A Soviet cameraman described the blast as "a powerful white flash" appearing over the horizon, followed by "a remote, indistinct and heavy blow, as if the earth had been killed".
He had just witnessed the world's biggest-ever explosion - a blast more powerful than the total of all the armaments used in the second world war.
It was the 58 megaton (58m tons of TNT) Tsar Bomba, the "King of Bombs", being tested by the Soviet Union 40 years ago this month.
The Tsar Bomba exploded 3,700 metres above the ground and the shockwave from the blast circled the earth three times. One observer commented that "in districts hundreds of kilometres from ground zero, wooden houses were destroyed, and stone ones lost their roofs".
The flash could be seen 600 miles away even though heavy cloud covered the site of the detonation - the Arctic island of Novaya Zemlya or "New Land". The mushroom cloud rose more than 43 miles into the sky.
But where had such a giant come from? And why?
It was largely a result of the cold war chilling steadily throughout 1961. US President John F. Kennedy had responded to Soviet Premier Khrushchev's calls to support wars of national liberation by promising that Washington would "oppose any foe to assure the survival and success of liberty".
In April, Washington launched the disastrous Bay of Pigs invasion of Cuba, and, in Berlin, a wall was erected that would divide the city for three decades. Khrushchev believed the Soviets should negotiate over Berlin "from a position of strength" - and this was the genesis of Tsar Bomba.
In June 1961, Khrushchev met his senior nuclear weapons scientist Andrei Sakharov, and ordered him to develop a 100 megaton hydrogen bomb. Khrushchev wanted the bomb ready for testing in September to create maximum impact at the 22nd Communist Party Congress in Moscow.
Sakharov returned to Arzamas-16, a secret Soviet nuclear laboratory, to construct the device, together with four of the country's top nuclear scientists: Victor Adamskii, Yuri Babaev, Yuri Trunev and Yuri Smirnov.
Although the bomb was not ready by September, its design and construction took just 14 weeks, and, by mid-October, the giant bomb, weighing 27 tons, was ready for testing.
On a cold autumn day, Major Andrei Durnovtsev manoeuvred his Tu-95 bomber towards Novaya Zemlya; his cargo was the Tsar Bomba. Over the island, he dropped the bomb and it fell effortlessly, attached to a parachute so large that the quantity of nylon needed for it had seriously disrupted the Soviet hosiery industry.
Years of testing had taught the nuclear powers of the day (US, UK, Russia and France - China would not test until 1964) that nuclear explosions were large affairs, but Tsar Bomba was different.
Nuclear bombs produce an electronic disturbance known as the electro- magnetic pulse, which severely damages or destroys electronic circuits, and Tsar Bomba knocked out all long-range communications around the Arctic sea for more than an hour. Staff at the Olenya airforce base where the bomber was scheduled to return had no radio confirmation that the aircraft was safe or that the bomb had been detonated.
The arctic terrain of Novaya Zemlya, the area below the explosion, was transformed from a rugged snow- and ice-covered landscape into what one observer called "an immense skating rink".
However, there is confusion over whether Tsar Bomba exploded at full power. Although Khrushchev had requested the bomb to have an explosive power of 100 megatons, atomic experts are un-decided as to whether the device tested was a "slimmed-down" version of the weapon, or if the bomb malfunctioned and did not explode at full capacity.
However, the military leadership was largely unimpressed: Tsar Bomba might have been a striking scientific achievement but they were unconvinced it was of any strategic value. It is more effective to destroy a city by using many small hydrogen bombs targeted on the periphery, rather than by dropping one large bomb in the centre. Even so, a few of the huge bombs were built and stockpiled.
If the military potential was doubtful, the political effect was vivid. Khrushchev chose the Party Congress to tell his party, his citizens and the world of the explosion. Khrushchev teased the west, saying he hoped "we are never called upon to explode these bombs over anybody's territory", but privately he spoke of Tsar Bomba hanging "over the heads of the imperialists, like a sword of Damocles".
Washington's response was resolute, despite the tests catching the US off-guard. Kennedy said that "in terms of military strength, the US would not trade places with any nation on earth", and that it was not necessary "to explode a 50-megaton nuclear device to confirm that we have many times more nuclear power than any nation on earth".
While the leaders were talking tough, Sakharov was feeling uneasy. He later commented in his memoirs that he was perturbed by the test. The father of Tsar Bomba was convinced that "our work was crucial in preserving the parity necessary for mutual deterrence" but at the same time he was tormented by the environmental and political impact of his actions.
Almost exactly a year after the explosion, the US and the Soviet Union were locked in their most bitter stand-off yet, during the Cuban Missile Crisis - and the power of the King of Bombs to "preserve the parity for mutual deterrence" was put to its stiffest test.