Original subtitles

Narrator: Deep inside this mountain

Lies a labyrinth of tunnels.

How did its heavy-duty design help defeat the nazis?

Jackson: They would've bricked up the inside,

So there was no visible sign that anybody had been inside.

Narrator: 1,500 miles away,

A sinister disease...

Thousands of people were dying.

Narrator: ...Forces a city to build underground

In a race to save lives.

This is something to be proud of.

Narrator: And could one of the world's largest cave systems

Actually be a dragon's lair?

Kanduc: It is extremely rare.

This is the only place in the world where you can find it.

Narrator: Beneath our feet lie extraordinary chambers,

Caves, vaults, and tunnels...

The span and the size is just crazy.

Narrator: ...Some designed and built by humans,

Others formed over thousands of years,

But how were they created and adapted --

By who and why?

You've got to face your fears.

Narrator: Throughout history, subterranean life

Has captured our imagination.

We're going further and deeper

To unearth their mysteries, stories...

And secrets.

In gibraltar, along Spain's southern shore,

A limestone ridge dominates the coastline.

Hidden within plain sight

Lies a mysterious world --

A subterranean city carved out with precision.

There was rumors about this secret chamber.

Narrator: Beneath this distinctive landscape

Lies no less than 34 miles

Of hidden tunnels and caves.

It is almost impossible to fathom

What is actually inside the rock.

Narrator: Overlooking the straits

Between north africa and europe,

Gibraltar's role in world war ii is immeasurable.

Garcia: In November 1942,

The most important decisions were taken here in secret.

Narrator: How was this incredible feat achieved,

Why were mysterious bunkers critical to the war effort,

And what covert matters of national security

Have only recently been discovered?

Jackson: All the men who were sworn to secrecy,

They were brought up here not knowing where they were.

♪

Narrator: The rock of gibraltar --

This majestic entrance to the mediterranean

Has been key to britain's military strength since 1704.

Jackson: It's always been of key strategic importance to britain,

Sitting where it does in the mouth of the mediterranean.

You control the straits.

You control what's in and out.

It's a very important place.

Narrator: And since they arrived here several centuries ago,

British troops have excavated

Through gibraltar's limestone rock,

Joining existing caves

And excavating new tunnels and spaces.

They've created an underground fortress

Which controls access to the entire southern coast of europe.

Jackson: Gibraltar is a labyrinth of subterranean excavations,

Which stretches such a time period.

It stretches from the 1700s

To post-world war ii.

The system is fantastic.

Narrator: Yet the majority of subterranean excavation on gibraltar

Occurred during world war ii.

At the beginning of the war, in 1939,

There were approximately seven miles of tunnels

Inside the rock.

By the end of the war,

Troops had increased that to a staggering 34 miles.

Jackson: It's like coming to mecca for anybody

That's an enthusiast for subterranean.

We could almost call it a city underground.

Narrator: How was creating this underground system possible?

During world war ii, gibraltar was of particular importance

As britain's foothold on the european continent,

Which was succumbing to nazi forces.

Legend has it that as long as macaque monkeys

Remain on the rock,

It will stay under british control,

And during the war, winston churchill

Was so determined to hang onto this strategic position,

It is rumored he imported more just to keep their numbers up.

In addition to keeping up numbers

Of the good-luck macaques,

Thousands of troops were making the rock their residence,

And they needed food, equipment, and ammunition to do their jobs.

Since first moving to gibraltar in 1988,

Former british soldier pete jackson

Has become an expert on its military

And underground history.

I arrived in gibraltar not knowing at that time

What I was in for,

And viewing it from the outside, it is almost impossible

To follow what is actually inside the rock.

Narrator: And excavating this massive space for military needs

Was critical not just for housing

The additional 16,500 soldiers

But for building what could become

The ally's last defense in the war against the nazis.

Leading the charge was a military team

Called the royal engineers.

They were responsible for the strategic building needs

Required for combat,

And the needs of the rock were unique.

The royal engineers blasted 27 miles

Of carefully planned tunnels and underground spaces.

Jackson: The tunnels become far bigger for storage,

Brigade headquarters, for vehicle stores,

Generating stations, hospitals.

You can enter the tunnels in the dockyard at sea level

And make your way to the highest tunnels in gibraltar

Without putting your head above ground.

Narrator: To travel through the rock undetected,

A gigantic main road from the north to the south

Was excavated.

They named it after a familiar highway back in England.

♪

♪

Jackson: They used things that people would recognize.

You enter the great north road from its southern end

At maida vale,

We continue north within the tunnels,

We get to peterborough.

We're moving on from there within the tunnels,

We get to doncaster, and then on to durham.

So these are towns that lay theirselves along the line

Of the great north road in britain.

Narrator: Some of the stops along the great north road

Include engineering marvels

You wouldn't expect to see underground.

♪

Was turned into calpe hole generating station.

Narrator: This vast underground space generated vital electrical power

For the 34-mile-long military facility.

What we have here inside are three fullagar engines.

Engines like this can produce 1,035 kilowatts per engine.

Later on, a ruston industrial gas turbine

And jet engine was brought in

That produced a further 675 kilowatts.

This generating station is an unsupported chamber.

Narrator: What did it take to create these cavernous voids

In the rock and at what cost?

The largest explosion known to have taken place on gibraltar

Were 3,000 pounds of gelignite in one single blast.

I was lucky enough to show a gentleman round

Who was in charge of drilling the largest chamber

In gibraltar, what we call "reme chambers,"

And he said that even his most experienced miners

That were drilling it out said

They didn't know what the outcome would be

They said when they blasted it,

It was either gonna cut a chamber,

Or it was gonna cut the rock in half.

Narrator: The royal engineers did successfully

Cut through the rock.

They even created a space deep within this tunnel system

That was a national security secret

Hidden from the world for 50 years.

Narrator: Gibraltar --

Known to the world as "the rock,"

But it holds many secrets.

Inside is a labyrinth of tunnels,

And hidden within lies a secret room

That changed the course of history.

During world war ii,

Future us president general dwight d. Eisenhower

Set up his headquarters

In this humble space during operation torch.

♪

This was the first mass involvement of u.S. Troops

In the european north african theater of war.

From deep inside gibraltar, eisenhower

Coordinated the successful invasion of french north africa

To reduce pressure on allied forces in europe and egypt.

In November 1942, dwight eisenhower spent 12 days

In one of the offices here implementing operation torch.

It was very important because the most important decisions

Were taken here in secret,

So a lot of intelligence gathering was done,

And the top brass was briefed

Of what the next move should've been.

Narrator: According to eisenhower's autobiography,

He was not impressed with the lack of fresh air

And natural light.

While it couldn't offer creature comforts,

Eisenhower's command office was protected from enemy bombs

By rock in every direction.

We are literally in the middle of gibraltar --

North, south, east, west --

So we are in the heart of gibraltar.

It was a tin-roof construction, so in world war ii,

This must have looked very, very different,

Must have been quite dismal at the time.

Narrator: However, this hidden space in the heart of the rock

Is perfect for a new modern-day purpose.

Now functioning as a data center,

It keeps servers safe

In a climate-controlled environment.

Luis garcia works for a technology company

That benefits from the security of this space

Once occupied by the famous u.S. General.

This was refurbished in the 1980s.

Walking in, obviously,

You cannot but notice that you are underground,

But once you're in here, it looks like a normal office.

It's just that we haven't got any windows.

Narrator: And while the rock held many subterranean secrets

During world war ii,

There is one that was almost lost to history.

♪

Gibraltar's best-kept secret

Is located on the southernmost tip of the rock

Behind rusting metal and debris.

Jackson: For years, in gibraltar, there was rumors

About this secret chamber inside the rock

That was locally called "stay behind cave."

People were often out looking for it...

...But in 1997, members of the gibraltar caving group

Were in this tunnel.

They stopped just here to have a break,

And they felt wind coming through the corrugated iron.

Narrator: What exactly had these cavers stumbled upon?

They moved further down and took another couple of panels away,

And they found the way that the men would've been

Put into the chamber.

Narrator: Concealed inside was a secret cave.

What was the purpose of this hidden cell?

It was part of an elaborate counterintelligence ploy.

If the nazis captured gibraltar, this lair would allow

Allied soldiers to survive within to spy on the germans.

The bold plan was to seal six naval personnel

Within the chamber.

Jackson: They would've bricked up the inside,

And people on the outside would just see

All the corrugated iron, so there was no visible sign

That anybody had been inside.

Narrator: The original plan was to hide the courageous men

In here for one year,

But provisions and equipment

Were supplied to last seven years.

Known as "tracer cave,"

It was built to last with two toilets

And two ventilation systems operated by hand

Like a kind of exercise bike.

Jackson: It was a static bicycle, and instead of having a chain,

It's got a leather band so it wouldn't make a noise.

The men would need to stay in good physical condition.

To keep the noise down, on the floor,

There are cork tiles to deaden the sound.

Narrator: The men could charge batteries and stay active

While remaining completely silent.

This secret cave has tunnels that lead to outlook posts

On either side of the rock,

One pointing east to the mediterranean

And the other west out to the bay

And to the atlantic ocean beyond.

The original plan for the apertures was for them to be

30 centimeters wide and 15 centimeters high.

When they got to the east side --

This is part natural cave --

There's a small natural platform on the outside,

And the men could actually climb outside.

They're shielded from view from the germans anywhere

Because there was a lot of foliage around it,

So they could actually get outside,

Get some sunlight,

And monitor the shipping from there.

But on the western side, it was very exposed,

Very close to where there was a path

That the germans would be patrolling.

The slot itself is only about 15 centimeters long

And two centimeters high.

It had a concrete wedge, which normally would be slid

In there with a tiny pinprick hole.

They'd look through there,

Check that there were no germans in the area,

And then they'd carefully remove the wedge.

Once they gathered the information,

That information would need to be transmitted back to london,

But they couldn't have an aerial

Sticking out of the side of the rock

Because it would've given away the position.

Narrator: The immense risk of detection

Forced them to come up with an ingenious solution --

A retractable, flexible antenna.

During the day, it would've been pulled into that pipe there,

And then at night, they would compress the information down,

So you could send a lot of information

In a very short time.

Narrator: Luckily for the troops on gibraltar

And the allied forces, the rock was not overtaken,

And operation tracer was never put into action.

The incredible efforts and ingenuity of military personnel

In constructing this vast 34-mile-long subterranean city

Capable of housing up to 16,500 soldiers

For up to 16 months were instrumental in helping

Allied forces defeat the nazis in world war ii.

♪

In central europe,

A secret is buried underground,

Concealed for millions of years.

People are coming from all over the world.

Narrator: Slovenia is rich with subterranean wonders...

...And one discovery is fit for a king.

Glazar: I found paradise.

Narrator: Are mysteries still hidden belowground?

Narrator: And what mythical creature has been found thriving

In this underground labyrinth?

They're actually top of the food chain.

Narrator: Slovenia -- a stunning mountainous country

With 90% of its land

Towering 650 feet above sea level,

Yet beneath this limestone terrain lies a secret underworld

With 11,000 caves hidden below.

♪

In 1818, something unexpected was discovered.

Shortly after the napoleonic wars,

The region was unstable but rebuilding.

In preparation for a royal visit from the emperor of austria,

A local worker named luka cec unearthed a natural treasure.

♪

Narrator: Luka cec had discovered what is, to this day,

One of the largest cave systems in the world --

The postojna caves.

At 15 miles long, it was created by the pivka river,

Which has eroded down through the limestone rock

To create this stunning complex

Of caverns, halls, and passages.

Postojnska jama was actually formed

In a lot of limestone layers

Approximately 80 to 90 million years ago.

Narrator: These caves began forming

When dinosaurs still roamed the earth

And may still be home to ancient creatures

That exist nowhere else on the planet.

The biggest cavern is known as the great mountain.

The room is 330 feet long,

130 feet wide, and 100 feet tall.

This massive space is home to enormous cave formations.

Glazar: Stalactites and stalagmites are formed

By rainwater dripping from the ceiling.

This water will dissolve and transport

The limestone to the inside of the cave,

And the limestone will solidify into various shapes.

Narrator: These incredible formations have been here

Since long before man ever walked this earth

And could be home to a mysterious creature

That's lived here since the time of the dinosaurs.

Some of these limestone layers are 90 million years old.

Narrator: Luka cec, the 19th century worker

Who first discovered these caves,

Found something that not only impressed the austrian emperor

But attracts tourists from around the world.

The popularity of the postojna caves

Brought further excavation, investment, and innovation.

In fact, on may 25th, 1884,

The caves became just the third place in the world

To have permanent electrical wiring.

In 1901, the electricity supply in this underground world

Was more extensive than the city of london's.

Major renovation of the wiring after world war I

Saw the cave's dance hall decorated with this

Stunning murano-glass chandelier from Italy.

Electricity also made possible a motorized railway in 1923,

Creating the world's first cave train.

Because sending postcards from exotic locations

Was so popular at the time,

This tourist hot spot even had its own post office.

The magic of these caves drew people from across the globe.

Narrator: One of the biggest attractions was and still is

A stunning, extremely rare example

Of a massive, pure limestone stalagmite.

It stands an impressive 16 feet tall

And is simply called "brilliant."

The name of the formation is "the brilliant"

Because it's so pure white.

It's made of limestone -- pure limestone, no impurities.

Narrator: The discovery of postojna's caves was hugely significant

And not just for tourism.

Biologists have discovered a legendary monster here,

A creature that could actually help us fight

One of our deadliest diseases.

Narrator: Deep beneath the slovenian landscape

Are a staggering 11,000 caves

Home to magnificent structures and surprising creatures.

To this day, the search for secrets hidden in the depths

Of the postojna caves' underground world continues.

Exploring beyond the known limits of the cave system

Are divers like igor vrhovec

Who continue to venture into the unknown.

Narrator: For postojna's cave divers,

Each new expedition is another step into uncharted territories.

Narrator: Diving into the unknown is dangerous and time-consuming.

Narrator: It's dark with poor visibility.

For those whose life's work is dedicated to cave exploration,

There's a real sense of purpose to push further and deeper.

There is so much more to discover.

Narrator: And within the deepest, darkest parts of the caves,

Something was discovered --

A mysterious creature

That inspired fear in the explorers

Who first found them.

The first people that found it,

They actually thought it was baby dragons.

This is the biggest cave animal in the world.

They're actually top of the food chain.

Narrator: Known as an olm,

They can live up to a staggering 100 years

And have completely adapted to living in the cave system.

The olm's mutations are so surprising,

Charles darwin featured them

In his 1859 groundbreaking publication,

"on the origin of species."

Kanduc: They have very good sense of hearing and of smell,

So that's easier to find their prey,

And they prey on all the other cave animals

That we can find here.

♪

Here you can see the head.

If you look closely, you're gonna see the heart,

Which is basically located in the throat.

Behind its head, they look a bit like red ears.

These are the external gills.

Narrator: Given the scientific name proteus anguinus,

They aren't actually baby dragons

As the first explorers feared.

They are hunters, but this is as large as they ever grow.

They don't have eyes.

They hunt by using a combination of taste and smell

As well as vibrations in the water.

A new type of organ has also been analyzed on the olm's head,

Which suggests they can even register electric fields,

Helping them to detect other organisms.

The whole darker part of the body,

These are the intestines.

If you take a closer look at the front legs,

You're gonna see it has only three fingers.

On the hind legs, it only has two fingers.

Narrator: These creatures never know when their next meal will come

And have the ability to slow their metabolic rate,

Allowing them to survive for up to 10 years without feeding.

It is extremely rare to see cave salamanders

Actually eating in nature,

So you have to consider yourself lucky to get such a good shot.

Narrator: And in 2016, something unprecedented was witnessed

In this remote cave system.

Something very special happened in this aquarium.

The mama actually started to lay eggs, yeah.

She laid 64 eggs.

Twenty-one of the younglings hatched,

And this is actually an event

That has never before been seen in nature.

We were very lucky that it happened in this aquarium.

Narrator: Given their natural habitat,

Olms are extremely sensitive to light.

Together with their slow metabolic rate

And genetic makeup, they continue to reveal

Surprising characteristics to researchers.

They may be immune to cancerous cells, so they can't get cancer,

And this is the only place in the world

Where you can find them,

So there's still much research to be done on this subject.

I mean, it's amazing to comprehend

That in such an environment that anything would actually survive.

♪

Narrator: Far from slovenia, beneath london,

A subterranean world is being exposed to the light of day.

Giant holes are being bored out deep under the capital.

They're connecting to a vast, historic underground system

Rarely visible to the city dwellers above.

Fenn: This is the cathedral.

Narrator: This mysterious masterpiece

Has secretly saved millions of lives.

How was such a complex network

Constructed with victorian technology?

This was radical.

Narrator: What secrets did the victorians give us,

And why are they responsible for the london population

And economy that we know today,

And what can be done to support this international city

Into the 22nd century?

Narrator: Deep below london lies a historic marvel.

Until now, this victorian-era labyrinth

Has been kept a secret from the people above,

But what hides beneath was designed to save lives

And made way for an international city

Of epic proportions.

♪

It was in about 1831

That the bacterium cholera arrived from india.

Lots of people drew their drinking water from the thames,

But their sewage was flowing out of their houses

Into the thames by their tributaries.

They were drinking their own excrement.

Narrator: As london grew, so did the problem.

Bazalgette: Thousands of people were dying

In these outbreaks of cholera.

It actually would've got worse and worse and worse

Had there not been a cleansing of the city

And a citywide solution.

Narrator: In order to understand the inspiration

For the revolutionary engineering

That ultimately saved london,

You have to travel a few hours away to the city of liverpool,

Where in the 1840s, civil engineer james newlands

Made it his mission to clean up this port town.

If we were to think about liverpool in the 1830s,

It was filthy.

You had butchers slaughtering animals in the street,

Fishmongers throwing their guts around

And hoping for a heavy rain

To maybe wash them down into the mersey.

There was a sanitary report, and it highlighted liverpool

As being particularly filthy.

Narrator: At the time,

Liverpool was a key industrial hub for England,

But its reputation for squalor and disease

Affected its trade as a port.

The ability to solve this problem

Would be the only thing that could save the city.

In 1847,

James newlands was appointed the first borough engineer.

From this, it went from the basest form of filth

And disgusting environment

To really the height of modern sanitary thinking.

James newlands turned out to be, in some ways,

The sanitary savior of liverpool, didn't he?

Yes, he definitely did.

When he arrived in liverpool and was appointed,

He targeted a lot of specific issues

Such as street lime, pavements,

Communal bathhouses,

Which all played the small part

Of ultimately cleaning up liverpool

In conjunction with a major sewage system,

Which he created, the first of its kind.

Narrator: Construction of the sewage system began in 1848,

And by 1851, 17 miles of sewers had been built.

This increased to a staggering 300 miles by 1869.

Fenn: The construction of the world's first integrated

Sewer network completely changes the health

And the outlook for liverpool.

Lynch: Before systems like this were built,

The average life expectancy in liverpool was 19.

However, over the 10-year period

It took to construct,

The life expectancy doubled.

This was a standard design in the victorian era,

And it was built using an open-cut method,

So this would've been exposed,

Built by the various trades and laborers.

Then eventually, once it's complete,

Then they backfill the ground above it.

We've got bricks on the bottom

And then a stone arching over the top.

Yes.

And then that comes onto a brick channel on the bottom

That's carrying the effluence around.

Yeah, keeps the solids in the middle

Where the velocity is the greater.

The actual gradient is 1 in 113,

So that actually gives it the potential to self-cleanse,

So the floors will be enough

To ensure that it's kept as clean as you can see.

Even today, at peak flows,

This sewer can handle more than 7 million gallons per hour,

Enough to fill an olympic-sized swimming pool in 3 1/2 minutes.

This is the first time anyone had contemplated

Having an integrated sewer system,

Of course, anywhere in the world.

Like, this was radical.

It was a system

That many other people followed after,

And ultimately, it was built for a reason.

It achieved that reason, and it's still standing,

So it is quite magnificent.

The remarkable thing about liverpool's sewers

Is that we can still see

Parts of that original victorian brick network

Integrated into modern concrete systems.

Narrator: Liverpool's pioneering subterranean sewer project

Had transformed the lives of its people

And sustained the city to grow into the future.

Its success wasn't going unnoticed,

But could it help save the capital city and for how long?

This was a model that was then going to be picked up

By other towns and cities, most notably across london,

To manage this new, modern sanitary revolution.

It was designed to last.

It was designed to allow the city to grow.

Narrator: And it was james newlands' design

That would allow london resident joseph bazalgette

To solve his city's 1860s cholera epidemic

And support the city's people and future economic growth.

This is crossness sewage pumping station

On the south side of the river thames in london.

It's where bazalgette's mammoth subterranean system culminates.

Fenn: This is a cathedral,

A cathedral to municipal engineering.

We're at the end of bazalgette's major sewage improvement works

That he'd started in 1856 across london.

So here we have a map of london

Showing the bazalgette sewer network all in place.

So firstly, in the north of london,

There are three main sewers.

They all meet at abbey mills pumping station at the top.

Then in south london, we have three more,

And they come flying down, takes the waste

And puts it out to crossness,

Where it then hits the most tidal reaches of the thames

And can hold the waste and release it as the tide goes out.

Crossness was the southern guardian of that sewer network.

It was a matter of huge pride, and at the opening,

We saw prince albert, and lords and ladies came here,

So it was something to boast about.

It wasn't going to just be a anonymous building

Hidden away in the marshes.

We're surrounded by the cast-iron tracery,

And then all around here, we've got beam engines,

Beam engines with pumps that are being used to move

The effluent that's been taken through sewers.

Narrator: The pumps were used to lift outgoing sewage

Up from the sewers deep underground

And into vast holding pens.

Then at each high tide, 75,000 tons of waste would be

Released with the estuary tide out towards the north sea.

It wasn't a sufficient long-term plan,

But it was a significant improvement

Over the existing system

And proved that keeping the people healthy

Allowed the city and its economy to grow.

This is a massive piece of pumping work.

Everything here is just solid.

It's just gorgeous!

This is something to be proud of.

Narrator: Sir peter bazalgette

Is joseph's great-great-grandson.

Peter, hello!

So, colin, here is our cathedral of sewage.

Bazalgette was hardly talked about,

Hardly known compared to, say, brunel

Because most of his achievements are underground,

And you can't see them.

There was an article in one of the newspapers,

I think it was "the observer,"

That called him "the drain brain,"

And then, having read that,

I realized quite how profound his achievements were.

London, at the time, had a population

Of between 2 and 2 1/2 million,

And when bazalgette finally got the money

To execute the scheme,

He overspecs the whole scheme

For a population of about 4 1/2 million.

What joseph bazalgette did was save a city.

Narrator: Yet, in a major international metropolis,

This victorian-era technology isn't enough

To support london for long.

What could engineers do to save the city

From its dirtiest problems

And transform it into the major financial hub that it is today?

Narrator: The future of london and its economy

Is at the mercy of a victorian-era sewer network.

Although it is an engineering marvel,

It can't keep up with the city's modern needs.

Could the dirtiest problems be its downfall?

29 vertical shafts like this one

In hammersmith west london

Have been excavated to link up to

A brand-new 19-mile underground urban wastewater system.

This $5.25 billion subterranean network

Will run directly beneath the river thames

Serving 13 boroughs and the city of london

Because some of its basest needs aren't being met.

Sewer expert colin fenn is meeting john corcoran

Of the thames tideway project.

John, I've come down this huge shaft,

Which is so many steps deep.

How far belowground are we here?

Corcoran: Well, we're 35 meters deep

And so 12 1/2 meter downward to shaft.

How on earth did you construct such a shaft like this?

Well, we dig out a meter.

Then we spray a lining.

We spray concrete.

Then we continue, do another meter,

Spray that lining and carry on down.

Our main tunnel is 300 meters that way

Or will be ultimately.

We've completed this connection tunnel,

So this is 300 meters long,

And later on this year, underneath the river thames,

The larger main tunnel with a tunnel-boring machine

Will come through,

Eat off the side of our tunnel, just the very end of it.

While the larger main tunnel boring machine spans

Over 30 feet,

The smaller feeder tunnels like this

Need their own agile tunneling equipment.

This is the specialist excavator we use,

Call it a schaeff excavator.

It runs on electric.

The two big features of this

Which make it good for tunneling --

One is the special knuckle joint on the arm here,

Which means you can dig that tunnel profile,

And the second one is all the spoil

Comes into this conveyor system,

So the excavator driver doesn't have to keep rotating,

And the guys can excavate 1 meter in an hour.

We actually have four or five dumper trucks.

This machine keeps them busy, basically.

They keep loading it,

And they take it away as quick as possible,

And they're taken away by surface mud wagons

And used for lime reclamation.

You know, this tunnel has been designed to last 120 years,

And we want to absolutely minimize

The amount of maintenance

Because it's very, very difficult

To go into a sewage system once it's operational.

Currently, london has a problem, and it isn't sustainable.

When there's heavy rainfall,

And the storm-drain network fills with water,

It floods the sewer system,

Causing raw sewage to escape into the thames.

Corcoran: Pumping station here is activated, on average,

Once a week, so over the course of one year,

That's 2 million tons of sewage

Going into the river, which we will intercept.

So any fish in the river, anyone canoeing,

Anyone dipping their hand in the thames

Is likely to have a bit of a surprise

50 days of the year.

I've talked to people locally around hammersmith,

And they know when the sewage is coming out

Because it kills the fish.

We're gonna do something about this

And have this fantastic project.

Within a few years' time, we'll capture all that sewage,

And we'll have a much better, cleaner river thames.

And those are tons of sewage that will take a toll

On the people of london, their health and, ultimately,

Its economy if they can't get it under control

Because what the victorians originally taught us

When they created

Their revolutionary underground network

Is that keeping your people healthy

Allows a population to grow and an economy to flourish.

London now has almost 9 million people

And is one of the greatest cities in the world,

All thanks to victorian ingenuity.

Bazalgette: We need today to understand,

As much as they did in the victorian era,

That great public works are a legacy.

Fenn: This is something

That people couldn't have imagined years ago,

The technologies, the way that it's all come together,

Corcoran: And I'm looking forward to that day

When we start seeing wildlife coming into the river,

You know, and who knows?

We could see dolphins coming up and down the thames,

And wouldn't that be fantastic?

This infrastructure has been designed to last

Not just our generation,

Not just our children's generations

But for generations beyond that.