SIXTY years ago Russia unleashed hell on the world - a doomsday mega bomb that could flatten a city and kill millions.
The infamous Tsar Bomba ignited what is still the most powerful man-made explosion ever seen, with 3,300 times more energy than the nuclear bomb that fell on Hiroshima.
Declassified video showed the terrifying test detonation over a remote island in Russia's Arctic in 1961.
It serves as a powerful reminder of the danger the world faces as Russia ramps up its nuclear threats over Ukraine.
Vladimir Putin test fired a Satan-2 ICBM last week, and vowed to boost his "nuclear triad" in a chilling threat to the West.
He also suspended Russia's participation in the New START treaty on non-proliferation with the US.
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UN chief Antonio Guterres urged the tyrant to "step back from the brink" of nuclear conflict.
Russia has the world's largest stockpile of 6,000 nuke warheads and is racing to mount them on "unstoppable" hypersonic missiles.
None is as huge as the Tsar Bomba or "King of Bombs" - the nickname of an experimental warhead that changed the course of the Cold War.
Officially called "product 602" and code-named Ivan, the massive thermonuclear bomb was designed to show Soviet scientists had caught up with the US in destructive power.
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The city destroyer was so huge - 26ft long, 7ft wide and weighing 27 tons - it could not fit in a plane.
Top secret footage declassified in 2020 shows it slung below a specially converted Tu-95 bomber on its one and only test.
On October 30, 1961, the crew released the monster nuke at high altitude over Severny Island in Russia's far north.
A parachute slowed its fall before it detonated 2.4 miles above the ground with a blinding flash seen 630 miles away.
Footage shows a hellish mushroom cloud of dust and debris that rose 42 miles into the sky - seven times the height of Mount Everest.
The earth-shattering shockwave destroyed buildings on the sparsely populated island up to 34 miles away.
And it reportedly cracked windows as far away as Norway and Finland more than 1,000 miles from the blast site.
According to reports, the nine-strong Soviet crew on the Tu-95 bomber were given just a 50 per cent chance of surviving the blast.
It was 24 miles away by the time the bomb went off, but the shockwave forced it to drop more than half a mile in altitude - although it was able to land safely.
A second aircraft carried a laboratory crew of five tasked with monitoring the test. Both planes were coated with special white paint to reflect radiation.
Measurements showed the blast yield was 50 megatons - equivalent to 50million tons of TNT.
Thankfully the Soviets decided the monster bomb was too large to use, and smaller warheads delivered by missiles became the focus of Cold War arsenals.
But if the Tsar Bomba had ever been dropped on a built-up area, the results would have been devastating.
According to - a tool created by historian Alex Wellerstein - a 50 megaton blast today in London would kill 5.8million people and injure another 3.4million.
A detonation over Trafalgar Square would generate a six-mile-wide-mile fireball that would engulf the whole of central London and as far out as Kensington, Camberwell and Camden.
Anyone caught inside the fireball would be "vaporised".
The "heavy blast damage" zone would reach out 5.5 miles from the centre, destroying concrete buildings and killing almost everyone.
Greenwich, Streatham, Hammersmith and Cricklewood would be flattened.
The outer suburbs would not escape as the "moderate blast damage" radius stretches to 12.9 miles, according to the tool.
Most residential homes would collapse in Hounslow, Barnet, Chigwell and Bexleyheath, with widespread fatalities and no one escaping injury.
Windows would shatter over 30 miles away deep into Surrey and Essex - potentially shredding people who went to look after the flash.
As well as the destructive shockwave, the nuke would trigger an intense blast of thermal radiation.
It would cause third degree burns - possibly requiring amputation - up to 37 miles away.
People in Reading, Bishop's Stortford and Southend-on-Sea would be within the enormous danger zone.
The figures are based on a simulation of a 50 megaton blast like the two-stage Tsar Bomba.
However the original design was for a three stage device yielding 100 megatons.
Scientists left out the third stage on the test device - halving its power - because it would have generated enormous amounts of radioactive contamination.
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The creation of the megabomb was seen as a turning point in the Cold War and in part led to an international treaty banning nuclear weapons testing above ground.
The bomb's designer Andrei Sakharov was so horrified by his creation he went on to campaign against nuclear proliferation, earning him a Nobel Peace Prize.