the threat of radioactive fallout,

of mutually assured destruction and of the end of the world.

But they don't have to be:

any large enough explosion will cause a mushroom cloud.

And it's happened in Britain: right here.

This crater was once RAF Fauld, an old mining operation

converted to an underground bomb storage depot for the Royal Air Force.

Underground storage was meant to be safer:

one of the problems with munitions dumps is that the enemy

just needs to hit them with one bomb and the whole lot goes up.

This site seemed perfect: but there was a problem.

Because this was built in the 1930s for small bombs,

the sort that one person can carry,

the sort that you can move on a conveyor belt,

the sort that the Army can fire from a field gun across the trenches.

That's why these old quarries with all their tunnels and caverns were perfect.

But then the Second World War moved on,

and bombing became a job for the Air Force,

and the bombs got bigger and bigger and bigger, fast: half a ton, one ton, two tons.

And the war effort needed many, many more bombs than had been predicted.

Rapid changes in supply and demand meant that the underground depots were filled

far beyond anything like their original capacity.

By the end of 1944, the men here at RAF Fauld were storing stacks of high incendiary bombs

outside the mine until they could find space inside.

There was a war on.

They had to make it work.

Monday, 27th November, 1944.

The commanding officer was on leave.

The officer responsible for the underground stores was also on leave --

and it was his deputy's day off too.

Everyone who was down in the depot and in charge was inexperienced and massively overworked.

According to the official inquiry,

someone tried to remove a detonator from a live bomb with a brass chisel.

And brass chisels cause sparks.

In Morocco, 1500 miles away, seismographs recorded what felt like a distant earthquake.

A hundred miles away, people heard a rumble carried on the wind.

Four miles away, a shock wave blasted out windows.

And here: a mushroom cloud rose into the sky.

4,000 tonnes of bombs had gone up in one terrible chain reaction,

the largest explosion ever in the British Isles.

This crater was once a hill, with a farmhouse on top of it:

no trace of that farmhouse, or the people who lived there, was ever found.

The bombs that hadn't immediately exploded were thrown into the air

and rained down for miles around, along with millions of tons of debris.

A nearby dam collapsed, sending floodwaters down into factories and houses in a nearby valley.

The exact death toll will never be known; at least 60, perhaps 90.

It took months before all the bodies were recovered, and some were never found.

And in the Air Force reports, held classified for years and years after the explosion,

along with the names of the dead and page after page

itemising every bit of the destruction, there is this phrase:

"columns of black smoke and debris rose in gigantic mushroom form".

When you're rushed, when you're inexperienced, anyone can get complacent,

anyone can cut corners, anyone can make a mistake.

Even working with high explosives.

Now slowly, over years, the crater was cleared and reclaimed; decades later,

what remains is an odd scar on the landscape.

About half the bomb store actually managed to survive the explosion;

and while the bombs were removed, there are still tunnels and storage rooms hidden under the crater.

But after careless explorers started damaging the site,

all the known entrances were filled in.

And given the warnings about unexploded bombs still in the crater,

I'm not about to climb this fence and go any further.

There is one little interesting note, though,

right at the end of the story:

through the late 40s and the 50s,

the American military kept asking for records and details of the explosion.

How big was it, exactly?

What was the damage to the nearby land, and was it by earthquake or by blast wave?

What happens if you, essentially,

detonate something with the yield of a tactical nuclear bomb underground?

Britain never answered.

And America went on to make its own mushroom clouds.

[Translating these subtitles? Add your name here!]