National Geographic: The World's Biggest Bomb Revealed

As World War II ended with the atomic bombing of Japan, a secret war commenced,

ushering in one of the most terrifying periods in recent history.

Scientists in the Soviet Union and America set to work designing even bigger bombs,

culminating 50 years ago in the biggest man-made explosion of all time,

a single bomb which could wipe out millions of people.

The awesome power of these devices still casts a shadow.

In the Pacific, whole islands were vaporised or turned into a radioactive danger zone.

And the scientists themselves were often flying blind, pushing the science too far,

in the effort to stay ahead.

When scientists panicked, and started crawling up the beach in terror, as this thing kept going up and up.

Now we'll learn what really happened in this clandestine war.

From one of the leading scientists...

The cloud just kept rising and rising but what frightened me was the heat.

I never thought the heat was going turn off.

From a US marine, who barely escaped with his life...

I turned around to my buddy, and I said "Hey, I think we're goner."

He said "Yeah, I think you're right."

These explosions changed the course of the Cold War, shaping the world we live in today.

Half a century on, we'll uncover the story of this secret struggle;

the race to build the world's biggest bomb.

In October 1961,

technicians at a US military monitoring station close to Washington DC received startling news.

A shockwave on their seismograph appeared to come from a place far from any earthquake zone,

from Russian territory, inside the Arctic Circle.

There was only one possible explanation for what looked like a mini earthquake:

The Soviets had a bomb many times bigger than anything the US had built.

One of America's key scientists at that time remembers when the news came through.

We were impressed.

That meant they had a deliverable 50-megaton weapon. That was pretty scary!

A weapon more than 3000 times bigger than the Hiroshima bomb...

Agnew knew exactly what that meant, because he was there right at the beginning.

As a brilliant young physicist, he worked on America's wartime 'Manhattan Project.'

Agnew flew on the historic mission to Hiroshima,

in a B-29, shadowing the delivery aircraft 'Enola Gay'.

His task was to measure the yield from this first atomic bomb.

When the bomb went off, we saw the light. Then we felt two shockwaves, which surprised us.

Then we realised one of them was reflection from the ground.

Then we dashed over to look out of this little porthole.

And I took the pictures of the Hiroshima cloud.

His film is a unique record,

the only moving images of the Hiroshima bomb.

Agnew flew back to help prepare the next bomb.

He's pictured here, carrying the bomb core itself.

When dropped on Nagasaki, days later,

this small innocuous-looking device effectively ended World War II.

These bombs had killed more than 150.000 people.

Many more would perish from their injuries and the effects of radiation.

When he learned of their power,

Soviet leader Josef Stalin decided he too must have such a weapon.

The Cold War arms race was underway.

Two weeks after Hiroshima,

Stalin ordered scientists here, at Moscow's Lebedev Institute,

to deliver him an atom bomb.

Physicist Boris Alchula was part of an elite community of Soviet scientists.

His father helped design Stalin's first bombs.

My father really understood that they must have the bomb.

Because it is necessary to save our country, to save peace as they believed it.

He was brought up in an "Atomgrad",

a secret township of scientists 400km east of Moscow.

I lived there, and I can tell you they worked absolutely night and day to restore the balance.

We physicists and chemists said it would be just a matter of time in fact.

The politicians thought it would it would take them longer than we thought it would take them.

Agnew and his fellow scientits were tasked to develop ever bigger bombs

to ensure the United States stayed ahead of the soviets.

But testing these weapons would turn an island paradise into a raging inferno.

As America and the Soviet Union raced to develop ever more powerful nuclear bombs,

a series of vast experiments would be conducted here on Bikini Atoll,

one of the most remote, and for many years, most secret places on earth.

This coral atoll in the Marshal Islands lies 4300km south-west of Hawaii.

In 1946, the US decided this is where it would test the next generation of weapons.

Historian Richard Rhodes is a leading chronicler of the period, a Pulitzer Prize winner.

He says the atom bombs spelt excitement than even sex appeal.

The bikini was invented by a French designer in 1946.

They decided to name it after the sexiest place at that time on the planet,

which was where the United States was conducting nuclear weapons tests.

The Bahamas at that time had a kind of charisma. That, of course, it soon lost.

The bombs were big news.

And the people of Bikini seemed happy to be centre stage.

Even if it meant temporary upheaval.

These first pictures of the little island

show the 165 men, women and children as they prepare for the move to another small island,

a safe distance away.

The first test, in 1946, would be one of the most remarkable experiments ever conducted.

The Americans assembled millions of dollars worth of weapons and a fleet of 95 deserted warships.

Captured Japanese and German ships, plus old American vessels were lined up as targets,

with further data from animal subjects.

Different animals have been placed aboard the ships.

Some were shaved in order that the effects of heat and radiation on their skins could be observed.

Five, four, three, two, one... Fire!

As these warships and weapons were sacrificed, it sent out the clearest signal.

The rules of war had been re-written.

The atom bomb trumped yesterday's weaponry,

and only America knew the secret of how to build one.

Or so they believed...

Years before our experts believed it would be possible,

the Soviets in August 1949 tested their first atomic bomb.

And there was panic in Washington as a result.

The Soviet weapon was an almost exact copy of the Nagasaki bomb.

So could there be a spy inside America's super secret design centre in New Mexico - Los Alamos?

There were American idealistic physicists who gave American secrets to Soviet intelligence.

They had the idea that there must be balance.

It's very dangerous if only capitalists, imperialists, the United States, will have these weapons.

Two spies were arrested; and their treachery kicked this race into an even higher gear.

There was a great rush in Washington to find something that could reassert the balance.

And for some of the people in Washington, in fact those who prevailed,

the hydrogen bomb was that thing.

The bombs exploded so far at Bikini were all variations on the original atom bomb design.

But the force unleashed by a hydrogen bomb would soon make those devices look primitive.

There would be two key components:

an atom bomb, and a tank of hydrogen isotopes, such as tritium and deuterium.

The primary explosion would force together - or fuse - the bomb fuel, producing immense heat;

a thermonuclear reaction releasing energy measured in megatons,

releasing energy measured in megatons,

millions of tonnes of TNT.

The other important thing about a hydrogen reaction is that it's kind of like a fire;

the more fuel you add, the bigger the fire.

It has an unlimited potential size,

whereas the biggest fission weapon we ever built was half a megaton,

500.000 tonnes of TNT equivalent; really couldn't get any bigger than that.

We all agreed, at least the technical people,

that it would just be a matter of a few years

before the Russians would have the same capability.

The fact the Soviet Union had so quickly, with the help of espionage,

developed their first atomic bombs,

made it certain from the point of view of the United States Joint Chiefs of Staff

that yes, we must have this weapon.

But would it work?

It was down to Harold Agnew, now in his early thirties, and his Los Alamos colleagues,

to build the world's first thermonuclear device, codenamed "Mike".

I was a project engineer with a small group.

We actually put tritium in Mike, we were in charge of doing that.

We got the thing fabricated, got it shipped out.

It was an atomic bomb and a long cylindrical tank of liquid hydrogen,

surrounded by a cylinder that would funnel the huge burst of radiation that comes off an atomic bomb,

and turn the material around the hydrogen cylinder into an intense plasma,

a very very hot gas, which would then set off the hydrogen that was in the tank.

That was the Mike device.

By November 1952, Mike was ready for testing.

I was on the Curtis, which was a seaplane tender, 30 miles away, waiting for Mike to be detonated.

What frightened me was the heat.

We were just in a pair of shorts and this got hotter and hotter and hotter.

This cloud was 30 miles away, but it felt as if it was on top of us.

Here was a truly historic scientific milestone, the world's first man-made thermo-nuclear reaction.

But at 82 tonnes weight, what use was it?

Propaganda wrote the first hydrogen bomb was tested by the United States in 1952, it's called "Mike".

It was not a bomb; it was a construction like a big 3-storey house.

It was a bomb that you would have to, as Robert Oppenheimer said, deliver by ox-cart, or by ship.

That wasn't very useful; he wanted one you could deliver by air.

The race was now to develop a thermo-nuclear device which would fit inside an aircraft.

And despite their early lead, America seemed to be falling behind.

In 1953, the Soviets announced they'd developed the first practical H-bomb.

Could the US do the same?

Six months later, Los Alamos produced their answer to the Soviet bomb;

far more compact and based on the lightest metal that there is: lithium.

There are various versions of lithium, "isotopes" they're called,

and the one that was needed for this weapon was Lithium 6;

it had three protons, three neutrons; Lithium 6.

The fuel mixture, known as "lithium deuteride", was packed inside this aluminium cylinder.

Bikini was about to witness the most powerful explosion ever staged by the United States;

codename Castle Bravo.

But this time, things would go dramatically out of control.

They tested the bomb with liquid hydrogen but they'd never tested one with lithium deuteride.

And the measurements had been made wrong, as it turned out.

The scientists' miscalculation would have terrifying consequences;

for some of their own team,

for hundreds of Pacific islanders,

and for unsuspecting fishermen in the path of this massive explosion.

Almost 60 years ago, John R. Halderman was a young marine corporal.

Shipped out to the Pacific for this top secret mission,

he stood guard over America's super-bomb.

It looked like a big propane tank; about 5 feet in diameter and 20 feet long.

I wrote my name on it, being smart; signed it.

But we had to stay with that until it was all set up to go off.

And you weren't allowed on there if your name wasn't on the access list.

We had orders to shoot to kill.

Bikini Atoll is a ring of small islands.

Halderman stood guard in the northwest corner, where Bravo would actually detonate.

But it would be triggered from the island of Enyu, 35km away.

What happened next was documented in this once classified film.

Starting at H-48 hours, all personnel at Bikini Atoll were evacuated.

The only men to remain were the members of the firing party, protected in their bunker on Enyu Island.

The bunker is located just beside the island's landing strip.

It meant that if things went wrong, they could call in a helicopter for rescue.

But the men inside here felt safe,

protected not just by reinforced concrete, but massive blast doors.

Their job was to ensure that all monitoring devices were running

and finally prime the firing circuit itself.

If the scientists calculations were correct,

the lithium deuteride mixture would erupt with the force of 5 million tonnes of TNT.