Building Britain Britain Beneath Your Feet

This is the Britain we know.

A patchwork of fields, forests,

of rugged mountains and dramatic skylines.

But ours is also a land of secrets

that you can only see if you look at it in a new way.

From beneath.

I'm going to show you Britain as you've never seen it before.

The hidden world below our cities.

The engineering genius that powers the nation

and keeps the country moving.

I'll be going deeper and deeper underground

to explore this unknown Britain.

To experience its awesome wonders.

The scale and the drama of this place is just off the chart.

And it's dirtiest surprises.

I don't even want to know what that is.

It will change the way you think about our country.

I want to unlock the secrets of what's above ground

by understanding what's below.

How every city, every forest, even every field,

depends on an extraordinary hidden world beneath our feet.

'I'm on my way to one of the most

'amazing structures in modern Britain.'

I'm about to discover how what goes on below ground

can have a profound and unexpected impact on what happens above it.

The realm of the underworld has so often determined

how Britain's been built over the centuries.

One building towers over London.

The tallest ever constructed in Western Europe.

The Shard.

London has never, ever looked this good.

The Shard's over 300 metres tall,

but until recently, London had precious few skyscrapers.

It wasn't until the 1960s

that the first building over 20 storeys went up.

Now, just compare that to New York. Think about Manhattan,

where, they've got over 600 skyscrapers

that were built since the 1890s.

Now, what is the difference?

# One, two, get down. #

It's all to do with what's underground.

You might assume you're best to build on something solid,

and you'd be absolutely right.

New York sits on hard rock.

This makes it relatively easy to secure skyscrapers into the ground

and support their colossal weight.

Classic skyscrapers, like the Chrysler Building,

built as early as 1930.

And the Empire State Building a year later.

Then the tallest building in the world.

So, why doesn't London have a skyline like Manhattan's?

Well, it's all to do with what's underground.

Last time I did this was in about 1983.

-Right, I'm going to step back.

-I'd stand well back if I were you.

'Underneath most of London is this stuff.

'Soft, squidgy clay.

'A huge challenge for Roma Agrawal,

'a structural engineer who helped design The Shard.'

This is London clay, straight out of the ground.

This is the stuff that you would actually build on.

But is it the same clay that you'd get in your modelling shops

to make a pot, or a mug, or something?

Amazingly, the answer is yes.

All these pots and things that we make out of clay

and the stuff that we're putting our buildings on top of in London

are pretty much the same thing.

Let me get an idea, how difficult is it to actually build in London,

compared with somewhere like New York,

which is pretty much building straight onto bedrock?

Yeah, exactly. So New York is actually brilliant for skyscrapers.

You come in, you put the skyscraper on the rock

and the rock is very strong.

And the loads just go straight down into it.

In London, we need to do a bit more kind of gymnastics around the soil

to try and make sure that our loads are going down where it should be.

It's bizarre that this is the exact material

that you're building huge skyscrapers on.

It is very difficult to believe.

Yeah. It just seems... Why would you?

Well, we don't have a choice, do we, in London?

'Building on soft ground can have catastrophic consequences.'

In 2009 in Shanghai, China,

a 13-storey building was being constructed.

Its foundations couldn't support its weight.

And one day, it simply toppled over.

Despite the devastation, only one worker died.

'So, just how do you build the tallest building in Britain

'on soft, unstable clay?

'Well, to answer that, we're going to reveal how it would look

'from an entirely different angle.

'From beneath the ground.'

Imagine the earth is made of glass

and you could look up and see the street level above.

From down here, we can see a completely new subterranean world.

Tube trains thunder below the surface,

carrying millions of us to work each day.

And next to London Bridge Station, the base of The Shard,

sitting on the soft, unstable clay.

The Shard needs foundations,

and not just any old foundations.

18,000 tonnes of building is kept upright

by over 100 concrete piles.

And they're deep.

Most foundations only go down a few metres.

The Empire State Building's are just 16 metres deep.

But The Shard's are three times deeper.

An astonishing 53 metres down.

Deeper than Nelson's Column is tall.

So building London's new skyline has required

some of the most impressive foundations in the world.

Its 53-metre-deep foundations mean that The Shard remains,

and will remain, firmly rooted to the spot.

And thanks to these new techniques in digging ultra-deep foundations,

it means that London has had a real growth spurt in recent years.

We've seen all these new skyscrapers popping up all over the place.

You've got the Cheesegrater building

and you've got the Walkie-Talkie building and The Gherkin.

Who would have thought that that soft London clay

would have such a profound effect on the London skyline?

This may be one of the great sights of modern Britain,

yet the real wonder is the secret world that lies beneath.

Across the country, it's often the natural landscapes

that dominate our view.

Forests and fields, rolling hills,

ancient mountains.

But some of our most spectacular wonders are invisible,

hidden underground.

This is Gaping Gill in North Yorkshire.

A hole in the ground that swallows a river.

People once believed this was a gateway to hell.

More people have summated Everest than have abseiled into Gaping Gill,

and I can understand why.

When you look over the edge, it's just a black void into nothingness.

And that stream that you can see going over the edge

that doesn't look too dramatic,

it actually turns into a waterfall twice the height of Niagara Falls.

There is only one way, really, to explore it,

and that's to go over the edge.

I hope this looks nice from where you're sitting

because from where I am, it is bloody terrifying.

Ah! My God, just look at it!

The scale and the drama of this place is just off the chart.

It's much bigger and more impressive than I could imagine.

'With a vertical drop of over 100 metres,

'this is the tallest waterfall in Britain.

'A hidden natural wonder.

'And standing here, you can feel its raw, elemental power.'

And that water that pours down, it's not just here for dramatic effect.

That water is a real force of nature.

It's carved out Gaping Gill.

We think of the ground as pretty solid,

but down here, water has created a void as vast as York Minster.

So, just why did this gigantic underground cathedral

form in this particular place?

'Well, the water isn't quite what it seems.'

I've got some acid and a little pipette.

And if I put a little bit on the limestone,

you should see it fizz away

as it starts to dissolve the limestone. Let's have a go.

That's the one. Yeah, there you go. You can start to see it fizz away

as it eats away the limestone.

That's what's happening in here, but very, very slowly.

'The water that pours down here is, in fact, a weak acid.

'It forms when rainwater mixes with carbon dioxide in the air and soil

'to make carbonic acid.

'The same stuff that's in fizzy drinks.

'And that acid reacts with the particular type of rock

'that exists here. Limestone.'

It's this process, drawn out over 30 million years,

that's carved out Gaping Gill.

Hidden beneath these limestone hills.

'To escape the cavern, I've had to crawl for over a kilometre

'through a labyrinth of narrow passages.'

What's this? Oy-oy-oy!

From Britain's biggest underground cavern...to the smallest.

See, getting down was the easy bit.

Getting up is...is really difficult!

As I emerge from the womb,

I feel like I'm being born again.

That was one of the hardest day's filming ever.

It's really interesting because just in terms of fear factor,

you have that terrible fear of heights

combined with dreadful claustrophobia.

On every level, it was dreadful!

Across Britain, there are many more vast cave systems.

One stretches 90 kilometres,

nearly twice as long as the Channel Tunnel,

linking Cumbria, Lancashire and Yorkshire.

In fact, wherever you find limestone like that at Gaping Gill,

chances are there's a hidden network of caves and rivers below.

But if you live in a city, you might be just as surprised

at the extraordinary things you'd find beneath your streets.

Nowhere is this more true than Bristol.

You just need to know where to look.

I'm on my way to a place few people ever go, or even know about.

'This is the River Frome.

'And I'm about to pass into a lost world

'hidden below the city of Bristol.'

Here we go.

'Down here, the river's been entirely covered over,

'made invisible to the world above.

'My guide on this unique journey is explorer, Dave Talbot.'

I've a feeling we are well and truly off the Bristol tourist trail now.

We certainly are, yeah.

Over the last 200 years, this river's been

sort of built and covered over more and more.

-The last section was done in 1930.

-Yeah.

And since then, very few people have been down here.

'As the city expanded, the River Frome got in the way.

'So it was simply built over. Creating this secret world.'

It's quite odd. You've got a lovely river that comes through Bristol,

it's very picturesque, and then suddenly, they just decide,

"Oh, we'll just cover it over now!"

'Without building over The Frome,

'many of the roads, houses and shopping centres

'of modern Bristol couldn't have existed.

'A mile in and we're passing under the main shopping streets.

'And no-one knows we're below them.'

Every so often, you can hear cars above us.

Yeah, you can. It's quite eerie, isn't it,

to think that you're underneath the road.

'Another half mile or so, the tunnel begins to get lower

'as we pass under the very heart of Bristol.'

Actually, the thing that strikes me when you come down here is that

it really is a case of human needs versus nature.

So over the last couple of hundred years, obviously,

Bristol became very successful and expanded.

And as a result, the whole river has had to be completely confined

and hidden away from view.

'A bit further, and the headroom's even tighter.

'To be honest, I'm getting a bit nervous down here.

'But finally, after a treacherous six-hour journey,

'we emerge in Bristol harbour.'

Did you remember the key, Dallas?

There we are. Escape from The River Frome.

'Who'd have thought you could canoe under Bristol

'through a hidden underworld that most people don't know exists?

'And across Britain, millions of us

'go about our daily lives above lost rivers.'

There's the River Sheaf of Sheffield.

The River Farset of Belfast.

The River Sherbourne of Coventry.

And in London, you might think of The Thames as the only river,

but there's another hidden underground

that's transformed the city above.

The River Fleet.

'The only visible signs above that this river still exists

'is the street that bears its name.

'But The Fleet has been put to work.

'It performs a role so vital that without it,

'London could not be the city it is now.'

I've got to say, actually, looking down here,

it doesn't look like much of a river at all.

There's just a short ladder going down.

But if you want to explore the River Fleet these days,

you have to go underground.

'I'm entering a world that's not for the faint-hearted,

'or the weak-stomached.'

Here we are, this is the River Fleet.

It looks like a sewer, smells like a sewer.

Whoo!

'That's because it is a sewer.

'Back in the early 19th century, the open River Fleet

'was essentially a cesspit carrying disease through London.

'So it was decided to cover it up.

'And using 318 million bricks,

'Victorian engineers turned it into this.

'It means that millions of us can now live

'without the risk of disease in a few square miles.

'But it also hides some pretty gruesome surprises.

'Dave Dennis is one of an army of underground workers

'that keep The Fleet flowing.

'Surely, one of the least-enviable jobs in Britain.'

So this is the main sewer tunnel.

Are there tributary sewer tunnels that come off it, or is this it?

Yeah. Yeah, there's loads.

Obviously, down very small side roads, you get a main sewer,

and they drop into a trunk sewer, which we're standing in.

And if you look over here, there's a small junction,

which basically is a small sewer.

What comes out of here, this is human waste?

Straight from the toilet, that is. Direct from the toilet.

-Man!

-Direct from the customer.

-Oh, the smell!

-Yeah, I know.

DALLAS COUGHS

Oh, man!

'Every underground labyrinth has its monster, as I'm about to discover.

'Down here, it's not a minotaur,

'it's something far, far worse.'

Oh, God!

Oh, the stink!

Oh, my God, that is horrendous!

Is this white stuff just fat?

This is pure fat that's solidified with other material.

This has come from all over London

-and it congeals here because it's a bit of a bottleneck.

-Right.

You have to come and break it up?

-We have to come down and break it up for it to flow downstream.

-OK.

'Down here lives the fatberg.

'A mixture of rancid fat,

'human excrement and other unmentionables.'

Oh, Christ!

You all right?

DAVE CHUCKLES

Do you want to get out, Dallas?

Oh, God, the smell!

-Look at the size of that!

-This is a...

-So, this is...this is a fatberg?

-This is a fatberg.

-Look at the size of it!

-See the worms in it?

It's got worms in it?!

-Oh!

-DALLAS COUGHS

'Nothing could have prepared me for the overpowering smell.'

DALLAS COUGHS AND GAGS

You all right there?

What did you have for breakfast(?) DAVE LAUGHS

Yeah, might see it.

This is one big berg.

Where does that rate on the berg scale?

-That's a...

-Is that a big one?

-Pretty good one.

Oop! It's breaking up quite nicely, though.

Shall I give it a...?

Yeah, give it a...give it a go.

This is worm heaven.

-So...

-What the hell is that?

-God knows!

-I don't even know what that is.

This is the most disgusting thing

I've ever done in my life, without a shadow of a doubt.

The worst thing is, yeah,

getting a bit of splashback and getting it in your mouth.

DAVE CHUCKLES

So when you guys say, actually,

please don't pour fat down your sink, you actually mean...

-Bin it, don't cook it.

-..can you really not pour fat down your sink?

'What begins as an innocent bit of grease in your kitchen

'can quickly transform into the monster fatberg.

'This is a job I don't want to repeat any time soon.

'London's clean, fresh air has never smelt so good.'

It is only because of sophisticated sewer systems like we have in London

that millions of us can live together hygienically

and safely, well, relatively, anyway.

But, cities wouldn't work without all that underground

engineering and infrastructure.

Places like London and Glasgow

and Newcastle and Manchester just wouldn't exist.

'But there's more beneath our cities than the worlds we've built.

'If we go deeper, we can discover an older, more spectacular kingdom.

'One that shaped the lives of millions of us.'

Bath. One of Britain's most genteel cities,

and perhaps our most famous spa town.

Where, for centuries,

people have come to indulge themselves in the waters.

Yet you might never guess that beneath the city

is a trap door into a dramatic underworld.

'But that story starts out in the countryside.'

To understand how the city of Bath came to be,

you need to come up here, into the Mendip Hills.

Now, this is a pretty ordinary stream.

And it's only about 15 miles to Bath

from where I'm standing here, as the crow flies.

But this water is about to go on an extraordinary journey.

A journey that's going to take around about 10,000 years.

Soon, the stream plummets down a hole.

From here, the water travels deeper and deeper into the earth.

To follow its tortuous underground route,

surprisingly enough, I need to take to the air.

So I've hitched a ride with my new friend, Julian, in his plane

so we can follow the journey, albeit in much quicker time.

So behind me that way, you've got the Mendip Hills, where we were.

And straight ahead of me, it's only 15 miles or so, is the city of Bath.

Now what's going on is down to some very, very,

interesting geology beneath our feet.

'From up here, the landscape looks the picture of Middle England.

'But below the surface is a world closer

'to the furnace of volcanic Iceland.'

The water cascades down cracks in the rock,

pulled deeper towards the centre of the Earth.

It gets hotter the deeper it goes.

Until nearly two miles down, it's forced back up towards the surface.

A vast, natural steam engine hidden deep below ground.

And here we are over the city of Bath.

And this is where the water reappears 10,000 years later or so.

We've done it in, what, five minutes?

10,000 years!

It seems like an incredibly long time.

If you think about the Pyramids, for example, that's 4,000 years ago,

so 10,000 years, the rains in the Mendip Hills

have taken to get to Bath.

'This is where the water finally emerges.'

And if you look, just on the surface of the water,

you can see little bubbles where you can see it percolating through.

You can feel the heat against your face.

And the funny thing is, it's not the buildings themselves

that are the oldest thing here, it's actually the water.

Apparently, it's very medicinal.

Mm!

Really metallic.

I can taste that iron kind of taste.

I'm going to put that to one side, I think.

'When the Ancient Romans first arrived in Britain,

'the hot water bubbling up from the ground

'appealed to their fondness for bathing.

'And 2,000 years ago,

'they founded a city around these hot springs.

'Today, both city and springs are still thriving.

'And now I'm taking my first plunge into 10,000-year-old hot water.'

Seems quite decadent, doesn't it?

The idea of an open-air heated swimming pool in Britain.

But you've got to remember that the water in this pool

is all heated naturally inside the earth.

It gets to about 60 degrees.

And then when it comes up out of the ground, it's about 46 degrees,

and then here, they cool it to a nice balmy 33 degrees,

which is the ideal temperature for my morning constitutional.

But because of that high heat that occurs inside the Earth,

this is Britain's only true hot spring.

'This modern spa is a continuation of what the Romans began.'

I think the really interesting thing about Bath,

and it is the most beautiful city,

is that it is only here because of the geology.

'Bath may be the hottest spring in Britain,

'but there are others dotted around the country.'

So the fashion for taking the waters

led to a proliferation of many other spa towns.

But one city more than any other

has been shaped by even more ancient and violent forces.

Edinburgh. A beautiful city that's grown up around this.

The magnificent castle.

It dominates the skyline.

Up here, you can instantly see why this is the perfect place to put your castle.

We're nice and high, we've got very, very steep banks

and you can see for miles and miles and miles.

It's the perfect place to put a stronghold.

But there's something curious about the rock the castle stands on.

On three sides, the rock is a sheer face,

but on the fourth side, a ridge of land slopes gently downwards.

Seen from beneath, we can follow its course.

Over centuries, the city was built on this slope,

becoming the central spine of Edinburgh's Old Town,

the Royal Mile.

And at its head, the great rock where the castle stands above.

This is the gateway to an ancient and violent underworld

that holds the secret of Edinburgh itself.

350 million years ago,

what's now Edinburgh was sitting on a violent volcano.

Liquid magma surged upwards from beneath the earth.

Once it stopped, the magma that had made its way to the surface

cooled and formed solid rock.

This is called basalt, and it's one of the hardest rocks on earth.

And it's actually solidified lava.

So all of this would have been molten.

And if you look, you can see there's all these cracks. There's one here.

So this liquid rock would have cooled and contracted,

which is how you get these cracks.

'Castle Rock is all we can now see of the violent volcanic forces

'that shaped this city.'

The tip of a gigantic pillar of basalt

that stretches hundreds of metres down into the earth.

But that's not the end of Edinburgh's story.

During the Ice Ages, glaciers moved across the land.

They gouged out the softer rock around the hard, ancient volcano,

leaving just the Castle Rock

and a protected sloping ridge in its shadow.

Standing here, you can see all the evidence of this powerful

and ancient geological activity.

So you've got the Castle Rock itself, this volcanic plug

of very, very hard basalt that would have stood firm

in the face of the oncoming glacier.

And then behind it, you've got this tail of sloping ground

that would have survived where the Old Town is built.

'So if you're on the castle ramparts,

'you're standing on top of an ancient volcano.'

Sometimes, the rocks beneath us have made us rich.

We may not have diamonds hidden underground in Britain,

but we've something that's turned out to be far more valuable.

Coal.

Coal was the beating heart of the Industrial Revolution.

It drove the wheels of commerce and fired the factories.

It turned Britain into an industrial superpower

and helped grow the Empire.

Coal put the great into Britain.

And the best way to get it was deep underground.

For most of us, it's a world we can barely imagine.

An underground very few of us experience.

But in its heyday, mining was Britain's most important industry.

And for these miners, this wasn't Britain beneath their feet,

this WAS their Britain.

'I've come to West Yorkshire to one of the few collieries

'where you can still descend to a coalface.'

-How deep are we going to go now?

-140 metres underground.

-It's going to take approximately...

-That's pretty deep!

Collieries were much, much deeper than that.

You look at some of the Yorkshire coalmines, they were as deep as 1,000 metres.

So relatively, it's a relatively shallow coalmine.

'It certainly seems deep enough for me.

'Very few of us go this deep underground these days,

'but at its peak this was the daily commute to work

'for vast armies of miners.'

This way, Dallas.

'The riches that drew them down here

'had lain secretly buried for millions of years.'

I've got a lovely fossil here

of a tree fern that's come from a coalface.

You can make out the shape of the bark here.

And it's really good evidence of where coal actually comes from.

So trees would have died, they would have rotted down,

they would have become compressed.

Eventually becoming peat and then coal.

And it's funny to think that the only reason we have coal in Britain,

so much of it, is because 300 million years ago,

the entire country would have been covered in swampy forests.

And this is the result.

An underground world where miners spent their entire working lives.

This is just one tunnel, but if we could see through the earth,

then we could reveal the entire mine network.

This mine complex has a central shaft

that descends 140 metres into the Earth.

Branching off the main shaft are four galleries.

Each one tapping into a coal seam.

Along each working seam, a branching, intricate network

of many smaller tunnels spreads out into the coal.

There'd be teams of miners working on each coalface.

'Steve Guest is giving me a tiny insight

'into what life down here was like for the miners.'

The technique you're going to be using, Dallas,

down, pick low, undercut it

-and drop it down onto the floor.

-OK, let's have a go.

-So, if I sort of go in here, around about here?

-Yeah, that's right.

So we're chipping away at the bottom,

the idea that the coal seam above us would have fallen down,

but presumably, that would have also been very, very dangerous, as well.

Very, very dangerous. It's not only the coal seam that come down.

If you've got weak rock above, this is going to collapse on top of you.

Lots of fatalities on a daily basis.

This is how the industry was.

I'm trying to imagine what it would've been like

with dozens of miners in this cramped area.

Small conditions, yeah. All working together.

But this is what their daily work would be.

Man, it's frustrating! I just want to take a big swing and I can't!

-That's me done!

-You don't think you'd have had a career in mining?

Well...I'd like to say yeah,

but if I'm honest, I think I would have, er...

I think I would have struggled with this.

'In this mine alone, there were 240 miners working underground.

'In total, a staggering 1.25 million people

'once worked down mines like this.

'That's as many people who today live in Sheffield, Newcastle

'and Manchester combined.'

Across Britain, in the coalfields that lay under Yorkshire

and Lancashire, the Midlands and the North-East,

under South Wales and lowland Scotland,

there were over 3,000 mines.

And above ground, major cities grew on the back of coal

and the industry it fuelled.

The riches below defined much of the urban map of Britain

responsible for where millions of us live today.

Mining may be fading from our lives,

but as so often in our islands,

what's left can be put to eccentric use.

As time marches on, our needs and our priorities shift.

And here in the Lake District is a really good example of that.

'Under a mountain, a slate mine has been appropriated

'for a rather different purpose.

'With their usual pitch destroyed by flooding,

'the Threlkeld Cricket Club

'has been forced to some rather unusual measures

'to raise funds for the repairs.

'Extreme cricket.'

CHEERING AND APPLAUSE

I'm better in daylight.

I've always been bad at cricket

and I'm even worse playing it in a slate mine.

'However with such a small pitch size,

'even I should manage a four, or even a six.

'Surely I can hit something!'

MUSIC: Soul Limbo by Booker T & The MGs

Quick single! Yes!

CHEERING

'A very disappointing innings.

'And sad to say, my efforts are more Fred Flintstone

'than Freddie Flintoff.'

Some 97% of the land area of Britain is countryside.

It dominates the map.

Most striking of all, the great forests

that make Britain such a green and pleasant land.

But hidden beneath the surface is a secret world we barely know about

and that keeps most of Britain alive.

To find out how, I've come to Burghley Country Park in Lincolnshire,

where there's an oak tree in its prime.

And with enough space around its base

to explore exactly what lies below in a way I've never seen before.

Just like any building, trees have their own clever foundations

hidden beneath the ground to keep them upright.

We call them roots, of course.

And a tree like this will have the most substantial roots of all.

And for the first time, we want to reveal the root system

of such a majestic tree.

To do this, I need some help.

Sharon Hosegood is an expert at uncovering the secret life of trees.

Now, is this going to give us a pretty accurate measurement

of the tree's age?

It's as good as we've got without felling the tree,

-which would be absolutely awful.

-Wow, that would be little bit

extreme to get the age!

-6.3.

-6.3. It's a big 'un.

A girth of 6.3 metres makes this tree around 440 years old.

In other words, it was a seedling

when Elizabeth I had just come to the throne.

To explore these roots requires something very special.

Now, it might look like a pimped-up pushchair,

but this is a sophisticated piece of kit

that's going to let us peer into the ground.

Have you done any science on a tree of this significance before?

Not an oak. This is the biggest, oldest oak tree

that I know is being scanned in the UK.

Are you managing OK? I feel like I'm...

-Like I should help, or something!

-It's a one-woman job.

Tell me how it works. What's actually going on here?

Well, this piece of kit here, this tree radar looks like a pram,

but essentially, it's a ground-penetrating radar

which will pick up the roots

because they're full of water.

When we think about radar, we sort of think about aircraft radar.

We send out radio waves and they bounce back.

Is it doing the same thing as that?

It pretty much is. The principle's the same.

There's only a few of these in the world.

It'll show how deep they are, how far spread they are.

It picks up everything this diameter and above.

And to come here with this amazing, old, veteran tree

and to be able to see what's underground is a bit of a privilege.

'Sharon's got her work cut out.

'She goes round and round the tree in ever-increasing circles

'to well beyond where she thinks the roots will extend.'

Do we actually have a result? Am I allowed to see...?

We have a result. You are allowed to see it and here it is.

Oh, my God! That's great!

So here is the base of the tree.

And we can see the roots taper down quite quickly from the buttress.

I can't believe how clear it is.

I just thought it would be just a big mass of black

and you'd have to sort of tell me, "Oh, that's the root there."

But that... I mean, it's beautiful, isn't it?

Are these the biggest roots you've seen on a tree?

Well, this is certainly the biggest tree that I've scanned.

And I'm really pleased and surprised at the root density of this.

I mean, it was actually more than I imagined.

It shows that for a tree to be this old,

several hundred years old,

it needs to have a well-developed root system.

I'd like to go up to the tree and tell him,

-tell the tree that the news is good.

-Yeah, yeah.

-Tree, you're going to be OK.

-You're OK.

-We've done your medical, you're fine.

-Yeah.

You're going to live another 400 years.

'Enhancing Sharon's data, we can reveal how trees

'keep themselves upright

'in a very different way from, say, a building.'

The root system of our oak tree here

is likely to be one of the most impressive in the whole of Britain.

Some roots are as thick as a big branch.

And Sharon reckons the roots make up as much as a quarter

of the total weight of the tree.

The spread of roots underground

is even greater than the expanse of the branches.

A 30-metre crown,

and an even more impressive 34-metre spread of roots.

You might think the roots of such a tall tree

would have to go deep into the ground.

But even for such a massive oak, these are relatively shallow.

No more than a couple of metres deep.

In a way, the complete opposite of how you do foundations in a building.

So The Shard, which is very, very deep and very, very contained,

-here, you have shallow and spread out.

-Exactly.

And that manages the loading and it also helps the tree

get all the water and nutrients it needs from the soil.

Another reason why they tend to be shallow is tree roots need oxygen.

And the oxygen is found in the top metre or so,

before the ground gets too hard and consolidated.

So this is the perfect solution.

-Nature has found the perfect solution for the tree.

-It has.

'The roots are not only supporting the tree,

'they're also the tree's life-support system.

'The very reason why it could grow in the first place.

'Think of roots as more than just pipes

'drawing water out of the ground.

'There's something else far more interesting going on.

'Hopefully, I can see what that is.

'IF I can burrow under here.'

I'm actually under a tree.

This is a nearby tree that was growing onto a hillside

and then the hillside's just slipped away in a landslide,

revealing the roots underneath. And you can actually get inside.

You can see just how many roots there are

and how knotted and tangled it becomes.

But there's something else

that happens to roots when you're underground.

Let's have a little dig around here maybe.

I might be able to show you. Yeah, here we go. Now, look at this.

Hopefully, you'll be able to see this.

This, er...sort of white, stringy stuff that almost looks like cobweb.

That is what I'm interested in.

And that is actually a kind of fungus.

Fungi, which includes mushrooms, are odd.

They're neither plants, nor animals.

But they do have a nice trick.

Fungi break up dead plant material in the soil.

And this releases nutrients which the tree roots can then take in.

Without this process, the trees just couldn't grow.

But the fungi can't do that job alone.

They have some rather surprising helpers.

If you magnify soil 500 times, you'll see, hidden inside,

an army of microscopic animals and bacteria.

Working together, they're the ultimate recycling machine,

keeping the soil fertile.

So there's a whole, vast ecosystem underground

that's completely invisible to the naked eye.

And yet it sustains all of this,

the natural world we're so familiar with.

Today, there are around 100 million trees in Britain,

covering 10% of the land.

But there are far fewer forests than there once were.

We've cleared most of our trees

to make way for another living habitat.

One that covers over a quarter of the country.

Our farmland.

The distinctive patchwork of British fields looks familiar to all of us,

but sometimes, underneath, these, too, can hold surprises.

'You can't see what's down there,

'but there is one way to try to find out.'

Terry!

'Terry Herbert's been metal-detecting for over 20 years.

'And now, he's going to teach me how to hunt for buried treasure.'

Right. You've got a control box here.

You've got the VDI display unit.

You've got the coil, which finds you the items, that does.

That actually sends radio waves into the ground.

And if you hit a target, it comes back

and it gives you a reading on the meter.

Is it the type of hobby that you could give up your day job for

and actually make a bit of money on?

Well, you can do. I mean, some do.

On the beach, some people go to Spain and actually detect.

-You can earn quite a bit.

-Yeah.

So, what's the sort of most exciting thing that you've ever found?

Well, actually, the most exciting I ever found was a Saxon hoard.

It's the biggest haul of Anglo-Saxon gold and silver ever found.

In July 2009, Terry made the discovery of a lifetime.

An enormous collection of Anglo-Saxon treasure.

There were over 3,500 pieces

of decorated gold and silver buried underground.

I'm trying to imagine what it would have felt like

the first time you got a signal, like we're getting here,

and actually scrabbled around. And did you realise...?

You said it looked a bit like perhaps brass, or something.

Yeah. But at what point did you go, "Actually, no, that's gold"?

When I looked at it under me magnifying glass,

I noticed there was a pin.

So I thought, "Oh, this is a piece of gold."

I actually went back to me car and got me other machine out.

And, er...when I came back on the field with that,

it was just going off like a machine gun.

When archaeologists recovered the entire hoard and began to study it,

they realised the workmanship was exquisite.

Now it's considered one of Britain's most important archaeological finds.

The Staffordshire Hoard has been valued at a cool £3.25 million.

Terry received half of this, making him a millionaire overnight.

What happened to the money? What did you do with it?

I bought another machine.

I've actually bought three more machines, since then, like.

I think there's got to be another Saxon hoard somewhere in Britain.

No way have I just found the only one in this country.

There's got to be another one somewhere waiting to be found.

And it's going to take somebody with a metal detector to find it.

'Of course, the fields of Britain weren't cleared and cultivated

'just for Saxons to bury their hoards and for us to find them.

'They performed the very necessary task of feeding the country.'

And it seems every year, we demand the land produces

more and more food to put on our tables.

More than the soil can provide naturally.

And to find the magic ingredients to keep our fields fertile,

we've had to go beneath the ground once more.

I've saved the best till last.

Because here on the North Yorkshire coast,

I'm on my way to the deepest point you can reach under Britain.

OK, off we go. How long does it take to get all the way down?

-Takes about seven minutes, something like that.

-OK.

-Starting off slow and then we pick it up.

-Suddenly it speeds up!

This is Boulby Mine and it's the deepest mine in Britain.

And this lift shaft is over a kilometre straight down.

It travels at about ten metres a second.

Now we're getting to the bottom of the shaft.

'But the bottom of the shaft is just the start of my journey.'

This is so great. This is as deep as it is possible to go in Britain.

You can feel the wind, as well.

All the air that they're pumping in from up there.

There's a heck of a wind that comes down and just blows you.

'This is an amazing place.

'The mine is so vast, you've got to get around by truck.

'And I had no idea that the tunnels stretch out

'over five kilometres under the North Sea.'

It is an incredibly surreal experience being down here

because we're actually out to sea now,

we're actually underneath the seabed.

We've left Britain behind us.

And the tunnels just keep on going and going.

'It's a funny way to go to sea.'

We're 1,100 metres underground.

It's certainly the deepest place I've ever been in my life.

It's blisteringly hot, as well.

'The surface of the rock can be nearly 40 degrees centigrade.

'But what I'm really here for is what that rock contains.

'Now, it may not look much, but this is real buried treasure.

'And to get my hands on it,

'I'm going to operate this remote-controlled monster.

'Its jaws will do the hard for me.'

-So now you lift the head up.

-Ah.

Now, down a touch.

-So that's the head going down a bit.

-Yeah.

There we go, we're cutting in!

'These behemoths simple chew through the rock.'

This is what it's all about - freshly mined potash.

And it's this potash that they use to make fertiliser.

It's this stuff that reinvigorates the British landscape.

'They mine potash around the clock,

'carving out up to a million tonnes per year.

'This is the only place in Britain

'where we can get this valuable fertiliser.

'It's completely bizarre to think that down here

'is the stuff we need to put the food on our tables.

'But that's not all.

'They're mining for something else down here.

'Something strange and not of this Earth.'

Behind these doors, they're trying to get to the bottom

of perhaps the biggest mystery in all of science.

'Tucked away down here is Britain's deepest laboratory.

'And where you need to trade your green hat

'for a nice, clean, white one.'

-This is a full-on clean room.

-OK.

'Sean Paling is a physicist.

'He and his team down here are hunting for something mysterious.

'When scientists looked at galaxies in deep space, they found a problem.

'According to their theories, these spinning collections of stars

'should fly apart, but they don't.

'Something unseen is holding them together.

'Something they call dark matter.'

Dark matter is a name that we give to stuff that we think

exists in the universe that we can't see.

We think that when you look at the night sky,

the stars and the planets and galaxies,

the stuff that we know about makes up 15% of what's out there.

We think 85% of the mass in the universe is missing.

That's a lot not to know about.

Yes. I mean, it's an embarrassing lack of knowledge.

So far, no-one's found any trace of this dark matter.

Above ground, there's just too much light and radiation

getting in the way to detect it.

'But here, Sean hopes that the 1,000 metres of solid rock

'will stop any radiation from penetrating,

'but the dark matter will be able to get through.

'Down here is perhaps our best chance of detecting it.

'In our rapidly-changing world,

'it's knowledge itself that's become Britain's greatest resource.'

In making this programme,

I've seen a Britain I never knew existed. A hidden world.

I discovered what's underneath Britain's tallest building.

And ventured into a vast underground cathedral.

I've encountered the eccentric,

the surprising and the downright disgusting.

And I've seen the extraordinary ways that Britain below ground

has affected and shaped the countries and cities above.

And next time...

..I blast my way back underground.

And take to the skies

to reveal the secret networks and connections

that keep Britain moving.