohr brought, disagreed:

“Matter has been considered. It is thought very important that the heavy water shall be destroyed. Hope it can be done without too disastrous results. Send our best wishes for success in the work. Greetings.”

And so the plan began. The transport engineer coordinated the shipment so that it would be aboard the ferry on a Sunday morning, when the ship would be emptiest. Haukelid determined that the ferry which would be on that route was the aptly named SF Hydro; disguising himself as a worker, with his Sten gun hidden in a violin case, he rode it, timing its passage over the deepest part of the lake. He found a twenty-minute window in which the explosion had to happen.

For this precision, the time fuses Haukelid had wouldn’t be enough; he needed detonators and a clock. The detonators came from a local hardware store; Haukelid visited the owner at night and was turned away suspiciously, but one of his local friends succeeded. Alarm clocks were provided by Larsen and by a retired Vemork handyman, and modified to be the most traditional B-movie detonators you can imagine: the bells were removed, and the hammers would instead strike contact plates, completing a circuit. To test that mechanism, Haukelid set an alarm to trigger a few spare detonators outside his cabin in the mountains, and then fell asleep. A few hours later, the efficacy of explosives as a wake-up call was vividly proven, sending him leaping in surprise from bed, grabbing the nearest gun, and covering the door before he realized what had happened. He later noted: “the timing apparatus seemed to be working perfectly.”

Similarly, the explosives needed to be designed. How big a hole would be needed to sink the ferry quickly? “As the Tinnsjå is narrow, the ferry must sink in less than five minutes, or else it would be possible to beach her. I… spent many hours sitting and calculating how large the hole must be for the ferry to sink quickly enough.” He settled on a ring of plastic explosives, some twelve feet around.

Lake Tinnsjå, one of the deepest lakes in Europe. Credit: Wikimedia commons.

On Saturday, February 19th, Haukelid slipped into Rjukan together with Rolf Sörlie, another of the locals helping him. The city was packed with soldiers and SS; from a bridge over the river Måna, they saw the train parked near the lake, under heavy guard. Next they met their driver, in a car which Haukelid had arranged to “steal in the name of the King” and return in the morning. Once they got the car started – it took over an hour, as the car had been modified to run on methane – they picked up Larsen, who knew he would have to flee the country after this. He had come from a dinner party, where he had met a visiting violinist who was planning to leave on the morning ferry. Larsen had tried, unsuccessfully, to convince him to spend another day in the area and enjoy its skiing. A fifth man, another of Haukelid’s agents (unnamed to this day), joined them as well. Together, they approached the ferry. In Haukelid’s words:

Armed with Sten guns, pistols, and hand-grenades, we crept… down toward the ferry. The bitterly cold night set everything creaking and crackling; the ice on the road snapped sharply as we went over it. When we came out on the bridge by the ferry station, there was as much noise as if a whole company was on the march. Rolf and the other Rjukan man were told to cover me while I went on board to reconnoitre. All was quiet there. Was it possible that the Germans had omitted to place a guard at the weakest point in the whole route to the transport? Hearing voices in the crew’s quarters, forward, I stole to the companion[way] and listened. There must be a party going on down there, and a game of poker. The other two followed me onto the deck of the ferry. We went down to the third-class accommodation and found a hatchway leading to the bilges. But before we had got the hatch open we heard steps, and took cover behind the nearest table or chair. The ferry watchman was standing in the doorway.

Haukelid told the watchman that they were fleeing the Gestapo and needed to hide; he immediately helped them get the hatch open, telling them that they often smuggled goods there. The Rjukan man then kept talking with the watchman, keeping him busy as Haukelid and Sörlie went below. Standing in foot-deep water, they laid the ring of explosives along the bottom of the hull, towards the forward end of the ferry so that when water came on, the rudder and propeller would be pulled above the surface, and the boat would be unable to navigate to safety. The alarm clocks were taped to the hull; detonators attached to the timers; fuses connected the detonators to the explosive; the alarms were set for 10:45; and finally, the batteries were connected to the detonators, and the detonators to the fuses. The bomb was now set to go off, with only a third of an inch gap between hammer and contact plate preventing it from detonating under their feet.

They now convinced the watchman that they needed to return to Rjukan to collect some possessions. Haukelid considered warning him, but decided that it was too likely to endanger the mission; he could only thank him, and shake his hand. Ten minutes later, they abandoned the car; Haukelid and Larsen to ski forty miles to Kongsberg, from which they would take a train to the Swedish border; Sörlie to radio a report back to London; and the driver and the local man to simply stroll home, nonchalantly. The transport engineer had an even better alibi: he had asked doctors at the local hospital to operate on him for appendicitis, and spent the weekend having surgery. No questions were asked.

At 10:45 the next morning, with the ferry over the deepest part of the lake, the explosives went off. As passengers and crew frantically worked to release the lifeboats, the freight cars, with their heavy water aboard, broke loose from their chains, rolled over the edge, and sank like stones. The last hope of the Nazi atomic bomb project would spend the rest of the war at the bottom of Lake Tinnsjå.

Of the fifty-three people aboard the ferry, twenty-six drowned. The concert violinist was among the survivors: he made it into a lifeboat, and when he spotted his violin case floating by, someone fished it out for him.