(Ship's horn)
This is Coast.
ln the British lsles, we're familiar with wet weather blown in from the wild seas.
One benefit of a temperate climate is our wonderful labyrinth of rivers.
Giant waterways powered by rain, that all run to the coast.
As rivers and seas collide, great estuaries emerge.
Making our mark on these colossal watery spaces has taken centuries of struggle.
That's left a wealth of extraordinary stories waiting to be discovered along our estuaries.
We're braving three of our greatest.'
the Firth of Forth, the Thames and the Mighty Severn.
And we'll visit grand cities too.
Tessa discovers how the pulling power of estuaries gave the Victorians a capital idea.
Turn the Thames into a giant self-flushing loo.
Where rivers surge into sea lochs,
Miranda swims with the fishes.
The Scottish salmon industry spawned a business worth around half a billion pounds.
But how did they crack the secret of farming a wild sea fish?
And Mark's on the rollercoaster ride under...
Ahh! Wahey!
..and across the deadly tides of the Severn Estuary.
The story of how it was eventually crossed is one of extraordinary ingenuity
and also explosive tragedy.
We're here to explore what becomes of the coast when rivers and seas collide.
l'm starting my estuary odyssey a pebble's throw from Edinburgh on the Firth of Forth.
The scale of this seaway is staggering, it's impossible to take the whole thing in.
What l could really do with is something tall to climb up
so l can get a bird's eye view.
Only the engineering marvel of the Forth Rail Bridge
doesjustice to the sheer spectacle of the estuary.
As we're coming up here, you can see the rivets on this bridge that hold it together.
6.5 million rivets, and every one of them has been painted by hand.
- ls this it? - This is it, Nick.
Here we are on top of the Forth Bridge.
(Ship's horn)
Up here, right in the middle of the Firth of Forth,
you can get a real sense of the huge scale of this estuary.
l can see the Pentland Hills right over there,
there's the dark volcanic bump of Arthur's Seat
rising above the white buildings of Edinburgh.
Looking west, l can see all the way out to the open sea, the North Sea.
And looking inland in this direction, there's even more.
Here's the Forth Road Bridge arcing over the water in front of me,
behind it Rosyth naval base.
ln the far distance, l can just make out Grangemouth power station,
oozing smoke into the sky.
This estuary is so huge that even from this incredible vantage point,
inland it just fades into invisibility.
The only way of actually getting a true sense of its size is by looking at a map.
This is the mouth of the estuary marked by this little island, the lsle of May, here.
ln the other direction, 60 miles inland,
the water gets less and less salty, gets fresher and fresher
until you reach Stirling here, where this estuary is born.
Starting at its birthplace, l'm flying the length of the waterway.
Will the change in wildlife help pinpoint the elusive spot where river becomes sea?
My guide's marine ecologist Stuart Clough.
And as we pass over Stirling,
the river's very beautiful seen from above, it's like a huge coiled rope.
STUART: You're in classic lower river territory here.
Lower freshwater river, the place where the tide just starts to have its effect.
NlCK: And even now, the mud banks are starting to appear on the side.
STUART: And in those, you've got all kinds of worms and shellfish
that live within those sediments.
They become food for birds. lt's a fantastic environment.
ls it possible to identify the point at which this river, the Forth,
ceases to be a river and begins to be sea?
STUART: From a biologist's perspective, it's a continuum, it changes all the time.
On one hand, it's a no-man's land,
and on the other hand, it's a diverse and rich place with masses of life.
Life is rich where rivers and seas meet.
And where we flock, so does the wildlife.
As we move into saltwater, the big hitters start to surface.
Dolphins, seals and even whales have all been spotted here.
We're now over the sunlit seaside, aren't we, Stuart?
lt's completely changed.
Absolutely, yeah, we're right out in the outer estuary now.
The freshwater influence is a long way behind us,
the beaches are sandy...
NlCK: lf we were down at sea level now, what kind of birds would we be looking at?
Auks, like razorbills, guillemots and puffins,
you've got fulmar, you'll have kittiwakes, you'll have gannets -
real marine species that you'd never find in the freshwater parts of the estuary.
At the edge of the estuary, we get a box office few of the gannets of Bass Rock.
This swirling mass makes the most of food from the sea and shelter from the land.
Where are we now?
STUART: We're just adjacent to the lsle of May, very much the outer limit of the estuary.
We've flown the whole way from the freshwater of a river to the saltwater of the open sea,
over an extraordinary diversity of habitats both human and natural.
Estuaries are worlds of their own.
20 million of us, one third of the UK's population, live on an estuary.
Their flat shorelines are perfect for building,
so each of these coastal highways comes with its own gatekeeper.
Great cities surge up where mighty rivers plunge into the sea.
lt's fitting that the country's capital crowns the most hard-working waterway of all -
the titanic Thames.
For centuries, Londoners have swallowed up the benefits the estuary brings in.
The sea brought riches from abroad,
and the river supplies two-thirds of the city's drinking water.
But the Victorians found a newjob for old Father Thames...
..doing their dirty work.
Tessa's getting to grips with a grubby tale of triumph and tragedy.
The power of the tide gave an eminent Victorian engineer an extraordinary idea -
turn the Thames into a giant self-flushing loo.
TESSA: The tidal range of the river is huge, around eight metres.
This powerful ebb and flow gave rise to an ingenious sewer plan.
Release excrement as the tide turns,
and let the outgoing flow flush London's waste way out to sea.
The city's relationship with the sea spawned a sewer system that was the envy of the world.
Opened in 1 865 by the Prince of Wales,
this subterranean labyrinth elevated its mastermind, Joseph Bazalgette,
to become a hero of the Victorian age.
Bazalgette's master plan demanded a warren of waste pipes,
a network over 1,000 miles long to carry the capital's raw sewage out to the Thames.
lt took six years to build,
constructed so well, it still forms the backbone of London's sewer complex.
Over 300 million bricks, placed so precisely they form water-tight tunnels.
You know how to treat a girl, don't you, Rob?
l do, l take them only to the best spots.
lmpressive as this labyrinth is, it's only the means to a watery end.
The city's sewage still needed sweeping out to sea,
so it was piped towards the coast to pass the problem onto the tide.
The muck flowed downstream to arrive at the final triumph of the entire system,
the pumping station at Crossness.
This is staggering!
lt's like some sort of ballroom.
lt's a real indication of the level of pride they took in their work.
The beauty is just breathtaking.
And these huge pumps are even named after members of the royal family.
The pumping stations were the final stage of Balzalgette's grand plan.
They pushed the sewage up into huge reservoirs,
to be stored until the tide began to turn.
(Ship's horn)
When the tide started to ebb, they released the sewage into the Thames just there.
They relied on the surge of seawater to whisk Londoners' muck out of sight and out of mind.
This was Joseph Bazalgette's big tidal flush.
His plan to turn the Thames into one gigantic toilet bowl was complete.
Bazalgette was heralded as the city's saviour.
But is there a skeleton lurking in London's water closet?
Life may have been rosy for those in central London,
but it didn't smell so sweet for those living downstream.
Like a real-life toilet, the Thames is full of U-bends.
The waste wasn't clearing as fast as Bazalgette had imagined,
and the consequences turned out to be devastating.
lt's 3rd September 1 878.
The pleasure steamer the Princess Alice is on its way back to London,
crammed with passengers.
The day-trippers had been enjoying fresh air at the mouth of the estuary,
but returning to the city, near the sewage outlet, the pleasure steamer was struck by disaster.
(Creaking and splintering of wood)
lt collides with another boat.
Hundreds are flung into the river, many will be drowned.
But it's even worse than that.
Revealing the gruesome fate of those floundering in the estuary
is local historian Joz Joslin.
So the vessel's upended, and hundreds of people are in the water?
Yes, and lots of them are women and children.
And they're screaming.
Unfortunately it's not water that they're in, they're in actually sewage.
So there was no oxygen.
Lots of them died because there was no air to breathe.
Ohh!
So they're either being suffocated or drowning?
- Or poisoned. - How revolting!
- And the majority died? - Yes, the majority died.
So they said that every street in the East End of London had lost somebody,
because it was their Sunday school outings that were on board the vessel.
The pleasure boat sank close to the sewage works,
and the timing could not have been worse.
The Beckton sewage outlet pipe carrying all North London's waste
had just discharged its stinking load into the river.
Over 600 people choked to death in a toxic soup of human filth.
After the tragedy, Bazalgette's sewage system came under the spotlight.
Members of the local historical society read the words of their forefathers.
''There had been an accumulation of black greasy filth along the shore.
The filth settles on the steps as the tide goes down.''
''The river in hot weather is very bad.
ln some places, it smells so bad you cannot stand it.''
A commission of inquiry delivered a damning indictment, concluding.'
''lt is neither necessary orjustifiable
to discharge sewage in its crude state into any parts of the Thames. ''
The Pall Mall Gazette took Bazalgette to task, stating.'
''The natural man in him puts off the evil day of having to admit failure. ''
Luckily for Bazalgette, the muck didn't stick,
but London did pull the plug on his big tidal flush.
ln 1 887, a new system started.
Now the solid human waste was pumped into vessels like this.
The excrement was shipped out to the open sea and dumped.
They were known locally as ''Bovril Boats''...
amongst other things.
We used to call them... Well, never mind what we used to call them!
- What did you call them? - No, l'm not saying.
They used to come and moor, they had moorings for them,
and they would take the residue of it.
Cos all the fluids were taken off, so it was almost solid the stuff that they took out,
human detritus,
so that it wasn't into the river.
Sewage-carrying ships didn't just do the dirty work of London.
They were once a common sight on our estuaries,
cleaning up Glasgow, Belfast and other coastal cities.
London's Bovril Boats were finally pensioned off in 1 998.
Balzalgette's tunnels still bring raw sewage here to the Crossness Works,
but now the solid matter's burnt off to make electricity.
The liquid sewage is treated, it goes from this...
..to this.
The cleaned-up fluid still gets the big tidal heave-ho and is discharged into the Thames,
where the river and the sea collide.
NlCK: Tide and traffic on the Thames flow two ways.
ln deep waters at the estuary mouth,
ships from around the world come to unburden themselves on the docks at Sheerness.
But back in the 1 9th century,
a group of foreign stowaways snuck off a ship and never left.
They set up a secret community within the harbour walls.
This is the des-res of Britain's only colony... of scorpions.
But not the monster kind.
European Yellowtail scorpions arrived here from ltaly on a masonry ship some 200 years ago.
Now the offspring of those ltalian scorpions have found a British admirer.
That's amore
Hi, l'm Bex, and l'm a scorpo-holic.
l've been fascinated by scorpions since l was a teenager and been hooked ever since.
l'm here to see Britain's only colony of scorpions,
but l've got to wait for the sun to go down.
Ting-a-ling-a-ling, and you'll sing...
lt's properly dark now, so l'm going to see if l can find some.
l'm using a UV torch cos scorpions glow under ultraviolet light.
l think l've just spotted one.
Definitely an adult, probably out looking for something to eat.
lt is pretty cool, though, isn't it, having scorpions in the UK?
They are a member of the spider family, they have got eight legs, not six.
They eat woodlice.
They're ambush predators, so they will just sit and wait
and then something will come past and they'll just jump out and grab it
and subdue it with their claws rather than stinging it.
They don't generally use their stings, these ones.
That's amore
Very happy that we found some.
This is a tiny little incy one, it's so cute, and with tininess comes speed.
lt's been a great night, we've seen loads of scorpions.
But l think l'll put this one back before it legs it.
Bye, little fella.
That's amore
l think l'll leg it now, too.
That's amore
We're on a journey to discover what becomes of the coast when rivers and seas collide.
l'm exploring the Firth of Forth on Scotland's east coast,
where canny folk profited from their prime location, ideal for seaborne business.
And with rich seams of coal for power, the population boomed.
With more mouths to feed, getting enough fresh food was tricky,
so they looked to the sea to preserve their provisions.
You'll find the evidence at St Monans.
Here food processing created a curious landscape.
The shore is lined with lots and lots of very strange grass-covered humps,
with what seems to be a ruined building over there.
And up there, a stone windmill.
The ruins of industrial activity reveal themselves the more you look.
This land was remodelled by people making the most of one bounty from the sea
that isn't in short supply - salt.
Before refrigeration, salt was a valuable commodity,
preserving herring landed along the East Coast.
Scottish salt was also exported to England,
turning a tasty profit for the salt works.
Those strange hummocks come in sets,
each set of hummocks is the ruins of a pan house.
lnside each of those pan houses,
there was an iron pan about six metres by three metres, with coal fires beneath it.
Sea water was pumped, probably using this windmill,
from the sea in pipes up to each pan house.
Once it had been boiled off in the pans, you had salt.
A rare film brings the enterprise back to life.
Salt works once flourished along Scotland's east coast.
The last operation, at Prestonpans, didn't close its doors until 1 97 4.
lt was the abundance of coal along this estuary
that made it a good site for boiling up seawater.
A sample of seawater stirs up a mystery,
right at the heart of this forgotten industry.
Out there is the sea full of salt.
And l can certainly taste it.
This little brook running into the sea...
..doesn't taste salty at all.
So why is freshwater fresh...
..and why is seawater salty?
lt's one of those brilliantly simple, infuriating questions that kids ask.'
why is the sea salty?
l'm enlisting the help of a grown-up.
Simon Boxall's from the National Oceanography Centre.
He should be able to work it out.
We've all swum in the sea, we know it doesn't taste like freshwater, Simon.
But why is it salty?
You have to go right back to the beginning stage of the earth,
back several billion years.
lf you go back that far, the earth was a completely different place.
lt was full of volcanic eruptions,
there was lots of steam around,
but also there was a lot of sodium in the rocks.
And that sodium was being hit by the hydrochloric acid
that was given off by these volcanic vents.
lf we take these two very harmful chemicals -
on the one hand, you've got the element of sodium, very reactive,
on the other hand, there's chlorine, very dangerous and very reactive.
Put the two together and you create something, sodium chloride,
which is the sort of thing you sprinkle on your chips and certainly isn't harmful at all.
So you've got this hydrochloric acid pouring out of the volcanic vents
meeting the sodium hydroxide which is already lying in the rocks on the sea bed,
creating this stuff called sodium chloride, which is salt.
These ancient chemical reactions gave birth to our salty seas.
We can create those sort of primordial days.
We can actually take some hydrochloric acid,
the sort of stuff that came out of the vents of the volcanoes.
We've got some dilute sodium hydroxide, which represents the stuff that was in the rocks.
Between us, we can make some salt.
We can take these two quite nasty chemicals
and we can produce something really vital to life, in many ways.
This is hydrochloric acid, very dilute.
And we're going to pop it into this vessel here.
OK.
We're going to then add our sodium hydroxide.
Now, at the moment, basically, the sodium and the chlorine are combining.
That's giving off heat, can you feel that?
lt's warm, really warm. Wow!
We've effectively neutralised that acid, that sodium hydroxide,
and what we have in there now is basically water and salt.
We've compressed billions of years of the earth's evolution to make a miniature ocean.
Boiling off our DlYseawater leaves the prize ingredient.
So here it is - our very own home-made salt.
White crystals that washed wealth in from the sea to help feed an estuary.
The salty shoreline satisfies our appetite in surprising ways.
On sea marshes in Wales, sheep graze on grass made sturdy by regular salt baths.
This distinctly coastal vegetation gives the lamb a unique flavour.
But to explore the biggest effect our shoreline has on our waistline, head north to Scotland.
A natural wonder plays out where rivers meet the sea,
like here at Lochcarron.
Miranda is immersed in the secret life of the salmon.
l'm in the thick of Britain's favourite fish dish.
Our insatiable appetite for salmon
has spawned an industry in the UK worth around half a billion pounds a year.
But farming these fish is a lot more difficult than you think.
MlRANDA: Salmon are challenging to rear in farms because of their extraordinary life cycle.
They're born in freshwater rivers,
then migrate to the sea.
To survive in saltwater, a salmon's body goes through radical changes.
That transformation is very tricky to manage in captivity.
To see why, l'm seeking out the wild fish.
Take a winter walk along a salmon river
and there might be delicate little eggs lurking deep in the watery gravel.
So why do salmon out at sea bother to come all this way to lay their eggs?
l'm meeting wild salmon expert Bob Kindness.
What a place for the salmon to come back and lay their eggs in the gravel here.
BOB: They have to be spawned in fresh water.
The eggs would not survive other than being in fresh water, they wouldn't survive in the sea.
So the salmon has to make that journey, it has to come back from the feeding grounds,
usually back to its home river, back to the area where it itself was born.
Once hatched, these freshwater fish face a big problem.
They need to get out to sea where there's more food,
but their bodies are incapable of dealing with salt water.
Ajelly sweet experiment gives us a taste of their challenge.
This tiny jelly bear has a dense sugary body.
Soak him in water overnight and he looks like this, all swollen up.
This plump bear is our happily hydrated freshwater salmon.
But if l add salt to create seawater,
the fresh water is sucked out of the sweet, and it shrivels.
Death by dehydration is the challenge a salmon faces
going from the river down into the sea.
To avoid dehydration, salmon develop two defences.
Their scales toughen to stop freshwater leaching out of their body.
Also their kidneys and gills adapt so they can cope with life in saltwater.
To see the scale of the transformation, we've anaesthetized two of the fish.
Gosh, you can really see the difference, can't you now, side-by-side?
The speckled brown tiddler is a freshwater salmon,
the silver beauty below, just a year older, is ready for the sea.
A salmon can turn around and go back
if its body isn't adapting to life in the seawater,
and some fish take years to make it out to sea.
That's OKin the wild,
but how could fish farmers know when it was safe to move their salmon to saltwater pens?
ln the early 1 970s, years of experiments finally paid off.
l've got a patent here applied for in 1 97 1 .
This is a scientific recipe for the crucial step in the growing of captive salmon.
The Fish Equation says when the weight of the salmon, W,
x 1 00, divided by its length, L, cubed,
is equal to 0.8, it's ready for sea water.
Armed with the power to control nature,
salmon farming in Scotland rapidly became big business.
As the scale of production rose, the price of the fish started to plummet.
NEWSREEL: The market shows every sign of expanding as the supply increases.
And now, salmon is not just to be found in the exclusive fishmongers,
but can be bought vacuum-packed in the supermarkets.
MlRANDA: Salmon stopped being a treat saved for special occasions.
Now it was as cheap as chicken.
So how did fish farmers manage to mimic nature with their production line process?
The salmon start life in freshwater tanks.
When they're big enough to satisfy the Fish Equation,
they're sucked up and piped into trucks.
Trucks take them to boats,
boats ferry them to saltwater enclosures anchored at the sea end of the estuary.
The salmon are kept in these pens for up to 22 months,
but they're quite used to human company.
There are divers who regularly check on the fish and the nets that hold them in.
The salmon are constantly watched.
CC TV operators check they're eating - and make sure they're not being eaten - by seals.
Farmed fish carry more lice than wild ones,
so they're monitored for sickness too.
Our appetite for salmon makes all this effort profitable.
lf our demand for these fish is such that we need to farm them,
then salmon pens will become an even more common site where rivers and seas collide.
NlCK: We're on a journey around three of our mightiest estuaries...
..where tideways bring bounty from the sea deep inland.
So imagine the potential of connecting two great estuaries to form one super-highway.
That unifying vision excited the imagination of merchants in the Firth of Forth.
ln 1 768, businessmen began to build their own waterway,
an epic canal joining the Forth to another great estuary, the Clyde.
What those early entrepreneurs couldn't know was that their scheme to connect two estuaries
would one day lead to a revolution in fibre-optic communication.
The story starts with an extraordinary discovery on the waterway, some 1 80 years ago.
Let's go back to that fateful day in 1 834.
Young engineer John Scott Russell - this is him later in life -
was on the towpath of this canal.
Russell was watching a barge,
when suddenly its bow wave detached from the front of the boat and sped off on its own.
lt was a solitary hump of travelling water.
The engineer would become obsessed
trying to understand the solitary wave through mathematics.
But on first sighting, he had to act fast.
Russell jumped on his horse and gave chase to the wave.
Normally waves peter out or break up,
but this mysterious wave retained its shape and sped onwards.
After a chase of about two miles,
the solitary wave still showed no signs of petering out.
Eventually, it escaped Russell's pursuit altogether.
Russell would have recognised that this peculiar wave was similar to the Severn bore.
That solitary wave is produced when the tide collides with the River Severn,
sending a surge of seawater up the narrow funnel of the Bristol Channel.
Not unlike the narrow channel of the canal.
So Russell experimented with canal-shaped water tanks.
Mathematician Chris Eilbeck thinks he can recreate Russell's solitary wave.
- Keep our fingers crossed. - Off you go, l'm dying to see one.
- So you can see the wave there... - Look at that, you got it first time.
Just a hump of water on its own.
You can feel the force of it when it hits.
- Now it's gone all the way back. - lt's amazingly stable.
lt'll keep going for miles.
The canal's dimensions were perfect for keeping the wave stable,
deep enough for the wave not to steepen too sharply and break,
but shallow enough that it didn't flatten out.
Amazingly, these waves remain self-contained and intact,
even when they collide.
Three, two, one, go!
CHRlS: And you can see that after the collision, they just reappear on the other side.
Chris, are you saying that the waves didn't just bounce off each other
but went through each other?
Yes that's correct, they go through each other and reappear on the other side.
- Undiminished? - Yes, and mathematically exact.
Unbelievable.
These remarkable packets of travelling energy have been christened solitons.
Now is the time to scale our experiment up.
Let's try and recreate the full wonder of the soliton wave
on this 800-foot-long straight section of Scottish canal.
But it won't be easy.
The wave John Scott Russell chased back in 1 834
was created when a horse-drawn boat came to a sudden stop,
releasing its bow wave.
We're trying a more modern approach - a motor boat loaded with people
to provide ballast, and to bear witness.
Now, Rachel, you're sitting in the front of the boat here with some top hats.
You're clearly expecting something to happen. What have you got the hats for?
We've got the hats as a tribute to John Scott Russell, who discovered the first soliton wave.
We'll be throwing them up in the air with joy when we recreate it, is the plan.
l'm very impressed by your optimism.
Solitons are rare, not easily produced.
The boat must gain enough speed to form a sizeable bow wave then come to a sudden stop.
Hopefully that'll release a soliton.
lf it does, l'm going to chase it, just like Russell,
only l'll be racing it on foot, not on horseback.
l've run up mountains, and l've run a few marathons.
But l've never run against a wave before. l'm not sure l'm going to have to.
Oh, my God!
l beat it over a few hundred metres, but it's still going.
And l'm not going to be able to keep going as long as that wave.
(Cheering)
John Scott Russell designed special boats to ride his wave at high speed along canals.
But sadly, most people preferred a new form of transport - the railway.
Russell waved goodbye to his bright idea.
True to its nature, though, the soliton wasn't finished.
Today, the solitary waves first seen on this canal aren't only found in water, they travel as light.
Scientists have created soliton waves from tiny pulses of light.
Fired down the next generation of fibre-optic cables,
these self-sustaining waves can carry communications for thousands of miles,
travelling on endlessly between continents,
just as John Scott Russell's wave ran on out of sight of me.
- So his day has come. - His day has finally come.
And if you were to meet him now, what would you say to him?
l would tell him that his idea has really come of age and everybody talks about him,
everybody talks about the soliton.
lt's a really big thing in science, so he'd be delighted, l think.
- Satisfied scientist? - Yes.
On a journey around our estuaries, we've arrived at the mighty Severn.
Here the Atlantic Ocean surges in to collide with Britain's longest river.
The Severn Estuary has a staggering rise of tidal water, some 1 5 metres in all.
The sea swirls in strange patterns here.
lts currents would wash the bodies of wrecked sailors to the same spot again and again -
the village of Brean.
Those unknown souls delivered to the doorstep of Brean needed burial,
a sorrowful ritual remembered in song.
Folk singer and storyteller June Tabor recalls The Brean Lament.
The first thing that strikes you about being here...it's timeless.
lt could be any time between now and 200 years ago.
And the old timbers of this ship going nowhere ever again.
The men who were on this ship, did they survive?
The waters, they washed them ashore
Ashore
And they never will sail the seas no more
We led them along by the churchyard wall
And all in a row we buried them all
The song The Brean Lament
describes what happened quite commonly along this coast
when bodies were washed up.
You have to give them burial
but not in the main churchyard, in a separate sailors' graveyard.
lt was quite strongly believed along this stretch of coast
that the sea might decide to reclaim their bodies as its own.
They didn't want the dead of the village being taken at the same time.
Possibly as a way of appeasing the sea,
the sailors' boots were buried below the tide line,
so the sea would have something to take.
We led them along by the churchyard wall
Where all in a row we buried them all
But their boots
We buried below the tide
On Severnside
NlCK: The Severn Estuary used to pose a fearsome challenge
on any journey between England and Wales.
The two countries were divided by this massive tear in our coastline.
Avoiding it meant a diversion deep inland.
Even so, only hardy travellers would brave the deadly waters.
Today, a concrete solution spans this vast channel.
But conquering the Severn was a bold venture fraught with peril, as Mark is about to discover.
Dashing over the estuary from Wales to England,
commuters take the elegant crossings their lives depend on for granted.
But imagine a world before this bridge was possible -
a world without steel cables,
without reinforced concrete,
when the sea reigned supreme.
That was the challenge faced by the Victorians to cross the River Severn.
The formidable collision of river and sea facing the early engineers can still be experienced.
lt's one of the most dangerous seaways in the world, and l'm just a little bit excited.
The Severn Area Rescue Association is going to pit me against the ebb tide.
Cast off!
The power of the tide here is just extraordinary!
When the tide goes out, it's like a maelstrom.
The waters were an immense challenge, but by the1 840s, crossing the river by boat was old hat.
An irresistible new force was spreading across Britain - the railways.
Come hell or high water, estuaries weren't going to stand in the way of progress.
The great Victorian engineer lsambard Kingdom Brunel is a hero of mine.
He'd already managed to cross the Avon Gorge with a mighty suspension bridge.
When his railway came to Bristol, he wanted to cross into South Wales
and planned an even bigger suspension bridge.
Here are the preliminary sketches.
The biggest problem was the sheer scale of the span that Brunel required - over 1,000 feet.
He left a little note in his notebook which says, ''ls 1 ,1 00 feet practicable?''
Brunel's bridge was never built.
But if taking trains over the water defeated the best brain of the age,
how about going underneath?
A tunnel, was that the answer?
Digging deep to create a railway under the water, this was very bold, big thinking.
This is one of the original drawings of the tunnel from around 1 887,
and you can see how the track comes down
underneath the deepest part of the Bristol Channel here in The Shoots,
then gradually up to the Welsh side.
So what we've got here is around seven miles of railway track.
That passage under the estuary is now a vital link between England and Wales,
carrying over 250 trains a day.
Passengers are oblivious to a catastrophe that nearly sank the tunnel before the first train ran
and is a problem that still lurks below.
So here we go.
l've been granted access to a shadowy waterworld few get to see.
lt's great, we're just coming into the cutting, the portal's ahead
and we're about to go under the sea.
Ahh! Wahey! lsn't that fantastic?
We're heading for the deepest point in the tunnel.
Just 50 feet above us, millions of gallons of water are swirling around,
the river Severn and the sea are in full flow.
Keeping the water out here is hard enough,
but can you imagine if there was a flood down here?
With an estuary hanging over their heads, engineers knew there'd be seepage of seawater
but it was freshwater that nearly drowned the project.
Nobody expected this.
A raging torrent - they'd broken through to an underground spring.
ln October 1 879, water began to pour into the tunnel.
The workers fled for their lives.
The disaster struck when a shaft dug on the Welsh coast
cut into an underground river deep below the surface.
For four years, the engineers made desperate attempts to block the freshwater spring,
but every effort proved futile.
And it's been flooding in at this alarming rate ever since.
lf they couldn't stop the spring water, they'd have to live with it.
The only solution was pumps, massive ones like this,
that pump the water out as fast as it comes in, right up to the surface.
Leighton Jenkins helps keep the tracks dry today.
So what would happen if the pumps actually failed?
Every second counts.
As soon as the pumps stop, we'd have to inform the control within ten minutes
just to shut the tunnel itself,
and within 20 minutes, we've water coming up through the tracks.
Every second absolutely counts.
MARK: But have they yet failed?
No, not as far as l know, no. Not while l'm on the shift, anyway. (Laughs)
The railways had proved irresistible, with rival Victorian companies vying for routes.
By the time it was finished, the tunnel already had a competitor.
ln 1 879, trains had started to roll over the estuary.
But the bridge's sturdy uprights, always an obstacle to shipping,
would ultimately prove its downfall.
Do you see?
That's a tower where a railway bridge once crossed the Severn Estuary.
l've got a photograph that shows the stanchions marching across the river.
lt's now totally destroyed.
The raging waters where river and sea smash together
would deliver a fatal blow to the rail bridge.
ln October 1 960, the Arkendale, carrying oil,
and the Wastdale, laden with petrol,
were heading for a combustible collision.
The Arkendale was carried in by the surging tide.
That powerful current would drive it into the Wastdale on a foggy night at Sharpness Docks.
As Alan Hayward knows.
They were coming upriver, intending to come into the docks here,
but they were accidently swept past.
And then they collided and became, in effect, stuck together.
Disabled ships in thick fog,
carrying 600 tonnes of inflammable cargo at the mercy of a swirling sea,
propelled them to disaster.
They were desperate to separate from each other,
fighting by steering in different directions but it just didn't work,
and they only had about four minutes before they would reach the railway bridge.
The rail bridge across the Severn loomed out of the fog.
A collision with the ships carrying oil and petrol was now inevitable.
A lot of sparks would have been created which ignited the petrol in one of the vessels.
The fuel, of course, spilt out over the river, so the whole river became a mass of flame.
First mate Percy Simmonds was aboard one of the tankers.
His son Chris was 1 3 at the time.
CHRlS: l try to imagine that night and what he was going through.
lt must have been just terrible with the flames.
l'm just sure he was determined to make it across this river somehow
and make it back to us.
Daylight and a low tide revealed wrecks of the fuel tankers
smouldering on a sandbank.
Soon the first body was found.
They identified the body there.
They let Mum know...
..that, yeah, it was definitely Perce.
Chris's father Percy died along with four others on that terrible evening.
The damaged bridge was too expensive to repair, it was demolished.
But each day when the tide recedes, scars of tragedy are revealed.
CHRlS: Out there, of course, are two hulks buried now in the sands.
They've been washed over by countless tides, but they're still there.
They're there as monuments,
they're here as a reminder to all of us.
lt's immensely humbling to be next to such a vast body of brooding water,
even on a calm day like this.
One can feel the power where rivers and sea collide.
Britain's mighty estuaries have pushed engineers to breaking point and beyond.
ln Scotland, there's a salty graveyard of overreaching ambition - the Tay.
This immense river disgorges more water into the sea than the Thames and Severn combined.
ln the shadow of today's rail crossing is a spine of stumps -
the footings of the first Tay Bridge.
On a stormy night in December 1 879, it collapsed.
60 passengers died...
..and so did the myth of infallible Victorian engineers.
The blame was put on designer Thomas Bouch, lax maintenance and poor ironwork.
Hard lessons learned just down the coast on the Firth of Forth.
Following the disaster at the Tay Bridge,
that designer's plans for this crossing were thrown out.
The Forth Rail Bridge was beefed up into a massive cantilevered skeleton,
built not of iron, but steel.
This bridge is really a memorial, a masterpiece in steel
to the poor souls who lost their lives in the Tay Bridge tragedy.
Surging waters urge us on to fresh endeavours.
And we're not alone in finding creature comforts around the fringes of our great seaways.
The tide brings in the bounty that makes our estuaries brim with vitality.
Safe havens that offer boundless prospects.
Where rivers collide with the sea, our coast comes alive and opportunity awaits.