0:00:02 > 0:00:03This is the Britain we know.
0:00:04 > 0:00:08A patchwork of fields, forests,
0:00:08 > 0:00:12of rugged mountains and dramatic skylines.
0:00:14 > 0:00:16But ours is also a land of secrets
0:00:16 > 0:00:19that you can only see if you look at it in a new way.
0:00:21 > 0:00:22From beneath.
0:00:25 > 0:00:28I'm going to show you Britain as you've never seen it before.
0:00:34 > 0:00:36The hidden world below our cities.
0:00:38 > 0:00:41The engineering genius that powers the nation
0:00:41 > 0:00:44and keeps the country moving.
0:00:51 > 0:00:53I'll be going deeper and deeper underground
0:00:53 > 0:00:55to explore this unknown Britain.
0:00:58 > 0:01:01To experience its awesome wonders.
0:01:01 > 0:01:06The scale and the drama of this place is just off the chart.
0:01:06 > 0:01:08And it's dirtiest surprises.
0:01:11 > 0:01:13I don't even want to know what that is.
0:01:13 > 0:01:16It will change the way you think about our country.
0:01:18 > 0:01:21I want to unlock the secrets of what's above ground
0:01:21 > 0:01:23by understanding what's below.
0:01:23 > 0:01:27How every city, every forest, even every field,
0:01:27 > 0:01:31depends on an extraordinary hidden world beneath our feet.
0:01:43 > 0:01:45'I'm on my way to one of the most
0:01:45 > 0:01:47'amazing structures in modern Britain.'
0:01:49 > 0:01:52I'm about to discover how what goes on below ground
0:01:52 > 0:01:57can have a profound and unexpected impact on what happens above it.
0:01:57 > 0:02:00The realm of the underworld has so often determined
0:02:00 > 0:02:03how Britain's been built over the centuries.
0:02:05 > 0:02:07One building towers over London.
0:02:07 > 0:02:11The tallest ever constructed in Western Europe.
0:02:11 > 0:02:12The Shard.
0:02:18 > 0:02:20London has never, ever looked this good.
0:02:26 > 0:02:28The Shard's over 300 metres tall,
0:02:28 > 0:02:33but until recently, London had precious few skyscrapers.
0:02:33 > 0:02:35It wasn't until the 1960s
0:02:35 > 0:02:37that the first building over 20 storeys went up.
0:02:37 > 0:02:40Now, just compare that to New York. Think about Manhattan,
0:02:40 > 0:02:44where, they've got over 600 skyscrapers
0:02:44 > 0:02:46that were built since the 1890s.
0:02:46 > 0:02:48Now, what is the difference?
0:02:48 > 0:02:51# One, two, get down. #
0:02:52 > 0:02:54It's all to do with what's underground.
0:02:56 > 0:02:59You might assume you're best to build on something solid,
0:02:59 > 0:03:02and you'd be absolutely right.
0:03:02 > 0:03:05New York sits on hard rock.
0:03:06 > 0:03:10This makes it relatively easy to secure skyscrapers into the ground
0:03:10 > 0:03:12and support their colossal weight.
0:03:13 > 0:03:16Classic skyscrapers, like the Chrysler Building,
0:03:16 > 0:03:18built as early as 1930.
0:03:19 > 0:03:22And the Empire State Building a year later.
0:03:22 > 0:03:24Then the tallest building in the world.
0:03:27 > 0:03:30So, why doesn't London have a skyline like Manhattan's?
0:03:30 > 0:03:32Well, it's all to do with what's underground.
0:03:34 > 0:03:37Last time I did this was in about 1983.
0:03:37 > 0:03:40- Right, I'm going to step back. - I'd stand well back if I were you.
0:03:40 > 0:03:43'Underneath most of London is this stuff.
0:03:43 > 0:03:45'Soft, squidgy clay.
0:03:45 > 0:03:48'A huge challenge for Roma Agrawal,
0:03:48 > 0:03:51'a structural engineer who helped design The Shard.'
0:03:51 > 0:03:54This is London clay, straight out of the ground.
0:03:54 > 0:03:57This is the stuff that you would actually build on.
0:03:57 > 0:03:59But is it the same clay that you'd get in your modelling shops
0:03:59 > 0:04:01to make a pot, or a mug, or something?
0:04:01 > 0:04:03Amazingly, the answer is yes.
0:04:03 > 0:04:07All these pots and things that we make out of clay
0:04:07 > 0:04:10and the stuff that we're putting our buildings on top of in London
0:04:10 > 0:04:12are pretty much the same thing.
0:04:14 > 0:04:17Let me get an idea, how difficult is it to actually build in London,
0:04:17 > 0:04:19compared with somewhere like New York,
0:04:19 > 0:04:22which is pretty much building straight onto bedrock?
0:04:22 > 0:04:25Yeah, exactly. So New York is actually brilliant for skyscrapers.
0:04:25 > 0:04:27You come in, you put the skyscraper on the rock
0:04:27 > 0:04:29and the rock is very strong.
0:04:29 > 0:04:31And the loads just go straight down into it.
0:04:31 > 0:04:34In London, we need to do a bit more kind of gymnastics around the soil
0:04:34 > 0:04:37to try and make sure that our loads are going down where it should be.
0:04:37 > 0:04:40It's bizarre that this is the exact material
0:04:40 > 0:04:43that you're building huge skyscrapers on.
0:04:43 > 0:04:44It is very difficult to believe.
0:04:44 > 0:04:46Yeah. It just seems... Why would you?
0:04:46 > 0:04:49Well, we don't have a choice, do we, in London?
0:04:51 > 0:04:55'Building on soft ground can have catastrophic consequences.'
0:04:57 > 0:04:59In 2009 in Shanghai, China,
0:04:59 > 0:05:02a 13-storey building was being constructed.
0:05:05 > 0:05:08Its foundations couldn't support its weight.
0:05:08 > 0:05:10And one day, it simply toppled over.
0:05:13 > 0:05:16Despite the devastation, only one worker died.
0:05:22 > 0:05:26'So, just how do you build the tallest building in Britain
0:05:26 > 0:05:28'on soft, unstable clay?
0:05:28 > 0:05:32'Well, to answer that, we're going to reveal how it would look
0:05:32 > 0:05:34'from an entirely different angle.
0:05:34 > 0:05:36'From beneath the ground.'
0:05:39 > 0:05:42Imagine the earth is made of glass
0:05:42 > 0:05:45and you could look up and see the street level above.
0:05:49 > 0:05:53From down here, we can see a completely new subterranean world.
0:05:58 > 0:06:00Tube trains thunder below the surface,
0:06:00 > 0:06:03carrying millions of us to work each day.
0:06:06 > 0:06:09And next to London Bridge Station, the base of The Shard,
0:06:09 > 0:06:12sitting on the soft, unstable clay.
0:06:16 > 0:06:17The Shard needs foundations,
0:06:17 > 0:06:20and not just any old foundations.
0:06:23 > 0:06:2618,000 tonnes of building is kept upright
0:06:26 > 0:06:29by over 100 concrete piles.
0:06:34 > 0:06:35And they're deep.
0:06:40 > 0:06:42Most foundations only go down a few metres.
0:06:44 > 0:06:47The Empire State Building's are just 16 metres deep.
0:06:49 > 0:06:51But The Shard's are three times deeper.
0:06:56 > 0:06:59An astonishing 53 metres down.
0:06:59 > 0:07:02Deeper than Nelson's Column is tall.
0:07:03 > 0:07:06So building London's new skyline has required
0:07:06 > 0:07:09some of the most impressive foundations in the world.
0:07:14 > 0:07:18Its 53-metre-deep foundations mean that The Shard remains,
0:07:18 > 0:07:20and will remain, firmly rooted to the spot.
0:07:20 > 0:07:24And thanks to these new techniques in digging ultra-deep foundations,
0:07:24 > 0:07:27it means that London has had a real growth spurt in recent years.
0:07:27 > 0:07:30We've seen all these new skyscrapers popping up all over the place.
0:07:30 > 0:07:32You've got the Cheesegrater building
0:07:32 > 0:07:35and you've got the Walkie-Talkie building and The Gherkin.
0:07:35 > 0:07:38Who would have thought that that soft London clay
0:07:38 > 0:07:42would have such a profound effect on the London skyline?
0:07:49 > 0:07:53This may be one of the great sights of modern Britain,
0:07:53 > 0:07:56yet the real wonder is the secret world that lies beneath.
0:08:00 > 0:08:03Across the country, it's often the natural landscapes
0:08:03 > 0:08:05that dominate our view.
0:08:05 > 0:08:08Forests and fields, rolling hills,
0:08:08 > 0:08:10ancient mountains.
0:08:10 > 0:08:14But some of our most spectacular wonders are invisible,
0:08:14 > 0:08:16hidden underground.
0:08:24 > 0:08:26This is Gaping Gill in North Yorkshire.
0:08:27 > 0:08:30A hole in the ground that swallows a river.
0:08:34 > 0:08:37People once believed this was a gateway to hell.
0:08:58 > 0:09:02More people have summated Everest than have abseiled into Gaping Gill,
0:09:02 > 0:09:04and I can understand why.
0:09:04 > 0:09:07When you look over the edge, it's just a black void into nothingness.
0:09:07 > 0:09:10And that stream that you can see going over the edge
0:09:10 > 0:09:11that doesn't look too dramatic,
0:09:11 > 0:09:17it actually turns into a waterfall twice the height of Niagara Falls.
0:09:17 > 0:09:21There is only one way, really, to explore it,
0:09:21 > 0:09:22and that's to go over the edge.
0:09:25 > 0:09:28I hope this looks nice from where you're sitting
0:09:28 > 0:09:31because from where I am, it is bloody terrifying.
0:10:16 > 0:10:19Ah! My God, just look at it!
0:10:20 > 0:10:25The scale and the drama of this place is just off the chart.
0:10:27 > 0:10:30It's much bigger and more impressive than I could imagine.
0:10:33 > 0:10:36'With a vertical drop of over 100 metres,
0:10:36 > 0:10:39'this is the tallest waterfall in Britain.
0:10:39 > 0:10:41'A hidden natural wonder.
0:10:43 > 0:10:48'And standing here, you can feel its raw, elemental power.'
0:10:49 > 0:10:53And that water that pours down, it's not just here for dramatic effect.
0:10:53 > 0:10:57That water is a real force of nature.
0:10:57 > 0:10:59It's carved out Gaping Gill.
0:11:02 > 0:11:04We think of the ground as pretty solid,
0:11:04 > 0:11:10but down here, water has created a void as vast as York Minster.
0:11:12 > 0:11:16So, just why did this gigantic underground cathedral
0:11:16 > 0:11:17form in this particular place?
0:11:20 > 0:11:22'Well, the water isn't quite what it seems.'
0:11:27 > 0:11:31I've got some acid and a little pipette.
0:11:31 > 0:11:33And if I put a little bit on the limestone,
0:11:33 > 0:11:37you should see it fizz away
0:11:37 > 0:11:40as it starts to dissolve the limestone. Let's have a go.
0:11:42 > 0:11:46That's the one. Yeah, there you go. You can start to see it fizz away
0:11:46 > 0:11:48as it eats away the limestone.
0:11:49 > 0:11:52That's what's happening in here, but very, very slowly.
0:11:53 > 0:11:57'The water that pours down here is, in fact, a weak acid.
0:12:03 > 0:12:07'It forms when rainwater mixes with carbon dioxide in the air and soil
0:12:07 > 0:12:09'to make carbonic acid.
0:12:09 > 0:12:11'The same stuff that's in fizzy drinks.
0:12:12 > 0:12:15'And that acid reacts with the particular type of rock
0:12:15 > 0:12:18'that exists here. Limestone.'
0:12:21 > 0:12:25It's this process, drawn out over 30 million years,
0:12:25 > 0:12:27that's carved out Gaping Gill.
0:12:30 > 0:12:32Hidden beneath these limestone hills.
0:12:37 > 0:12:40'To escape the cavern, I've had to crawl for over a kilometre
0:12:40 > 0:12:43'through a labyrinth of narrow passages.'
0:12:43 > 0:12:45What's this? Oy-oy-oy!
0:12:46 > 0:12:50From Britain's biggest underground cavern...to the smallest.
0:12:54 > 0:12:56See, getting down was the easy bit.
0:12:56 > 0:13:01Getting up is...is really difficult!
0:13:04 > 0:13:07As I emerge from the womb,
0:13:07 > 0:13:11I feel like I'm being born again.
0:13:18 > 0:13:20That was one of the hardest day's filming ever.
0:13:20 > 0:13:24It's really interesting because just in terms of fear factor,
0:13:24 > 0:13:27you have that terrible fear of heights
0:13:27 > 0:13:30combined with dreadful claustrophobia.
0:13:30 > 0:13:32On every level, it was dreadful!
0:13:35 > 0:13:39Across Britain, there are many more vast cave systems.
0:13:43 > 0:13:45One stretches 90 kilometres,
0:13:45 > 0:13:48nearly twice as long as the Channel Tunnel,
0:13:48 > 0:13:51linking Cumbria, Lancashire and Yorkshire.
0:13:53 > 0:13:57In fact, wherever you find limestone like that at Gaping Gill,
0:13:57 > 0:14:02chances are there's a hidden network of caves and rivers below.
0:14:05 > 0:14:09But if you live in a city, you might be just as surprised
0:14:09 > 0:14:12at the extraordinary things you'd find beneath your streets.
0:14:14 > 0:14:17Nowhere is this more true than Bristol.
0:14:17 > 0:14:18You just need to know where to look.
0:14:22 > 0:14:26I'm on my way to a place few people ever go, or even know about.
0:14:29 > 0:14:31'This is the River Frome.
0:14:31 > 0:14:34'And I'm about to pass into a lost world
0:14:34 > 0:14:36'hidden below the city of Bristol.'
0:14:44 > 0:14:45Here we go.
0:14:48 > 0:14:51'Down here, the river's been entirely covered over,
0:14:51 > 0:14:54'made invisible to the world above.
0:14:54 > 0:14:57'My guide on this unique journey is explorer, Dave Talbot.'
0:15:00 > 0:15:05I've a feeling we are well and truly off the Bristol tourist trail now.
0:15:05 > 0:15:06We certainly are, yeah.
0:15:06 > 0:15:09Over the last 200 years, this river's been
0:15:09 > 0:15:12sort of built and covered over more and more.
0:15:12 > 0:15:15- The last section was done in 1930. - Yeah.
0:15:15 > 0:15:17And since then, very few people have been down here.
0:15:19 > 0:15:23'As the city expanded, the River Frome got in the way.
0:15:23 > 0:15:28'So it was simply built over. Creating this secret world.'
0:15:33 > 0:15:36It's quite odd. You've got a lovely river that comes through Bristol,
0:15:36 > 0:15:39it's very picturesque, and then suddenly, they just decide,
0:15:39 > 0:15:40"Oh, we'll just cover it over now!"
0:15:44 > 0:15:46'Without building over The Frome,
0:15:46 > 0:15:49'many of the roads, houses and shopping centres
0:15:49 > 0:15:52'of modern Bristol couldn't have existed.
0:15:54 > 0:15:58'A mile in and we're passing under the main shopping streets.
0:15:58 > 0:16:00'And no-one knows we're below them.'
0:16:02 > 0:16:06Every so often, you can hear cars above us.
0:16:06 > 0:16:08Yeah, you can. It's quite eerie, isn't it,
0:16:08 > 0:16:11to think that you're underneath the road.
0:16:11 > 0:16:15'Another half mile or so, the tunnel begins to get lower
0:16:15 > 0:16:18'as we pass under the very heart of Bristol.'
0:16:18 > 0:16:21Actually, the thing that strikes me when you come down here is that
0:16:21 > 0:16:25it really is a case of human needs versus nature.
0:16:25 > 0:16:28So over the last couple of hundred years, obviously,
0:16:28 > 0:16:32Bristol became very successful and expanded.
0:16:32 > 0:16:37And as a result, the whole river has had to be completely confined
0:16:37 > 0:16:38and hidden away from view.
0:16:41 > 0:16:44'A bit further, and the headroom's even tighter.
0:16:45 > 0:16:48'To be honest, I'm getting a bit nervous down here.
0:16:48 > 0:16:51'But finally, after a treacherous six-hour journey,
0:16:51 > 0:16:54'we emerge in Bristol harbour.'
0:16:57 > 0:16:59Did you remember the key, Dallas?
0:16:59 > 0:17:02There we are. Escape from The River Frome.
0:17:04 > 0:17:07'Who'd have thought you could canoe under Bristol
0:17:07 > 0:17:10'through a hidden underworld that most people don't know exists?
0:17:11 > 0:17:14'And across Britain, millions of us
0:17:14 > 0:17:16'go about our daily lives above lost rivers.'
0:17:19 > 0:17:21There's the River Sheaf of Sheffield.
0:17:21 > 0:17:24The River Farset of Belfast.
0:17:24 > 0:17:26The River Sherbourne of Coventry.
0:17:26 > 0:17:30And in London, you might think of The Thames as the only river,
0:17:30 > 0:17:32but there's another hidden underground
0:17:32 > 0:17:34that's transformed the city above.
0:17:36 > 0:17:37The River Fleet.
0:17:41 > 0:17:44'The only visible signs above that this river still exists
0:17:44 > 0:17:46'is the street that bears its name.
0:17:47 > 0:17:50'But The Fleet has been put to work.
0:17:50 > 0:17:53'It performs a role so vital that without it,
0:17:53 > 0:17:55'London could not be the city it is now.'
0:18:00 > 0:18:03I've got to say, actually, looking down here,
0:18:03 > 0:18:05it doesn't look like much of a river at all.
0:18:05 > 0:18:07There's just a short ladder going down.
0:18:07 > 0:18:09But if you want to explore the River Fleet these days,
0:18:09 > 0:18:11you have to go underground.
0:18:19 > 0:18:21'I'm entering a world that's not for the faint-hearted,
0:18:21 > 0:18:24'or the weak-stomached.'
0:18:32 > 0:18:35Here we are, this is the River Fleet.
0:18:35 > 0:18:37It looks like a sewer, smells like a sewer.
0:18:38 > 0:18:40Whoo!
0:18:40 > 0:18:43'That's because it is a sewer.
0:18:43 > 0:18:46'Back in the early 19th century, the open River Fleet
0:18:46 > 0:18:50'was essentially a cesspit carrying disease through London.
0:18:53 > 0:18:55'So it was decided to cover it up.
0:18:55 > 0:18:58'And using 318 million bricks,
0:18:58 > 0:19:01'Victorian engineers turned it into this.
0:19:04 > 0:19:06'It means that millions of us can now live
0:19:06 > 0:19:09'without the risk of disease in a few square miles.
0:19:10 > 0:19:13'But it also hides some pretty gruesome surprises.
0:19:15 > 0:19:18'Dave Dennis is one of an army of underground workers
0:19:18 > 0:19:21'that keep The Fleet flowing.
0:19:21 > 0:19:24'Surely, one of the least-enviable jobs in Britain.'
0:19:24 > 0:19:26So this is the main sewer tunnel.
0:19:26 > 0:19:29Are there tributary sewer tunnels that come off it, or is this it?
0:19:29 > 0:19:30Yeah. Yeah, there's loads.
0:19:30 > 0:19:33Obviously, down very small side roads, you get a main sewer,
0:19:33 > 0:19:37and they drop into a trunk sewer, which we're standing in.
0:19:37 > 0:19:39And if you look over here, there's a small junction,
0:19:39 > 0:19:42which basically is a small sewer.
0:19:42 > 0:19:44What comes out of here, this is human waste?
0:19:44 > 0:19:46Straight from the toilet, that is. Direct from the toilet.
0:19:46 > 0:19:49- Man!- Direct from the customer.
0:19:49 > 0:19:51- Oh, the smell!- Yeah, I know.
0:19:51 > 0:19:52DALLAS COUGHS
0:19:52 > 0:19:54Oh, man!
0:19:56 > 0:20:01'Every underground labyrinth has its monster, as I'm about to discover.
0:20:01 > 0:20:04'Down here, it's not a minotaur,
0:20:04 > 0:20:06'it's something far, far worse.'
0:20:09 > 0:20:10Oh, God!
0:20:10 > 0:20:12Oh, the stink!
0:20:12 > 0:20:15Oh, my God, that is horrendous!
0:20:15 > 0:20:17Is this white stuff just fat?
0:20:17 > 0:20:21This is pure fat that's solidified with other material.
0:20:21 > 0:20:23This has come from all over London
0:20:23 > 0:20:26- and it congeals here because it's a bit of a bottleneck.- Right.
0:20:26 > 0:20:28You have to come and break it up?
0:20:28 > 0:20:31- We have to come down and break it up for it to flow downstream.- OK.
0:20:33 > 0:20:35'Down here lives the fatberg.
0:20:35 > 0:20:37'A mixture of rancid fat,
0:20:37 > 0:20:40'human excrement and other unmentionables.'
0:20:40 > 0:20:42Oh, Christ!
0:20:42 > 0:20:43You all right?
0:20:43 > 0:20:45DAVE CHUCKLES
0:20:45 > 0:20:47Do you want to get out, Dallas?
0:20:49 > 0:20:50Oh, God, the smell!
0:20:51 > 0:20:54- Look at the size of that! - This is a...
0:20:54 > 0:20:56- So, this is...this is a fatberg? - This is a fatberg.
0:20:56 > 0:20:58- Look at the size of it! - See the worms in it?
0:20:58 > 0:21:00It's got worms in it?!
0:21:03 > 0:21:05- Oh! - DALLAS COUGHS
0:21:06 > 0:21:10'Nothing could have prepared me for the overpowering smell.'
0:21:10 > 0:21:12DALLAS COUGHS AND GAGS
0:21:12 > 0:21:13You all right there?
0:21:14 > 0:21:18What did you have for breakfast(?) DAVE LAUGHS
0:21:18 > 0:21:20Yeah, might see it.
0:21:20 > 0:21:23This is one big berg.
0:21:23 > 0:21:25Where does that rate on the berg scale?
0:21:25 > 0:21:27- That's a...- Is that a big one? - Pretty good one.
0:21:27 > 0:21:29Oop! It's breaking up quite nicely, though.
0:21:29 > 0:21:31Shall I give it a...?
0:21:31 > 0:21:33Yeah, give it a...give it a go.
0:21:33 > 0:21:34This is worm heaven.
0:21:35 > 0:21:37- So...- What the hell is that?
0:21:37 > 0:21:39- God knows! - I don't even know what that is.
0:21:39 > 0:21:41This is the most disgusting thing
0:21:41 > 0:21:44I've ever done in my life, without a shadow of a doubt.
0:21:46 > 0:21:47The worst thing is, yeah,
0:21:47 > 0:21:50getting a bit of splashback and getting it in your mouth.
0:21:52 > 0:21:53DAVE CHUCKLES
0:21:54 > 0:21:56So when you guys say, actually,
0:21:56 > 0:21:58please don't pour fat down your sink, you actually mean...
0:21:58 > 0:22:01- Bin it, don't cook it.- ..can you really not pour fat down your sink?
0:22:03 > 0:22:06'What begins as an innocent bit of grease in your kitchen
0:22:06 > 0:22:10'can quickly transform into the monster fatberg.
0:22:11 > 0:22:14'This is a job I don't want to repeat any time soon.
0:22:21 > 0:22:24'London's clean, fresh air has never smelt so good.'
0:22:27 > 0:22:31It is only because of sophisticated sewer systems like we have in London
0:22:31 > 0:22:34that millions of us can live together hygienically
0:22:34 > 0:22:38and safely, well, relatively, anyway.
0:22:38 > 0:22:41But, cities wouldn't work without all that underground
0:22:41 > 0:22:43engineering and infrastructure.
0:22:43 > 0:22:45Places like London and Glasgow
0:22:45 > 0:22:48and Newcastle and Manchester just wouldn't exist.
0:22:52 > 0:22:56'But there's more beneath our cities than the worlds we've built.
0:22:56 > 0:23:02'If we go deeper, we can discover an older, more spectacular kingdom.
0:23:02 > 0:23:04'One that shaped the lives of millions of us.'
0:23:09 > 0:23:13Bath. One of Britain's most genteel cities,
0:23:13 > 0:23:16and perhaps our most famous spa town.
0:23:16 > 0:23:18Where, for centuries,
0:23:18 > 0:23:21people have come to indulge themselves in the waters.
0:23:21 > 0:23:23Yet you might never guess that beneath the city
0:23:23 > 0:23:26is a trap door into a dramatic underworld.
0:23:28 > 0:23:30'But that story starts out in the countryside.'
0:23:35 > 0:23:38To understand how the city of Bath came to be,
0:23:38 > 0:23:41you need to come up here, into the Mendip Hills.
0:23:41 > 0:23:43Now, this is a pretty ordinary stream.
0:23:43 > 0:23:46And it's only about 15 miles to Bath
0:23:46 > 0:23:49from where I'm standing here, as the crow flies.
0:23:49 > 0:23:53But this water is about to go on an extraordinary journey.
0:23:53 > 0:23:57A journey that's going to take around about 10,000 years.
0:23:59 > 0:24:02Soon, the stream plummets down a hole.
0:24:07 > 0:24:11From here, the water travels deeper and deeper into the earth.
0:24:13 > 0:24:16To follow its tortuous underground route,
0:24:16 > 0:24:19surprisingly enough, I need to take to the air.
0:24:22 > 0:24:26So I've hitched a ride with my new friend, Julian, in his plane
0:24:26 > 0:24:29so we can follow the journey, albeit in much quicker time.
0:24:29 > 0:24:33So behind me that way, you've got the Mendip Hills, where we were.
0:24:33 > 0:24:37And straight ahead of me, it's only 15 miles or so, is the city of Bath.
0:24:37 > 0:24:39Now what's going on is down to some very, very,
0:24:39 > 0:24:42interesting geology beneath our feet.
0:24:44 > 0:24:48'From up here, the landscape looks the picture of Middle England.
0:24:48 > 0:24:51'But below the surface is a world closer
0:24:51 > 0:24:53'to the furnace of volcanic Iceland.'
0:24:57 > 0:24:59The water cascades down cracks in the rock,
0:24:59 > 0:25:03pulled deeper towards the centre of the Earth.
0:25:03 > 0:25:06It gets hotter the deeper it goes.
0:25:06 > 0:25:10Until nearly two miles down, it's forced back up towards the surface.
0:25:12 > 0:25:16A vast, natural steam engine hidden deep below ground.
0:25:22 > 0:25:24And here we are over the city of Bath.
0:25:24 > 0:25:30And this is where the water reappears 10,000 years later or so.
0:25:30 > 0:25:32We've done it in, what, five minutes?
0:25:32 > 0:25:3410,000 years!
0:25:34 > 0:25:37It seems like an incredibly long time.
0:25:37 > 0:25:40If you think about the Pyramids, for example, that's 4,000 years ago,
0:25:40 > 0:25:43so 10,000 years, the rains in the Mendip Hills
0:25:43 > 0:25:45have taken to get to Bath.
0:25:56 > 0:25:59'This is where the water finally emerges.'
0:26:05 > 0:26:08And if you look, just on the surface of the water,
0:26:08 > 0:26:11you can see little bubbles where you can see it percolating through.
0:26:11 > 0:26:14You can feel the heat against your face.
0:26:14 > 0:26:18And the funny thing is, it's not the buildings themselves
0:26:18 > 0:26:21that are the oldest thing here, it's actually the water.
0:26:21 > 0:26:23Apparently, it's very medicinal.
0:26:26 > 0:26:27Mm!
0:26:27 > 0:26:29Really metallic.
0:26:29 > 0:26:31I can taste that iron kind of taste.
0:26:31 > 0:26:33I'm going to put that to one side, I think.
0:26:37 > 0:26:40'When the Ancient Romans first arrived in Britain,
0:26:40 > 0:26:42'the hot water bubbling up from the ground
0:26:42 > 0:26:46'appealed to their fondness for bathing.
0:26:46 > 0:26:47'And 2,000 years ago,
0:26:47 > 0:26:50'they founded a city around these hot springs.
0:26:51 > 0:26:55'Today, both city and springs are still thriving.
0:26:55 > 0:26:59'And now I'm taking my first plunge into 10,000-year-old hot water.'
0:27:10 > 0:27:12Seems quite decadent, doesn't it?
0:27:12 > 0:27:16The idea of an open-air heated swimming pool in Britain.
0:27:16 > 0:27:19But you've got to remember that the water in this pool
0:27:19 > 0:27:21is all heated naturally inside the earth.
0:27:21 > 0:27:23It gets to about 60 degrees.
0:27:23 > 0:27:27And then when it comes up out of the ground, it's about 46 degrees,
0:27:27 > 0:27:31and then here, they cool it to a nice balmy 33 degrees,
0:27:31 > 0:27:36which is the ideal temperature for my morning constitutional.
0:27:36 > 0:27:39But because of that high heat that occurs inside the Earth,
0:27:39 > 0:27:42this is Britain's only true hot spring.
0:27:45 > 0:27:49'This modern spa is a continuation of what the Romans began.'
0:27:51 > 0:27:55I think the really interesting thing about Bath,
0:27:55 > 0:27:57and it is the most beautiful city,
0:27:57 > 0:28:00is that it is only here because of the geology.
0:28:03 > 0:28:05'Bath may be the hottest spring in Britain,
0:28:05 > 0:28:08'but there are others dotted around the country.'
0:28:09 > 0:28:12So the fashion for taking the waters
0:28:12 > 0:28:15led to a proliferation of many other spa towns.
0:28:16 > 0:28:18But one city more than any other
0:28:18 > 0:28:22has been shaped by even more ancient and violent forces.
0:28:25 > 0:28:29Edinburgh. A beautiful city that's grown up around this.
0:28:30 > 0:28:31The magnificent castle.
0:28:33 > 0:28:34It dominates the skyline.
0:28:40 > 0:28:44Up here, you can instantly see why this is the perfect place to put your castle.
0:28:44 > 0:28:47We're nice and high, we've got very, very steep banks
0:28:47 > 0:28:50and you can see for miles and miles and miles.
0:28:50 > 0:28:52It's the perfect place to put a stronghold.
0:28:55 > 0:28:59But there's something curious about the rock the castle stands on.
0:29:01 > 0:29:03On three sides, the rock is a sheer face,
0:29:03 > 0:29:08but on the fourth side, a ridge of land slopes gently downwards.
0:29:12 > 0:29:15Seen from beneath, we can follow its course.
0:29:18 > 0:29:22Over centuries, the city was built on this slope,
0:29:22 > 0:29:25becoming the central spine of Edinburgh's Old Town,
0:29:25 > 0:29:26the Royal Mile.
0:29:28 > 0:29:32And at its head, the great rock where the castle stands above.
0:29:35 > 0:29:39This is the gateway to an ancient and violent underworld
0:29:39 > 0:29:41that holds the secret of Edinburgh itself.
0:29:47 > 0:29:50350 million years ago,
0:29:50 > 0:29:53what's now Edinburgh was sitting on a violent volcano.
0:29:55 > 0:30:00Liquid magma surged upwards from beneath the earth.
0:30:00 > 0:30:03Once it stopped, the magma that had made its way to the surface
0:30:03 > 0:30:06cooled and formed solid rock.
0:30:10 > 0:30:13This is called basalt, and it's one of the hardest rocks on earth.
0:30:13 > 0:30:16And it's actually solidified lava.
0:30:16 > 0:30:18So all of this would have been molten.
0:30:18 > 0:30:23And if you look, you can see there's all these cracks. There's one here.
0:30:23 > 0:30:26So this liquid rock would have cooled and contracted,
0:30:26 > 0:30:28which is how you get these cracks.
0:30:31 > 0:30:35'Castle Rock is all we can now see of the violent volcanic forces
0:30:35 > 0:30:37'that shaped this city.'
0:30:40 > 0:30:42The tip of a gigantic pillar of basalt
0:30:42 > 0:30:45that stretches hundreds of metres down into the earth.
0:30:48 > 0:30:50But that's not the end of Edinburgh's story.
0:30:52 > 0:30:57During the Ice Ages, glaciers moved across the land.
0:30:59 > 0:31:04They gouged out the softer rock around the hard, ancient volcano,
0:31:04 > 0:31:06leaving just the Castle Rock
0:31:06 > 0:31:09and a protected sloping ridge in its shadow.
0:31:12 > 0:31:14Standing here, you can see all the evidence of this powerful
0:31:14 > 0:31:16and ancient geological activity.
0:31:16 > 0:31:20So you've got the Castle Rock itself, this volcanic plug
0:31:20 > 0:31:23of very, very hard basalt that would have stood firm
0:31:23 > 0:31:25in the face of the oncoming glacier.
0:31:25 > 0:31:29And then behind it, you've got this tail of sloping ground
0:31:29 > 0:31:34that would have survived where the Old Town is built.
0:31:35 > 0:31:37'So if you're on the castle ramparts,
0:31:37 > 0:31:40'you're standing on top of an ancient volcano.'
0:31:46 > 0:31:49Sometimes, the rocks beneath us have made us rich.
0:31:51 > 0:31:55We may not have diamonds hidden underground in Britain,
0:31:55 > 0:31:58but we've something that's turned out to be far more valuable.
0:32:04 > 0:32:06Coal.
0:32:06 > 0:32:10Coal was the beating heart of the Industrial Revolution.
0:32:10 > 0:32:14It drove the wheels of commerce and fired the factories.
0:32:14 > 0:32:17It turned Britain into an industrial superpower
0:32:17 > 0:32:19and helped grow the Empire.
0:32:21 > 0:32:23Coal put the great into Britain.
0:32:24 > 0:32:27And the best way to get it was deep underground.
0:32:28 > 0:32:32For most of us, it's a world we can barely imagine.
0:32:32 > 0:32:34An underground very few of us experience.
0:32:36 > 0:32:40But in its heyday, mining was Britain's most important industry.
0:32:41 > 0:32:44And for these miners, this wasn't Britain beneath their feet,
0:32:44 > 0:32:46this WAS their Britain.
0:32:49 > 0:32:53'I've come to West Yorkshire to one of the few collieries
0:32:53 > 0:32:55'where you can still descend to a coalface.'
0:32:57 > 0:32:59- How deep are we going to go now? - 140 metres underground.
0:32:59 > 0:33:02- It's going to take approximately... - That's pretty deep!
0:33:02 > 0:33:04Collieries were much, much deeper than that.
0:33:04 > 0:33:07You look at some of the Yorkshire coalmines, they were as deep as 1,000 metres.
0:33:07 > 0:33:10So relatively, it's a relatively shallow coalmine.
0:33:14 > 0:33:16'It certainly seems deep enough for me.
0:33:17 > 0:33:20'Very few of us go this deep underground these days,
0:33:20 > 0:33:24'but at its peak this was the daily commute to work
0:33:24 > 0:33:26'for vast armies of miners.'
0:33:31 > 0:33:33This way, Dallas.
0:33:36 > 0:33:39'The riches that drew them down here
0:33:39 > 0:33:43'had lain secretly buried for millions of years.'
0:33:43 > 0:33:44I've got a lovely fossil here
0:33:44 > 0:33:47of a tree fern that's come from a coalface.
0:33:47 > 0:33:50You can make out the shape of the bark here.
0:33:50 > 0:33:53And it's really good evidence of where coal actually comes from.
0:33:53 > 0:33:56So trees would have died, they would have rotted down,
0:33:56 > 0:33:58they would have become compressed.
0:33:58 > 0:34:02Eventually becoming peat and then coal.
0:34:02 > 0:34:06And it's funny to think that the only reason we have coal in Britain,
0:34:06 > 0:34:10so much of it, is because 300 million years ago,
0:34:10 > 0:34:13the entire country would have been covered in swampy forests.
0:34:16 > 0:34:17And this is the result.
0:34:17 > 0:34:22An underground world where miners spent their entire working lives.
0:34:22 > 0:34:26This is just one tunnel, but if we could see through the earth,
0:34:26 > 0:34:29then we could reveal the entire mine network.
0:34:30 > 0:34:33This mine complex has a central shaft
0:34:33 > 0:34:35that descends 140 metres into the Earth.
0:34:36 > 0:34:39Branching off the main shaft are four galleries.
0:34:39 > 0:34:42Each one tapping into a coal seam.
0:34:44 > 0:34:47Along each working seam, a branching, intricate network
0:34:47 > 0:34:51of many smaller tunnels spreads out into the coal.
0:34:52 > 0:34:55There'd be teams of miners working on each coalface.
0:34:59 > 0:35:01'Steve Guest is giving me a tiny insight
0:35:01 > 0:35:04'into what life down here was like for the miners.'
0:35:06 > 0:35:08The technique you're going to be using, Dallas,
0:35:08 > 0:35:12down, pick low, undercut it
0:35:12 > 0:35:15- and drop it down onto the floor. - OK, let's have a go.
0:35:15 > 0:35:18- So, if I sort of go in here, around about here?- Yeah, that's right.
0:35:23 > 0:35:25So we're chipping away at the bottom,
0:35:25 > 0:35:28the idea that the coal seam above us would have fallen down,
0:35:28 > 0:35:31but presumably, that would have also been very, very dangerous, as well.
0:35:31 > 0:35:34Very, very dangerous. It's not only the coal seam that come down.
0:35:34 > 0:35:38If you've got weak rock above, this is going to collapse on top of you.
0:35:38 > 0:35:41Lots of fatalities on a daily basis.
0:35:41 > 0:35:42This is how the industry was.
0:35:45 > 0:35:47I'm trying to imagine what it would've been like
0:35:47 > 0:35:49with dozens of miners in this cramped area.
0:35:49 > 0:35:52Small conditions, yeah. All working together.
0:35:52 > 0:35:54But this is what their daily work would be.
0:35:58 > 0:36:01Man, it's frustrating! I just want to take a big swing and I can't!
0:36:07 > 0:36:11- That's me done!- You don't think you'd have had a career in mining?
0:36:11 > 0:36:14Well...I'd like to say yeah,
0:36:14 > 0:36:16but if I'm honest, I think I would have, er...
0:36:16 > 0:36:19I think I would have struggled with this.
0:36:21 > 0:36:26'In this mine alone, there were 240 miners working underground.
0:36:26 > 0:36:30'In total, a staggering 1.25 million people
0:36:30 > 0:36:32'once worked down mines like this.
0:36:32 > 0:36:36'That's as many people who today live in Sheffield, Newcastle
0:36:36 > 0:36:38'and Manchester combined.'
0:36:41 > 0:36:44Across Britain, in the coalfields that lay under Yorkshire
0:36:44 > 0:36:47and Lancashire, the Midlands and the North-East,
0:36:47 > 0:36:50under South Wales and lowland Scotland,
0:36:50 > 0:36:53there were over 3,000 mines.
0:36:53 > 0:36:57And above ground, major cities grew on the back of coal
0:36:57 > 0:36:59and the industry it fuelled.
0:37:00 > 0:37:04The riches below defined much of the urban map of Britain
0:37:04 > 0:37:07responsible for where millions of us live today.
0:37:12 > 0:37:14Mining may be fading from our lives,
0:37:14 > 0:37:17but as so often in our islands,
0:37:17 > 0:37:20what's left can be put to eccentric use.
0:37:21 > 0:37:26As time marches on, our needs and our priorities shift.
0:37:26 > 0:37:29And here in the Lake District is a really good example of that.
0:37:32 > 0:37:35'Under a mountain, a slate mine has been appropriated
0:37:35 > 0:37:37'for a rather different purpose.
0:37:39 > 0:37:41'With their usual pitch destroyed by flooding,
0:37:41 > 0:37:44'the Threlkeld Cricket Club
0:37:44 > 0:37:46'has been forced to some rather unusual measures
0:37:46 > 0:37:49'to raise funds for the repairs.
0:37:49 > 0:37:51'Extreme cricket.'
0:37:57 > 0:38:00CHEERING AND APPLAUSE
0:38:00 > 0:38:02I'm better in daylight.
0:38:02 > 0:38:04I've always been bad at cricket
0:38:04 > 0:38:06and I'm even worse playing it in a slate mine.
0:38:06 > 0:38:09'However with such a small pitch size,
0:38:09 > 0:38:12'even I should manage a four, or even a six.
0:38:14 > 0:38:16'Surely I can hit something!'
0:38:16 > 0:38:20MUSIC: Soul Limbo by Booker T & The MGs
0:38:26 > 0:38:28Quick single! Yes!
0:38:38 > 0:38:40CHEERING
0:38:40 > 0:38:42'A very disappointing innings.
0:38:44 > 0:38:47'And sad to say, my efforts are more Fred Flintstone
0:38:47 > 0:38:48'than Freddie Flintoff.'
0:39:00 > 0:39:04Some 97% of the land area of Britain is countryside.
0:39:05 > 0:39:07It dominates the map.
0:39:10 > 0:39:13Most striking of all, the great forests
0:39:13 > 0:39:17that make Britain such a green and pleasant land.
0:39:18 > 0:39:22But hidden beneath the surface is a secret world we barely know about
0:39:22 > 0:39:24and that keeps most of Britain alive.
0:39:28 > 0:39:32To find out how, I've come to Burghley Country Park in Lincolnshire,
0:39:32 > 0:39:34where there's an oak tree in its prime.
0:39:34 > 0:39:37And with enough space around its base
0:39:37 > 0:39:41to explore exactly what lies below in a way I've never seen before.
0:39:44 > 0:39:48Just like any building, trees have their own clever foundations
0:39:48 > 0:39:50hidden beneath the ground to keep them upright.
0:39:50 > 0:39:52We call them roots, of course.
0:39:52 > 0:39:56And a tree like this will have the most substantial roots of all.
0:39:56 > 0:40:00And for the first time, we want to reveal the root system
0:40:00 > 0:40:02of such a majestic tree.
0:40:10 > 0:40:12To do this, I need some help.
0:40:12 > 0:40:17Sharon Hosegood is an expert at uncovering the secret life of trees.
0:40:19 > 0:40:21Now, is this going to give us a pretty accurate measurement
0:40:21 > 0:40:23of the tree's age?
0:40:23 > 0:40:25It's as good as we've got without felling the tree,
0:40:25 > 0:40:28- which would be absolutely awful. - Wow, that would be little bit
0:40:28 > 0:40:30extreme to get the age!
0:40:30 > 0:40:33- 6.3.- 6.3. It's a big 'un.
0:40:33 > 0:40:40A girth of 6.3 metres makes this tree around 440 years old.
0:40:42 > 0:40:44In other words, it was a seedling
0:40:44 > 0:40:47when Elizabeth I had just come to the throne.
0:40:50 > 0:40:53To explore these roots requires something very special.
0:40:56 > 0:40:58Now, it might look like a pimped-up pushchair,
0:40:58 > 0:41:01but this is a sophisticated piece of kit
0:41:01 > 0:41:03that's going to let us peer into the ground.
0:41:05 > 0:41:09Have you done any science on a tree of this significance before?
0:41:09 > 0:41:12Not an oak. This is the biggest, oldest oak tree
0:41:12 > 0:41:15that I know is being scanned in the UK.
0:41:15 > 0:41:18Are you managing OK? I feel like I'm...
0:41:18 > 0:41:21- Like I should help, or something! - It's a one-woman job.
0:41:21 > 0:41:23Tell me how it works. What's actually going on here?
0:41:23 > 0:41:26Well, this piece of kit here, this tree radar looks like a pram,
0:41:26 > 0:41:29but essentially, it's a ground-penetrating radar
0:41:29 > 0:41:31which will pick up the roots
0:41:31 > 0:41:34because they're full of water.
0:41:34 > 0:41:37When we think about radar, we sort of think about aircraft radar.
0:41:37 > 0:41:40We send out radio waves and they bounce back.
0:41:40 > 0:41:42Is it doing the same thing as that?
0:41:42 > 0:41:44It pretty much is. The principle's the same.
0:41:44 > 0:41:46There's only a few of these in the world.
0:41:46 > 0:41:50It'll show how deep they are, how far spread they are.
0:41:50 > 0:41:53It picks up everything this diameter and above.
0:41:54 > 0:41:57And to come here with this amazing, old, veteran tree
0:41:57 > 0:42:00and to be able to see what's underground is a bit of a privilege.
0:42:02 > 0:42:05'Sharon's got her work cut out.
0:42:05 > 0:42:08'She goes round and round the tree in ever-increasing circles
0:42:08 > 0:42:12'to well beyond where she thinks the roots will extend.'
0:42:17 > 0:42:19Do we actually have a result? Am I allowed to see...?
0:42:19 > 0:42:23We have a result. You are allowed to see it and here it is.
0:42:23 > 0:42:25Oh, my God! That's great!
0:42:25 > 0:42:28So here is the base of the tree.
0:42:28 > 0:42:33And we can see the roots taper down quite quickly from the buttress.
0:42:33 > 0:42:35I can't believe how clear it is.
0:42:35 > 0:42:37I just thought it would be just a big mass of black
0:42:37 > 0:42:40and you'd have to sort of tell me, "Oh, that's the root there."
0:42:40 > 0:42:42But that... I mean, it's beautiful, isn't it?
0:42:42 > 0:42:45Are these the biggest roots you've seen on a tree?
0:42:45 > 0:42:49Well, this is certainly the biggest tree that I've scanned.
0:42:49 > 0:42:54And I'm really pleased and surprised at the root density of this.
0:42:54 > 0:42:56I mean, it was actually more than I imagined.
0:42:56 > 0:42:59It shows that for a tree to be this old,
0:42:59 > 0:43:01several hundred years old,
0:43:01 > 0:43:04it needs to have a well-developed root system.
0:43:04 > 0:43:07I'd like to go up to the tree and tell him,
0:43:07 > 0:43:09- tell the tree that the news is good. - Yeah, yeah.
0:43:09 > 0:43:11- Tree, you're going to be OK. - You're OK.
0:43:11 > 0:43:13- We've done your medical, you're fine.- Yeah.
0:43:13 > 0:43:15You're going to live another 400 years.
0:43:16 > 0:43:20'Enhancing Sharon's data, we can reveal how trees
0:43:20 > 0:43:22'keep themselves upright
0:43:22 > 0:43:24'in a very different way from, say, a building.'
0:43:30 > 0:43:32The root system of our oak tree here
0:43:32 > 0:43:36is likely to be one of the most impressive in the whole of Britain.
0:43:38 > 0:43:41Some roots are as thick as a big branch.
0:43:43 > 0:43:46And Sharon reckons the roots make up as much as a quarter
0:43:46 > 0:43:48of the total weight of the tree.
0:43:53 > 0:43:55The spread of roots underground
0:43:55 > 0:43:58is even greater than the expanse of the branches.
0:44:02 > 0:44:04A 30-metre crown,
0:44:04 > 0:44:08and an even more impressive 34-metre spread of roots.
0:44:14 > 0:44:16You might think the roots of such a tall tree
0:44:16 > 0:44:19would have to go deep into the ground.
0:44:21 > 0:44:25But even for such a massive oak, these are relatively shallow.
0:44:25 > 0:44:27No more than a couple of metres deep.
0:44:37 > 0:44:40In a way, the complete opposite of how you do foundations in a building.
0:44:40 > 0:44:43So The Shard, which is very, very deep and very, very contained,
0:44:43 > 0:44:45- here, you have shallow and spread out.- Exactly.
0:44:45 > 0:44:48And that manages the loading and it also helps the tree
0:44:48 > 0:44:51get all the water and nutrients it needs from the soil.
0:44:51 > 0:44:54Another reason why they tend to be shallow is tree roots need oxygen.
0:44:54 > 0:44:57And the oxygen is found in the top metre or so,
0:44:57 > 0:44:59before the ground gets too hard and consolidated.
0:44:59 > 0:45:01So this is the perfect solution.
0:45:01 > 0:45:04- Nature has found the perfect solution for the tree.- It has.
0:45:10 > 0:45:13'The roots are not only supporting the tree,
0:45:13 > 0:45:17'they're also the tree's life-support system.
0:45:17 > 0:45:19'The very reason why it could grow in the first place.
0:45:21 > 0:45:23'Think of roots as more than just pipes
0:45:23 > 0:45:25'drawing water out of the ground.
0:45:25 > 0:45:28'There's something else far more interesting going on.
0:45:28 > 0:45:30'Hopefully, I can see what that is.
0:45:32 > 0:45:33'IF I can burrow under here.'
0:45:36 > 0:45:38I'm actually under a tree.
0:45:38 > 0:45:41This is a nearby tree that was growing onto a hillside
0:45:41 > 0:45:44and then the hillside's just slipped away in a landslide,
0:45:44 > 0:45:47revealing the roots underneath. And you can actually get inside.
0:45:47 > 0:45:49You can see just how many roots there are
0:45:49 > 0:45:52and how knotted and tangled it becomes.
0:45:52 > 0:45:54But there's something else
0:45:54 > 0:45:57that happens to roots when you're underground.
0:45:57 > 0:45:59Let's have a little dig around here maybe.
0:45:59 > 0:46:03I might be able to show you. Yeah, here we go. Now, look at this.
0:46:03 > 0:46:05Hopefully, you'll be able to see this.
0:46:05 > 0:46:11This, er...sort of white, stringy stuff that almost looks like cobweb.
0:46:11 > 0:46:13That is what I'm interested in.
0:46:15 > 0:46:18And that is actually a kind of fungus.
0:46:22 > 0:46:24Fungi, which includes mushrooms, are odd.
0:46:27 > 0:46:29They're neither plants, nor animals.
0:46:31 > 0:46:33But they do have a nice trick.
0:46:33 > 0:46:36Fungi break up dead plant material in the soil.
0:46:38 > 0:46:42And this releases nutrients which the tree roots can then take in.
0:46:44 > 0:46:47Without this process, the trees just couldn't grow.
0:46:48 > 0:46:51But the fungi can't do that job alone.
0:46:51 > 0:46:54They have some rather surprising helpers.
0:46:54 > 0:46:59If you magnify soil 500 times, you'll see, hidden inside,
0:46:59 > 0:47:02an army of microscopic animals and bacteria.
0:47:05 > 0:47:08Working together, they're the ultimate recycling machine,
0:47:08 > 0:47:11keeping the soil fertile.
0:47:14 > 0:47:17So there's a whole, vast ecosystem underground
0:47:17 > 0:47:19that's completely invisible to the naked eye.
0:47:19 > 0:47:21And yet it sustains all of this,
0:47:21 > 0:47:24the natural world we're so familiar with.
0:47:26 > 0:47:30Today, there are around 100 million trees in Britain,
0:47:30 > 0:47:33covering 10% of the land.
0:47:33 > 0:47:36But there are far fewer forests than there once were.
0:47:38 > 0:47:40We've cleared most of our trees
0:47:40 > 0:47:43to make way for another living habitat.
0:47:43 > 0:47:45One that covers over a quarter of the country.
0:47:46 > 0:47:48Our farmland.
0:47:51 > 0:47:56The distinctive patchwork of British fields looks familiar to all of us,
0:47:56 > 0:48:00but sometimes, underneath, these, too, can hold surprises.
0:48:02 > 0:48:04'You can't see what's down there,
0:48:04 > 0:48:07'but there is one way to try to find out.'
0:48:10 > 0:48:11Terry!
0:48:11 > 0:48:15'Terry Herbert's been metal-detecting for over 20 years.
0:48:15 > 0:48:19'And now, he's going to teach me how to hunt for buried treasure.'
0:48:19 > 0:48:21Right. You've got a control box here.
0:48:21 > 0:48:24You've got the VDI display unit.
0:48:24 > 0:48:27You've got the coil, which finds you the items, that does.
0:48:27 > 0:48:30That actually sends radio waves into the ground.
0:48:30 > 0:48:33And if you hit a target, it comes back
0:48:33 > 0:48:36and it gives you a reading on the meter.
0:48:36 > 0:48:39Is it the type of hobby that you could give up your day job for
0:48:39 > 0:48:42and actually make a bit of money on?
0:48:42 > 0:48:44Well, you can do. I mean, some do.
0:48:44 > 0:48:48On the beach, some people go to Spain and actually detect.
0:48:48 > 0:48:51- You can earn quite a bit.- Yeah.
0:48:51 > 0:48:54So, what's the sort of most exciting thing that you've ever found?
0:48:56 > 0:49:00Well, actually, the most exciting I ever found was a Saxon hoard.
0:49:04 > 0:49:08It's the biggest haul of Anglo-Saxon gold and silver ever found.
0:49:10 > 0:49:15In July 2009, Terry made the discovery of a lifetime.
0:49:16 > 0:49:19An enormous collection of Anglo-Saxon treasure.
0:49:19 > 0:49:22There were over 3,500 pieces
0:49:22 > 0:49:26of decorated gold and silver buried underground.
0:49:27 > 0:49:29I'm trying to imagine what it would have felt like
0:49:29 > 0:49:32the first time you got a signal, like we're getting here,
0:49:32 > 0:49:34and actually scrabbled around. And did you realise...?
0:49:34 > 0:49:37You said it looked a bit like perhaps brass, or something.
0:49:37 > 0:49:40Yeah. But at what point did you go, "Actually, no, that's gold"?
0:49:40 > 0:49:43When I looked at it under me magnifying glass,
0:49:43 > 0:49:44I noticed there was a pin.
0:49:44 > 0:49:47So I thought, "Oh, this is a piece of gold."
0:49:47 > 0:49:49I actually went back to me car and got me other machine out.
0:49:50 > 0:49:53And, er...when I came back on the field with that,
0:49:53 > 0:49:55it was just going off like a machine gun.
0:49:58 > 0:50:03When archaeologists recovered the entire hoard and began to study it,
0:50:03 > 0:50:06they realised the workmanship was exquisite.
0:50:13 > 0:50:17Now it's considered one of Britain's most important archaeological finds.
0:50:19 > 0:50:25The Staffordshire Hoard has been valued at a cool £3.25 million.
0:50:27 > 0:50:31Terry received half of this, making him a millionaire overnight.
0:50:34 > 0:50:36What happened to the money? What did you do with it?
0:50:36 > 0:50:37I bought another machine.
0:50:37 > 0:50:41I've actually bought three more machines, since then, like.
0:50:41 > 0:50:45I think there's got to be another Saxon hoard somewhere in Britain.
0:50:45 > 0:50:48No way have I just found the only one in this country.
0:50:48 > 0:50:51There's got to be another one somewhere waiting to be found.
0:50:51 > 0:50:55And it's going to take somebody with a metal detector to find it.
0:51:00 > 0:51:04'Of course, the fields of Britain weren't cleared and cultivated
0:51:04 > 0:51:08'just for Saxons to bury their hoards and for us to find them.
0:51:08 > 0:51:13'They performed the very necessary task of feeding the country.'
0:51:13 > 0:51:15And it seems every year, we demand the land produces
0:51:15 > 0:51:18more and more food to put on our tables.
0:51:19 > 0:51:22More than the soil can provide naturally.
0:51:23 > 0:51:27And to find the magic ingredients to keep our fields fertile,
0:51:27 > 0:51:29we've had to go beneath the ground once more.
0:51:37 > 0:51:39I've saved the best till last.
0:51:39 > 0:51:41Because here on the North Yorkshire coast,
0:51:41 > 0:51:45I'm on my way to the deepest point you can reach under Britain.
0:51:49 > 0:51:52OK, off we go. How long does it take to get all the way down?
0:51:52 > 0:51:55- Takes about seven minutes, something like that.- OK.
0:51:55 > 0:51:57- Starting off slow and then we pick it up.- Suddenly it speeds up!
0:52:00 > 0:52:04This is Boulby Mine and it's the deepest mine in Britain.
0:52:04 > 0:52:07And this lift shaft is over a kilometre straight down.
0:52:07 > 0:52:10It travels at about ten metres a second.
0:52:12 > 0:52:14Now we're getting to the bottom of the shaft.
0:52:14 > 0:52:18'But the bottom of the shaft is just the start of my journey.'
0:52:22 > 0:52:26This is so great. This is as deep as it is possible to go in Britain.
0:52:26 > 0:52:27You can feel the wind, as well.
0:52:27 > 0:52:30All the air that they're pumping in from up there.
0:52:30 > 0:52:33There's a heck of a wind that comes down and just blows you.
0:52:36 > 0:52:39'This is an amazing place.
0:52:39 > 0:52:43'The mine is so vast, you've got to get around by truck.
0:52:43 > 0:52:46'And I had no idea that the tunnels stretch out
0:52:46 > 0:52:49'over five kilometres under the North Sea.'
0:52:51 > 0:52:54It is an incredibly surreal experience being down here
0:52:54 > 0:52:55because we're actually out to sea now,
0:52:55 > 0:52:57we're actually underneath the seabed.
0:52:57 > 0:52:59We've left Britain behind us.
0:52:59 > 0:53:01And the tunnels just keep on going and going.
0:53:05 > 0:53:07'It's a funny way to go to sea.'
0:53:08 > 0:53:10We're 1,100 metres underground.
0:53:11 > 0:53:15It's certainly the deepest place I've ever been in my life.
0:53:15 > 0:53:17It's blisteringly hot, as well.
0:53:19 > 0:53:23'The surface of the rock can be nearly 40 degrees centigrade.
0:53:24 > 0:53:27'But what I'm really here for is what that rock contains.
0:53:30 > 0:53:34'Now, it may not look much, but this is real buried treasure.
0:53:35 > 0:53:37'And to get my hands on it,
0:53:37 > 0:53:40'I'm going to operate this remote-controlled monster.
0:53:41 > 0:53:43'Its jaws will do the hard for me.'
0:53:45 > 0:53:46- So now you lift the head up.- Ah.
0:53:48 > 0:53:49Now, down a touch.
0:53:49 > 0:53:51- So that's the head going down a bit. - Yeah.
0:53:51 > 0:53:53There we go, we're cutting in!
0:53:57 > 0:54:00'These behemoths simple chew through the rock.'
0:54:02 > 0:54:05This is what it's all about - freshly mined potash.
0:54:05 > 0:54:08And it's this potash that they use to make fertiliser.
0:54:08 > 0:54:11It's this stuff that reinvigorates the British landscape.
0:54:14 > 0:54:16'They mine potash around the clock,
0:54:16 > 0:54:19'carving out up to a million tonnes per year.
0:54:19 > 0:54:21'This is the only place in Britain
0:54:21 > 0:54:25'where we can get this valuable fertiliser.
0:54:25 > 0:54:28'It's completely bizarre to think that down here
0:54:28 > 0:54:31'is the stuff we need to put the food on our tables.
0:54:36 > 0:54:38'But that's not all.
0:54:38 > 0:54:41'They're mining for something else down here.
0:54:41 > 0:54:44'Something strange and not of this Earth.'
0:54:45 > 0:54:48Behind these doors, they're trying to get to the bottom
0:54:48 > 0:54:51of perhaps the biggest mystery in all of science.
0:54:54 > 0:54:57'Tucked away down here is Britain's deepest laboratory.
0:55:00 > 0:55:02'And where you need to trade your green hat
0:55:02 > 0:55:04'for a nice, clean, white one.'
0:55:07 > 0:55:09- This is a full-on clean room.- OK.
0:55:09 > 0:55:11'Sean Paling is a physicist.
0:55:11 > 0:55:15'He and his team down here are hunting for something mysterious.
0:55:17 > 0:55:22'When scientists looked at galaxies in deep space, they found a problem.
0:55:22 > 0:55:26'According to their theories, these spinning collections of stars
0:55:26 > 0:55:29'should fly apart, but they don't.
0:55:29 > 0:55:32'Something unseen is holding them together.
0:55:32 > 0:55:34'Something they call dark matter.'
0:55:39 > 0:55:41Dark matter is a name that we give to stuff that we think
0:55:41 > 0:55:44exists in the universe that we can't see.
0:55:44 > 0:55:46We think that when you look at the night sky,
0:55:46 > 0:55:48the stars and the planets and galaxies,
0:55:48 > 0:55:52the stuff that we know about makes up 15% of what's out there.
0:55:52 > 0:55:55We think 85% of the mass in the universe is missing.
0:55:55 > 0:55:57That's a lot not to know about.
0:55:57 > 0:56:01Yes. I mean, it's an embarrassing lack of knowledge.
0:56:04 > 0:56:08So far, no-one's found any trace of this dark matter.
0:56:09 > 0:56:12Above ground, there's just too much light and radiation
0:56:12 > 0:56:14getting in the way to detect it.
0:56:14 > 0:56:18'But here, Sean hopes that the 1,000 metres of solid rock
0:56:18 > 0:56:21'will stop any radiation from penetrating,
0:56:21 > 0:56:24'but the dark matter will be able to get through.
0:56:24 > 0:56:28'Down here is perhaps our best chance of detecting it.
0:56:28 > 0:56:30'In our rapidly-changing world,
0:56:30 > 0:56:34'it's knowledge itself that's become Britain's greatest resource.'
0:56:38 > 0:56:40In making this programme,
0:56:40 > 0:56:44I've seen a Britain I never knew existed. A hidden world.
0:56:45 > 0:56:48I discovered what's underneath Britain's tallest building.
0:56:51 > 0:56:54And ventured into a vast underground cathedral.
0:56:56 > 0:56:59I've encountered the eccentric,
0:56:59 > 0:57:02the surprising and the downright disgusting.
0:57:04 > 0:57:08And I've seen the extraordinary ways that Britain below ground
0:57:08 > 0:57:11has affected and shaped the countries and cities above.
0:57:13 > 0:57:14And next time...
0:57:17 > 0:57:20..I blast my way back underground.
0:57:20 > 0:57:22And take to the skies
0:57:22 > 0:57:27to reveal the secret networks and connections
0:57:27 > 0:57:29that keep Britain moving.