We're taking a city break.
Around our isles, the coast is ringed with cities.
From the smallest...
to the grandest -
their lifeblood's the sea.
A city can thrive on its coastal connections,
but it's a relationship that can bring both prosperity and pain.
Mark's on board to see how navigation
put one city at the centre of the world's sea charts.
This is the story of how London,
a city 50 miles from the open sea,
became the capital of global navigation.
And Tessa's going underground
to investigate a secret wartime threat.
A forgotten but extraordinary story how, in World War II,
we stopped the Nazis flooding London.
Ruth exposes an urban epidemic spread by the sea.
As the sailors flood into the city,
a silent danger looms.
And I'll be immersed in a tidal wave of trade...
..as I uncover the unsung port all our cities rely on.
Join us to put the sea back into the city.
This is Coast!
For centuries, the beating hearts of our coastal cities
were the docklands that fed them.
But today these urban landscapes have transformed
to suit changing times.
In Cardiff, redundant docks have been dammed
to create a freshwater lake -
a perfect pond for pleasure.
Bristol's great floating harbour is now a home of heritage.
And in Liverpool, apartments not ships
stack up along the waterfront.
So where has the trade that once filled these docks disappeared to?
I'm finding out what happens when the sea moves out of the city.
A journey that's brought me to the Humber,
to explore an unsung hero of our isles -
the mega port at Immingham.
Some seaports grow up slowly over centuries,
but Immingham sprang up as a super port,
practically a city in its own right.
More tonnes of cargo arrive here than at any other port in the UK.
Its towering skyline marks a dramatic new chapter
in the story of the sea and our cities.
Pedal power suits this mini city.
Immingham's so big it has its own road and rail network.
A bevy of workers beaver away to keep our coastal trade rolling
day and night.
Every cog, ship,
train and crane
dances to the tune of our country's needs.
Go back a century, though,
and this working landscape looked very different.
At the turn of the 20th century,
Immingham was a sleepy backwater.
How did it spring up into this city of enterprise?
Immingham was transformed into an industrial powerhouse
by mountains of coal.
In the early 1900s, coal was being mined at a ferocious rate -
black treasure sought across the globe.
To convey our great export,
super-sized ships were called on.
The Humber's urban ports at Grimsby and Hull
couldn't expand sufficiently.
So, in 1912, Immingham was purpose-built,
opened in pomp and circumstance
by the King himself.
100 years on, coal mines have closed
but Immingham is still awash with the black stuff.
Today, though, the traffic's going in reverse.
Surprisingly, coal is still king here,
but rather than shipping it out,
huge boats now bring coal in.
They're importing tonnes of it.
This might look like a minnow nudging a whale,
but these tiny tugs are incredibly powerful,
and they have to be, because some of the ships sailing into Immingham
weigh 200,000 tonnes.
Overseeing this mountainous operation
is port director John Fitzgerald.
How much coal's actually passing through this port now?
Well, this year we're on track for about 14 million tonnes.
I'm quite surprised because, you know, I'm not the only one
who thought that Britain was decarbonising
and heading towards renewable fuels,
and yet we're surrounded by...
literally, mountains of the black stuff.
Nearly 50% of all the electricity generated in the UK
is generated from coal,
and that, effectively, means that the coal here at Immingham
is powering well over three million homes.
As we struggle to end our dependence on fossil fuels,
Immingham helps keep our lights on.
Out there in the North Sea,
there's always another coal ship waiting to come in,
then another then another,
vessels queuing up to disgorge their cargoes
into this energy capital on England's east coast.
I'll embark on a 24-hour exploration of this vast site,
to see how the port's kept on call for cargo around the clock.
One advantage of using the sea as a highway...
..this city traffic flows without the traffic jams.
Deep-water city docks export British cars across the world.
But 300 years ago,
a maritime mystery made global sea trade much more difficult.
In the 18th century, sailors had no way of knowing
their precise position at sea.
How would cracking the question of navigation
put our capital on every mariner's map?
For centuries, London's made the most
of its narrow link with the sea.
Mark's discovering how the city became
the centre of the maritime world.
The most important location for any sailor at sea
is marked by a single line
drawn through a surprisingly urban setting -
London.
Or, more precisely, Greenwich.
This is the home of the Prime Meridian,
the reference line for every ship at sea.
But how did it end up here?
This is the story of how London,
a city 50 miles from the open sea,
became the capital of global navigation.
Our story starts some 300 years ago,
with a maritime mystery that perplexed sailors around the world.
It may sound unbelievable today,
but up until the 1700s, sailors could not determine
their exact position at sea -
it was all a matter of guesswork.
In sight of the coast, seafarers could navigate using landmarks.
But in the 18th century, global trade was growing.
For ships to cross oceans,
sailors needed a new way to work out their location.
To understand more, I'm meeting with historian Simon Schaffer.
Hi! Hi!
Simon, why in the 18th century
was there this sudden need to know where we were?
Up till then, most trade routes
and most military enterprises that mattered
had been local, they'd been European,
and that meant you could navigate along the coasts.
But now, from the late 1600s, early 1700s, Britain goes global,
big time, and that means
long-range voyages across oceans
where you could not stick to the coasts.
There were two places in particular that really mattered -
the West Indies, where the sugar trade was based,
and the East Indies and India,
where spices were imported.
Why where ocean voyages so difficult?
300 years ago, sailors could determine
how far north or south they were,
thanks to the sun.
This was their latitude.
But no-one knew how to measure how east or west they were -
longitude.
So, if solving longitude was so important,
why hadn't anyone come up with a solution?
Even the world's top scientists were stumped.
In desperation, the Government took a novel step.
In 1714, they threw the question open to the general public.
Men across the country brought their solutions to London
to be examined by a government committee like no other -
the Board of Longitude...
..a panel of the great and good,
Britain's brightest brains.
They offered a huge cash prize to anyone clever enough
to crack the riddle of longitude.
Unsurprisingly, hundreds of submissions flew in.
Inventive, beautiful but sadly harebrained schemes.
Decades passed with no practical solution.
The Board of Longitude became a national joke.
A very good example of this is this image made by William Hogarth
in the 1730s,
which is a picture of Bedlam -
the vast and sprawling London madhouse -
and Hogarth was put right in the middle of this picture
of a lunatic asylum -
a man scribbling, and the man is drawing a longitude scheme.
After years of head-scratching, what had the board come up with?
There was one concept they all agreed on.
The secret to determining longitude was time.
BELL TOLLS
As long as you know the time at a fixed point like Greenwich,
then you can work out where you are anywhere in the world.
Because the Earth rotates,
the sun rises in the east,
so midday will be earlier on this side of Greenwich
and later on this side.
These lines represent lines of longitude.
It takes two hours for the sun to travel
from one line to the next.
So, if it's noon at Greenwich,
it's...
2pm here...
and 10am here.
That time difference between local time
and Greenwich time
could be converted to distance from Greenwich.
Local time was easy -
when the sun was highest,
they knew it was noon.
But how could they keep track of time in Greenwich?
300 years ago, this was the only kind of clock sailors had.
Pendulum clocks just don't work on board boats.
They needed a new method to keep Greenwich time.
Fundamentally, there were two approaches that seemed viable.
One was carrying Greenwich time with you in a box -
that meant building a very reliable and accurate clock.
The other was observing the time of events in the heavens
using astronomy.
In 1764, the two leading methods
were trialled on a voyage to Barbados.
First up, the astronomical, or lunar, method.
This uses the clockwork nature of the sky at night
as a huge celestial timekeeper.
All you need is a sextant to measure
the angles between the moon and certain stars.
Once you've got your angle,
you can work out what time it is in Greenwich - easy.
But the maths back then, took over four hours.
Fiddly calculations aside,
the Barbados trial showed the lunar method
to be accurate to within one degree of longitude.
But that could still mean a ship was more than 60 miles adrift.
Surely there was another option?
Next up, a sea clock,
submitted by an unknown carpenter from Yorkshire, John Harrison.
The rocking motion of ships
interrupted the timekeeping of pendulum clocks,
but Harrison had spent years perfecting a new design.
Harrison's clock was like none other.
In fact, it wasn't a clock,
it was modelled on a watch.
Using springs instead of a pendulum,
the watch only lost 40 seconds
during the seven-week Barbados voyage.
At last, sailors could carry Greenwich time with them.
This incredible clock
had finally solved the problem of longitude.
The end, surely?
Harrison thought he'd cracked longitude -
the board, however, thought otherwise.
Harrison was the greatest clockmaker of the age,
and it had taken him years and years to build his marine watch.
That was ONE watch.
That scheme would only work if every mariner had a watch.
The board rewarded Harrison,
but refused to give him recognition for cracking longitude
until accurate sea-going clocks could be mass produced.
Harrison never lived to see it.
It took another 40 years for clocks to become commonplace on our seas.
In the meantime, sailors across the world
persevered with the lunar method.
And it was astronomical data
compiled here in Greenwich that became their bible,
with far-reaching consequences for the city.
The astronomers here in Greenwich
began to make books exactly like this.
This is the book called The Nautical Almanac.
On these pages, the distance of the moon from a list of stars
is tabulated with exquisite precision,
based on the longitude of Greenwich as zero.
Every mariner who used these tables
would be assuming that Greenwich was the origin of time,
and therefore the origin of longitude.
Those British books circulated around the globe.
All used Greenwich
as their reference point.
Most mariners acquired the habit of looking to London
to work out their location.
In 1884, a world conference
confirmed our capital's connection to the sea.
When it came to a vote, it was official -
Greenwich was sanctioned
as the Prime Meridian,
the line of zero degree longitude.
Greenwich remains the centre of the maritime world.
But where is the capital's once-thriving sea trade?
Out towards the edge of the Thames Estuary
a new mega port has been taking shape,
London Gateway.
Around our isles, we've constructed large out-of-town ports.
When every second counts,
how do they get goods off the sea
and into our cities?
I'm caught up in a hive of activity at Immingham.
Global sea trade demands that ships keep moving.
Delays cost companies and, ultimately, us.
The volume of traffic here is relentless.
Connecting everything are these roll-on/roll-off trailers.
A 24-hour operation, overseen by Mark Reeve.
We bring a vast amount of cargo in.
It can be anything from food, timber, steels,
chemicals, as well - import and export.
So a very wide and diversified cargo.
How long do you have to turn a ship around? To get it out again?
Something like the vessel that's come in from Esbjerg in Denmark today -
200-plus trailers in, 200-plus trailers out -
we can turn that around in six hours. That's shifting!
Yeah, that's going some. That's going some.
The container port is just one district
in this city-sized enterprise.
More fiddly cargo is delivered by the boatload
and moved by the tonne.
Heavy lifting - which presents me
with the opportunity to realise a boyhood ambition.
After ten years on Coast, I, Nick Crane,
finally get to do a story on cranes!
Crane driver Chris Jubb is showing me his elevated office.
Not a lot of space in here, is there, Chris?
No, they're only built for one, Nick.
What are you unloading here?
Today, we've got 13,000 tonnes of salt coming from Egypt.
Is that road salt? That's road salt, yeah.
What are the qualities you need to be a dockside crane-driver?
I've heard it likened to the same as an airline pilot
on takeoff and landing,
the concentration that's needed.
So how much salt can you lift up in one go?
It's 12 tonnes of salt in that grab. That's a lot.
On top of the world up here, aren't you, looking down on the port.
The pinnacle of a manual-grades career.
NICK LAUGHS
Chris makes it look easy,
but guiding this massive grabber into the precise position
to dump its load into the hopper
takes skill and experience.
How long to train me to do something like this?
You're looking at least a year.
Yeah, I can believe it.
I might not have a year,
but I can't come all this way without having a go.
Chris has a novel challenge in store for a novice like me.
Have I got the spatial awareness to drive a monster crane?
We're about to find out.
Well, there's an awful lot of controls in here.
The only two you'll be looking at today
are the basic controls -
which is the jib lever, for slewing left to right,
and your right-hand lever, which is for lifting the grab,
lowering the grab, opening the grab and closing the grab.
Shall I give it a go? Give it a go, yeah.
Nice and gently. Slow...slow. Ease the lever back gently.
You need a light touch as a crane-driver.
Them small movements are the key movements.
If you want to try and position yourself over that traffic cone there.
OK... We'll try and grab hold of the traffic cone.
Once you think you're somewhere over it,
just gently bring her down.
Now start looking out the window.
Am I going to knock it over now?
If I try and do a grab now... A bit lower maybe?
Oh! Now I've knocked it over. I'm doomed!
Oh, yes! Got it! Bingo.
Gently lift her up. Gently lift her up.
Well done. NICK LAUGHS
You're the first person I've ever taught to grab a cone before.
That's so difficult!
But could you do that for eight hours a day? I'd be drained.
I'd be absolutely drained.
I think I'll leave the heavy lifting to the professionals!
That was great fun. I've always wanted to drive a crane.
It's amazing what a hands-on business our sea trade still is.
Our great cities weren't only built on the back of trade.
On the south coast, major cities owed their existence to the Navy.
As the Royal fleet grew up,
so did a vast and varied population.
As they know in Portsmouth.
This is our oldest naval base.
In its docks, the city proudly displays ships that brought
victory to our isles.
But there's a less-celebrated story here, too.
In the 19th century,
naval ships and their sailors carried disease from overseas.
Ruth is exploring how the port put the city in peril.
# The captain's daughter, I suppose
# Could be called an English rose
# What would you think if I propose?
# The pox she gave to me a dose... #
It's February 1856,
and the Crimean War has just ended.
The Royal Navy is heading back into Portsmouth,
but as the sailors flood into the city,
a silent danger looms.
The Royal Navy was plagued with a sexually transmitted disease -
over a third of the men admitted to hospital had syphilis.
The statistics were staggering.
In one year alone,
the Royal Navy lost 77,000 working days
to sickness from syphilis.
The Government saw syphilis
as a threat to national security.
Our naval cities were on the front line of a war.
So, what exactly is syphilis?
The disease scandalised Victorian Britain,
and it still has the power to shock.
Medical historian Pauline Price has some disturbing images.
The primary symptoms would be hard lumps called chancres,
and then you would end up
perhaps coming onto this stage, this is the secondary syphilis,
and you've got a lot of pustules around the chest, on the face,
and that would make you feel very tired, very ill.
That might disappear after about six weeks,
and then you might have
a period of 5 to 20 years where it was latent -
hadn't gone away, but you had no symptoms.
And then you would start developing the tertiary symptoms,
and these are even worse,
and you can imagine the results of these is usually death.
So, what could people do to treat it?
They would have various forms of mercury treatment -
they might make it into pills,
a lotion.
Mercury's a really poisonous substance, isn't it?
The effects on the body are truly vile.
Well, that was the Victorian idea,
that you were purging your system, so if you salivated,
if you vomited, then you're getting out of the system
the things that are making you ill.
And if you did go through with the treatment,
would it actually cure the syphilis?
No, it wouldn't. They just didn't have the science.
So you used this because it was the best they had.
With no cure in sight,
the silent killer spread.
Our cities were soon a breeding ground for the pox.
This is a copy of a caricature from the early 1800s
showing a typical Portsmouth dockside scene -
lewd goings-on, sailors rollicking with prostitutes,
an inebriated woman being carried off.
And it was these loose women, rather than the sailors,
that were to be the focus of the Government crackdown.
As the Government sought ways to control the epidemic,
a shadowy underworld came under the spotlight.
I'm with historian Fern Riddell.
We know that in Portsmouth in this period
there were about 2,000 prostitutes working.
There's a real strong link, it seems,
between sailors and prostitutes, isn't there?
They'd be somewhere the men could come and stay,
they'd look after their money, give them a social life.
And it really was very much a companionable relationship.
Because there were no barracks for the sailors, at all, in town? No.
When they arrived off the ships there was nowhere for them to stay?
None at all. You wanted somewhere you could actually lay your head,
and some company different from the men that you'd stayed with
the entire time you were at sea.
# Get six of me, comrade To carry my coffin...#
As syphilis spread, popular songs made clear who was held to blame.
# ..Bad luck to the girl that gived him the pox... #
"Bad luck to the girl that gived him the pox."
The sailors blamed the prostitutes for their sickness,
and the Government did, too.
In July 1864, Parliament passed the first Contagious Diseases Act -
this is it here.
Its purpose? To control prostitution and venereal disease
in order to increase the efficiency of the Navy.
The Act introduced draconian new measures
to control sexually transmitted disease...
aimed squarely at women.
Under the Act, any woman who was suspected of being a prostitute
could be examined - forcibly -
in the most intimate way.
While sailors walked free,
women suspected of having the disease
were sent to aptly named lock hospitals.
The shame of the city,
these prison-like wards have long since disappeared.
But Fern can paint a picture.
They were really stark and depressing places to go,
and each woman was responsible for their own treatment.
So they'd go from having a horrible, really horrible, internal exam
when they were proved to have a contagious disease,
to then treating themselves,
to then being examined by a doctor
to see if the treatment was working.
And that was for their entire time
that they were incarcerated in the lock hospitals,
which could be from three to six to even nine months.
One woman was especially outraged by the law.
Josephine Butler was a Christian from a middle-class background.
Heedless of potential embarrassment,
she attacked the Contagious Diseases Act.
She toured the country to argue the prostitutes' cause.
I'm following in Josephine Butler's footsteps.
Despite the odds stacked against her,
Josephine set up a campaign group to repeal the Act targeted at women.
She took to the stage to voice her dissent.
"Women turn to prostitution out of starvation, hunger.
"Two pence is the price in England of a young girl's honour."
What were her main arguments?
Well, she has three main points.
So, firstly, she manages to inspire compassion for a class of women
that most people would have ignored.
Secondly, she exposes this horrific double standard
of the fact that you would treat women, but you wouldn't treat men.
And that, I think, really grabbed a lot of people's attention.
"We must protest against the purchase
"of physical health of soldiers,
"at the cost of introducing so much darkness and immortality."
This is what really grabs the whole community together -
the fact that she exposes the Government as really being
incredibly immoral.
And they're basically legalising and regulating prostitution,
which to the Victorians was absolutely unthinkable.
# Come, my good friends and a story I'll relate
# I spied a brave comrade all dressed in white flannel
# Dressed in white flannel and cruel was his fate... #
It was this final argument that struck home.
By regulating prostitution,
the Government appeared to be condoning it.
A groundswell of popular opinion
backed Josephine Butler's cause,
but it took 19 years' hard campaigning
until Josephine finally received a telegram
It's dated 1886, and simply read,
"Repeal received royal ascent."
No longer could a women be forcibly examined
or locked up in a hospital without her consent.
# There's a hole in his boots...
# Bad luck to the girl... #
In 1905, the real enemy to our cities was identified - not women,
not the sailors that visited them, but this -
Treponema pallidum, the spiral-shaped bacterium
responsible for syphilis.
But only after the Second World War,
when penicillin became widely available,
did the threat to our cities finally subside.
# I might have been cured
# By those pills of white mercury
# Now I'm a young man cut down in my prime... #
We're exploring the sea and the city.
A journey that's brought me to the Humber.
Just 13 miles from the port of Immingham,
a finger of land reaches into the sea.
This is Spurn Head.
At its tip is a control centre...
..directing cargo ships safely into Immingham's busy port.
Now, I'm following this flow of traffic,
and it holds some surprises.
This is a gateway for global commerce.
But it isn't only cargo that washes up here -
these huge ships are also homes.
Seafarers are forever in transit between destinations,
so ports like this become temporary cities for the stateless.
Yesterday, the Greek-owned Elena Ve,
with its Filipino crew arrived from Russia.
These globe-trotting ships spend months at sea.
Far from their own cities,
what do the crew do when they arrive on our shores?
I'm meeting Colum Kelly.
We're going on board the Elena which has come from Russia
bringing some coal.
Colum is Immingham's very own chaplain.
He helps throw an anchor to a restless flow of visitors.
As the Elena's vast hold gives up its cargo...
..the crew are getting a break in the mess room.
Hi, guys. ALL: Hey.
What's for dinner tonight?
Pork. Pork. Yes.
Who's been longest on this ship? Contract?
These guys. These guys? How many months?
About nine months. Nine months.
So you must miss your families dreadfully in that time?
Yes, that's what we do when we are in port.
We try to find internet access
just to have contact with our families.
Well, tonight's your lucky night
because I've brought you some internet access -
and the good news is, it's free!
ALL: Yay!
It's not often an internet connection gets a round of applause,
but this ship and its 20-strong Filipino crew
have been at sea for weeks.
Column's Wi-Fi is a long-awaited lifeline home.
This is my daughter. Your daughter?
Yes, sir. I miss her so much.
I bet. How old is she?
Er, eight years old, sir.
Well, you're a very lucky man. Thank you, sir.
Is it difficult for you to stay in touch with your family
when you're on the ship? Yes, of course, sir.
The longest voyage I have ever experienced in a vessel is 42 days.
42 days on one voyage? Yes.
Where were you going from? From America to China.
In port, they only get six hours off at a time.
Even so, the chaplain tries to give them a good taste of Britain.
You must have had some strange requests over the years?
There was an Indian crew, and I said,
"Well, do you want to go to the cinema, to a supermarket,
"or want to go into the city and do a big shopping?"
And the captain says,
"They'd like to be taken somewhere where they could walk on grass."
Oh. Heartbreaking.
How simple a request is that?
With the coal disgorged, this crew is ready to sail again.
But where in the world,
they won't know until word comes
from the owner in Athens.
For centuries, the coming and going of boats
has brought opportunities
for city folk...
..giving them freedom to travel the world.
To find a city that's been shaped
by that spirit of adventure,
there's nowhere better to look
than Liverpool.
This city not only transported goods across the sea,
but also people.
The quayside once bustled with liners,
offering Liverpool's youth a route to adventure.
Transatlantic trade has long dried up,
but the sea still inspires the young to run free.
Today, their routes are just a little bit more imaginative.
I'm Ryan Doyle, and this is my city - Liverpool.
Liverpool is just one of the coolest cities -
every major road runs to the coast.
It's like a city that's had half of it opened up to the ocean,
so you don't have that inner-city claustrophobia.
Free-running is expressing yourself through movement
so you can set yourself a destination
and try and get there as creatively as you can.
It's just exploring what the body's capable of.
The sea... It's in my blood.
My father was half Irish, my mother is half Irish.
I don't know why, just something about the Irish Sea.
I'm connected to it.
The Liverpool docks - it's a maze of architecture,
unexplored architecture, that we need to use to our advantage.
You know, you don't really get to know an environment
until you've jumped all over it!
A lot of these kids like to play Spider-Man
and play all the PlayStation games,
but I like to go out and actually physically be Spider-Man.
The River Mersey flowing right through Liverpool.
If we want, we can go on a boat and just hit the open seas
because a lot of free-runners
are always up for the adventure,
and there's nothing more that says
adventure than the open sea.
We're on a metropolitan maritime journey.
A story of profit...
and loss.
For centuries, the sea has provided for our cities.
But when the tide turns,
our greatest ally can become our worst enemy.
Head down our east coast,
and chunks of land have been eaten by ferocious waves.
We've witnessed the devastating impact of the sea
on small coastal settlements,
making it clear what catastrophe the water could wreak on our cities.
They must defend themselves from the sea in London.
People and property are protected by the Thames Barrier
and the city's stone embankments.
But go back 75 years, and there was a storm brewing in Europe.
Hitler wanted to bring the city to its knees.
Would the power of the tide be London's Achilles heel?
Tessa is uncovering a hidden chapter in the story
of the Second World War.
I've got with me a top-secret wartime document.
It says here, "Warning. This publication must not leave
"the custody of the person to whom it has been supplied,
"nor may it be taken or sent abroad
"without the authority of the Hydrographer of the Navy."
What's in this secret document?
London's tidal bible.
The surging tide raises the level of the Thames
by up to eight metres.
During the Second World War,
one man saw this twice daily deluge
as a threat to Britain itself.
The document has been written by this person, Peirson Frank,
and here he is,
London County Council's chief engineer.
Frank's fear of the high tide
sprang from bitter experience.
In 1928, the Thames had poured into the city,
a storm overwhelmed the Embankment,
the Underground flooded,
people in basement flats drowned.
Then Peirson Frank had been powerless.
12 years later, as the Blitz rained down,
Frank knew that if Hitler had the tidal bible,
accurately placed bombs
could demolish London's flood defences at high water,
unleashing the sea on the city.
London had nearly drowned by accident,
could Hitler now drown it by design?
If so, Peirson Frank knew where the Nazi bombers would strike.
The Embankment protects the heart of the city from the sea.
Archaeologist Gustav Milne has pieced together evidence
of a secret that's been kept for over 70 years.
This is the scar of a bomb
that struck here on 16th March 1941,
and the hole was very rapidly filled.
What makes you sure this is the site of a bomb?
Well, we know it's a bomb simply because all the granite facing
that was there is now spread out
over a great 18-metre arc,
right the way round here.
So, 70 years on, it's still lying on the riverbed?
Still on the riverbed.
A co-ordinated bomb attack on several targets at high tide
could have crippled the city.
Power and phone lines would be cut,
the water supply contaminated -
losing London might have meant losing the war.
Gustav has found scars from 122 bomb strikes
to London's flood wall.
Attacks that worried the Government so much
they took action to cover them up.
What I've got in here is the logbook of the snappily named
Thames Flood Prevention Emergency Repairs Service.
The team was led by Peirson Frank,
and it was so secret
no-one even knew it existed.
Gustav has spent years researching this covert team of workers.
If you look at this photograph,
it shows the team in action.
What you can see here is this great line of 5,000 sandbags,
and they are blocking up a gap in the riverside wall.
And why was this unit such a secret?
Because they didn't want to alert the Luftwaffe
to the extreme vulnerability of low-lying London,
and they didn't want to affect the morale of Londoners,
who had already been faced with being exploded to death,
so they didn't want to make them fear drowning, as well.
People's worst nightmare
was a flood underground,
below the Thames in the Tube.
During airstrikes, stations became makeshift bomb shelters.
But what if a tunnel was hit
and the river above poured in?
The horror was unimaginable,
but the fear very real.
How could those helpless underground be protected?
Thousands of people use this Tube every day,
but I wonder how many notice the remnants of a metal door.
When the air-raid siren sounded,
heavy steel floodgates slid into place...
..to seal off tunnels running under the Thames.
This is one of the actual flood-proof doors
fitted during the Blitz.
It's now out of use,
but during the war, every tunnel
that ran under the Thames had one of these.
If the tunnel behind was breached,
the Tube network SHOULD stay safe.
But at night, in the chaos of war,
it was hard to tell if bombs were falling in the river.
If a bomb hit the Thames and flooded one of those tunnels,
how would they know without literally opening the floodgates?
Once again, the city's engineers devised a remarkable counter measure.
Hydrophones.
Basically, underwater microphones -
and they were placed on the bed of the Thames,
close to every one of the Tube tunnels under the water.
LOUD EXPLOSION
It was an astonishing scheme.
Sound waves would detect where the bombs were landing.
So, how did it work?
Thames hydrographer John Dillon-Leetch can demonstrate.
What we have here is two hydrophones,
and if we take this target
and we say this might be a bomb during the Second World War
and we drop it into the water,
like so, the ripples there would represent sound waves.
And they would be timed from the time they're received here and here,
and by looking at the difference between the two times
we should be able to calculate
the approximate position of the bomb.
The wartime hydrophone signals were interpreted by skilled analysts
at South Kensington Tube station.
Test recordings show sound waves from a Thames tug boat.
ENGINE HUM, SPLASHING WATER
And even the sensitivity to a single rifle shot.
RIFLE SHOT
Then, on 9th September 1940,
the Luftwaffe loomed over London.
Sirens sounded.
SIRENS WAIL
The electronic ears under the Thames were made ready,
floodgates slammed shut.
Then analysts received the signal they'd been dreading.
Beneath the river, a section of the Northern Line had been hit.
Thanks to the hydrophones the flood gates were kept shut,
the water contained.
Families sheltering in the Tube escaped...none the wiser.
So, what of London's guardian angel, Peirson Frank?
It turns out his fear of Hitler using the tidal Thames
to destroy the city was well founded.
Just not in the way he had imagined.
Hitler didn't flood London,
but his air force did turn the tide against us.
Instead of hitting at high water,
on the night of 29th December 1940,
Hitler struck the city with incendiary bombs at low tide.
Firefighters struggled to get enough water from the Thames
to extinguish the blazes.
The city burned.
It was a tragic night for London,
but had Hitler turned the sea against the city,
then the story of the Blitz and even the outcome of the war
could have been very different.
We've been exploring the sea and the city.
An urban coastal landscape
continually transforming with the times.
Old city trade routes welcome
a flow of new ideas.
And in the mega port of Immingham,
a new opportunity awaits.
For 100 years, coal has been the kingpin of Immingham,
but this port is building its future on a new fuel.
Wow, this is absolutely enormous!
This soaring tower is being built to store a new generation of energy.
This is biomass - mainly wood pulp from sustainable forests.
Such biomass may provide up to a tenth of our electricity -
the pellets stored in these huge silos.
Our small isle would struggle to grow enough trees,
so we rely on the sea for imports.
Biomass could cement an exciting future for this port,
harnessing the coast
to the changing needs of our country.
Life around our shores continues to transform...
..but the precious links between sea and city endure.
For thousands of years, we've built settlements
along the edge of the sea -
the great provider,
the global highway.
Now, as ever before,
the coast lies at the centre of our national life.