AT 5.29am on July 16, 1945, a light like a second sun blossomed over New Mexico's Jornada del Muerto desert.

Those who witnessed the testing of the world's first atomic bomb that day were driven to almost poetic heights in their attempts to describe what they saw.

"The whole country was lighted by a searing light with the intensity many times that of the midday sun," wrote the US Army's Brigadier General Thomas F Farrell, in his official report.

"It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described."

Robert Oppenheimer, the theoretical physicist known as the father of the atomic bomb for his part in the Manhattan Project, resorted to quoting Hindu holy book the Bhagavad Gita: "Now I am become Death, the destroyer of worlds."

Within weeks, the United States had dropped two atomic bombs on Japan - at Hiroshima on August 6, and at Nagasaki three days later. Japan surrendered on August 15, and the Second World War was over.

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A massive column of billowing smoke, thousands of feet high, mushrooms over Nagasaki, Japan

Arguments still rage about whether using the bomb against Japan was justified (see panel below).

What is not in doubt is that, once the power to split the atom had been unleashed, it could not be contained again.

A world riven for years by war between the Allies and the Axis split into new blocs, dominated by the USA and the Soviet Union. Fear and mistrust led to the 'cold war', an arms race - and the stockpiling of enough nuclear weapons to destroy the world many times over.

"The possibility existed of bombing human civilisation past the point of recovery," says Dr Nick Ritchie, an expert on nuclear disarmament at the University of York.

In 1961, at the height of fears over an atomic war, a concrete bunker opened beneath a quiet side street in Acomb.

It was one of 29 across the country which made up a network of semi-secret posts primed to observe and monitor a nuclear attack.

A team of 60 highly trained people - mainly Royal Observer Corps volunteers - stood ready to drop what they were doing and retreat into this bunker, says Kevin Booth, senior northern curator of English Heritage, which now looks after it.

Their role would have been simple: to chart the locations of nuclear explosions on or above British soil, and to monitor the spread of nuclear fallout so the authorities could predict which areas of the country were likely to be 'a bit more safe' than others. "They were preparing for Armageddon," Kevin says.

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Kevin Booth inside the control room at York's cold war bunker

It may seem hopelessly naive to us today. But in the early Sixties, there was a genuine belief that a nuclear war was survivable, Kevin says.

A visit to the cold war bunker today brings home as nothing else can how terrifyingly close we came to catastrophe.

The bunker - like the 28 others nationwide - was itself the centre of a local network of smaller bunkers dotted across Yorkshire. Each had the ability to detect nuclear explosions and monitor radiation levels in the air.

Their data would have been telephoned to the central bunker in Acomb where, in the underground operations room, the location of nuclear bomb strikes and the progress of radioactive fallout would have been plotted on giant screens and shared across the nationwide network.

The bunker was in operation from 1961 until the end of the cold war in 1991 - and today, it has been preserved as it was on its last day.

Standing in the operation room you can't help trying to imagine what it would have been like for the 60 men and women if they'd had to be sealed in here as the world ended.

They wouldn't have been allowed to bring loved ones, says Kevin: it would have been just them and the awful knowledge of what was happening above.

The list of potential British targets the Soviets had was terrifying, Kevin says - virtually every city, large town, industrial area and airbase in the country. But the Soviets also knew that if they dropped just 10 decent-sized nuclear bombs on western Britain, the prevailing winds would drive radioactive fallout across the country, wiping out a large percentage of the population.

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One of the two dormitories in York's cold war bunker

The York bunker's last commandant was Jim Millington. By day, he worked for BT in Leeds. But he also held the rank of observer commander in the Royal Observer Corps.

He was a married man in his fifties with two sons. Would he really have been able to leave them to enter this bunker if the worst had happened?

Now 81, he answers without hesitation: yes. "I had given the undertaking."

In many ways, he says, it would have been like going off to war. And he genuinely believed that what he and his colleagues were trained for was worth doing. They'd have helped ensure the government had the best possible information on nuclear strikes and fallout so it could make decisions which would potentially save lives. "People would have needed to be warned. We had to be prepared."

The reality is that York, and this bunker, would probably have been wiped out in any nuclear strike, Kevin Booth says. But observers in other bunkers across the country would have continued monitoring the spread of radioactivity as, above them, the world ended.

Thankfully, it never came to that. Probably the closest was the Cuban missile crisis of 1962, Kevin says. Then, in the 1980s, we came close again when the Soviets misinterpreted NATO war exercises.

There were other, terrifying near misses - often caused by simple accidents or misunderstandings. A nuclear war could easily have broken out by accident, Kevin says. "It's a miracle that it didn't."

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Nuclear Bomb Test, Nevada Desert, June 4 1953

The danger isn't over. York's cold war bunker may have been decommissioned, but the threat of nuclear war is as real as ever, warns Dr Ritchie.

As a results of years of gradual decommissioning by Russia and the USA, there are far fewer nuclear weapons stockpiled now. But there are still more than enough to cause massive, catastrophic destruction. Russia and the US each have something like 800 warheads on 'hard alert', ready to be deployed within 15-20 minutes, Dr Ritchie says. It would take only 5-6 such bombs striking the UK to put us 'past the point of recovery'.

And in many ways, the world we live in now is more unstable than during the cold war. Then, there were essentially two sides stockpiling weapons. Now many more states have nuclear capability - all five permanent members of the UN security council, plus Pakistan, India and Israel.

The recent deal struck with Iran - under which the country promises to limit nuclear activity in return for the lifting of international economic sanctions - is positive, Dr Ritchie says.

But North Korea has up to 10 primitive fission bombs. And it is even possible that a terrorist organisation like ISIS could develop a bomb. The main challenge would be getting the enriched uranium or weapons-grade plutonium. With that, building a bomb wouldn't be difficult.

As long states - including ours - regard nuclear weapons as legitimate, the world will continue to be at risk, Dr Ritchie believes.

The best hope is that nuclear weapons come to be regarded internationally as simply unacceptable.

There are signs that is beginning to happen.

The Humanitarian Initiative of more than 150 countries has held three international conferences since 2013. At the most recent, in Vienna last December, more than 100 governments signed a Humanitarian Pledge calling on nations to remove legal obstacles to the 'prohibition and elimination' of nuclear weapons. Iran was among the nations which signed: the US, UK and Russia were not.

But there is momentum, Dr Ritchie says.

Until attitudes change, however, we continue to be at appalling risk.

"Day to day, the chances of something going wrong are small," Dr Ritchie says. "But if things do go wrong... the consequences would be catastrophic."

  • Read the full Humanitarian Pledge here:


Was dropping bombs on Japan the right thing to do?

In September 1946 a young RAF flight mechanic, Ernest Astley, was posted to Japan. He was to be stationed near Hiroshima, and as he flew over the city he looked down from the aircraft’s window. “We’d been flying over green, and then suddenly it was all brown,” he says. “Just nothing but the odd tree here and there.”

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Ernest Astley as a young RAF flight mechanic

A few days later, he took the train to see Hiroshima for himself. It was a year after the bomb. Everything was devastation: with just a few larger concrete buildings sticking up out of the rubble like fingers. He remembers seeing glass bottles welded together by the heat, and bicycles fused together by the intensity of the explosion.

He went back three more times over the next 18 months, and on his last visit spoke to some Japanese survivors. And, despite the awfulness of what he saw, he firmly believes dropping the bomb was the right thing to do.

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A devastated Hiroshima after the dropping of the bomb

More than 200,000 people died at Hiroshima and Nagasaki. But far more would have died had the war continued, believes Mr Astley, now 87 and living in Dunnington.

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Ernest Astley today

It is even possible that, during the cold war, the threat of nuclear reprisal may have helped maintain an uneasy peace, Mr Astley believes. That said, he fully accepts that the existence of nuclear weapons today makes the world a far more dangerous place.

That is why it is time to get rid of them, says York Against the War.

NATO claims its nuclear weapons, including Britain’s own Trident, are for defence, the group says. “But when your bombs defend you against every nation, then you can attack any nation,” said a statement from group members John Heawood, Maurice Vassie and Alan Gerrard. “Children living beneath US drones know that.”

With the UN powerless to enforce nuclear disarmament, the existence of nuclear-armed nations is a powerful incentive to other nations to develop the weapons, the group says.

The tragedy is that nuclear weapons are virtually useless against what threatens us today - terrorist groups like ISIS and al-Qaeda. “It’s time to cancel Trident,” they say.