Original subtitles

We are surrounded

by extraordinary feats of engineering

constantly pushing the boundaries of what's possible.

Without engineering, there'd be no modern world.

Gigantic cities, amazing infrastructure,

and ingenious inventions.

Engineering is the key to turn dreams into reality.

To reach these dizzying heights,

today's technology relies on breakthroughs

made by ancient engineers.

It's mind-boggling how they did this.

How did early civilizations

build on such a scale?

They raised the bar for construction in a way

that no one thought possible.

The sheer engineering ability, which is a self-impressive.

By defying the known laws of physics

and daring to dream big.

They constructed wonders of the world from gigantic pyramids

to awe-inspiring temples and mighty fortresses.

All with the simplest of tools.

Can you imagine the skills people would have needed

to build like this now?

Now, it's possible to unearth the secrets

of the first engineers.

They managed to construct edifices

that has survived the ravages of time.

And reveal how their genius

laid the foundations for everything we build today.

(dramatic music)

One type of engineering has shaped the world

for at least 12,000 years.

Brick by brick.

The way we live has been defined by the wall.

Human civilization is a product of walls.

Fundamental to everything we build,

the genius of the wall is often overlooked.

Concept of a wall is simple.

It's a boundary, it's a barrier.

Without walls there'd be no houses,

skyscrapers or churches.

Even the world's most pioneering construction

relies on its walls.

Today, architects and engineers

are designing new types of wall

needed to build next generation cities.

But the planet's greatest wall was built in ancient China.

When this early civilization took wall building

to epic proportions.

The great wall of China

is one of the most incredible engineering achievements.

2,000 years in the making,

the great wall was built

with the blood and toil of millions.

The workers paid a terrible price.

It was hell on earth.

Its purpose: to protect the nation

against barbarian enemies.

This was the stone dragon, the chilling symbol of a nation

that refused to compromise.

But how did the ancient Chinese build a wall

that spanned deserts, rivers and even mountain peaks?

They built a wall on a knife edge

on the top of a mountain.

It looks incredibly powerful and impressive.

It's just amazing.

Could there be secrets

waiting to be discovered within its very structure?

People began to wonder if there was something awful

hidden in the walls.

And did the great wall ultimately caused

the downfall of the dynasty it was designed to protect?

But this engineering masterpiece

was not the first wall ever constructed.

It came after centuries of architectural breakthroughs,

that began with the dawn of civilization.

20,000 years ago, the world was locked into an Ice Age.

At this time, humans still had no need for walls or houses,

because they were always on the move, struggling to survive.

In the Ice Age, life was really hard.

You've got extreme weather conditions,

but you're moving around that landscape

trying to make the most of some really scarce resources.

So you're taking everything you own with you,

you're trying to create shelter wherever you go.

All you have is basically your wits to protect yourself

and the others around you to help protect yourself.

Gradually thousands of years,

the earth began to warm up.

The ice sheets slowly retreated.

The world blossomed.

In an area of the Middle East known as the Fertile Crescent,

there were rivers, natural springs, rich soil

and an abundance of plants and animals.

For the hunter-gatherers who lived here,

this was now a world of plenty.

It becomes far easier for people to survive.

You can stop and stay where you are.

Life is less brutal.

The fundamental nature of human existence

was about to change dramatically.

After tens of thousands of years on the move,

people started to farm the land.

And these farmers needed permanent homes.

So in places like the Fertile Crescent,

we start to see the beginnings

of what we really understand as urban life.

In place of rough shelters,

the first fixed structures appeared,

and even these early buildings

would reveal some incredible engineering genius.

At the center of this revolution was Jericho

in modern day Palestine.

Archeologists long believed a muddy hill was that remained

of the Stone Age settlement.

But when they started digging this site,

they made an astounding discovery:

the remains of circular houses with solid walls.

They'd found the world's first city.

Incredibly, 10,000 years ago, these Stone Age peoples

had already engineered something

that would become the building block of global civilization.

Bricks!

(brick crashing)

There are two key ingredients for brick-making

and the Middle East has each in abundance:

mud and sunshine.

So bricks are essentially made from clay.

Clay is a very fine-grained soil

that you can find pretty much all over the world.

So it's a very easy material to locate and to work with.

The ancients molded clay

into rectangular blocks and left them in the sun.

Drying made them stronger

and they could then be pushed out in little units.

And with this engineering leap, the world changed forever.

Bricks were here to stay.

These individual blocks

were glued together with mortar,

making the first known walls.

When you're building with a brick,

what you're essentially doing

is taking the earth beneath your feet,

finding a way of turning that into a wall.

Humanity's 10,000 year love affair with a brick

is evident across the globe.

From the first handmade mud varieties

to the mass produced examples of today.

Down the centuries, bricks have hardly changed.

I think bricks are absolutely amazing.

And the reason they're much the same

over the history of building

is that they're all broadly the same size,

and for a really good reason.

The reason that brick is the size that it is,

is it fits in your hand.

The world today produces

an estimated 1.5 trillion bricks a year.

Instead of some dried, modern bricks are hardened in kilns

at over 1,800 degrees Fahrenheit.

Once fired, they become as tough as rock.

Bricks keep homes cool in summer, warm in winter

and to some they're even beautiful.

I love bricks!

I mean, what isn't there to love about bricks?

I love seeing a brick building.

There's something romantic about it.

The brick is a timeless technology.

But the people that built the first brick walled homes

had a shotgun store.

Their new shelters could actually make them more vulnerable.

As archeologists further explored the ruins of Jericho,

they made another discovery.

Inventing the brick was just the beginning.

The thing about stopping in one place

and expanding all the things you can do,

like agriculture, keeping cattle,

starting your first crops, that makes you prosperous.

And if you're prosperous, you need to defend that stuff

from the people that wants to take it from you.

So early settlers

then made a second great engineering leap:

creating a massive defensive wall surrounding their town.

Jericho became the first known walled settlement

anywhere in the world.

Its final wall was over 11 feet tall,

visible for miles around,

a statement of power and a warning to anyone

who wanted to attack.

What the wall does is not just make us safe and secure,

but it provides the symbolism of that,

because the wall defines this as home,

or this is my place, and that is outside my home.

So human civilization is literally a product of the wall.

With Jericho, the defensive wall had arrived.

In the next few centuries, great walled cities

sprang up across the region,

their defenses becoming ever bigger, ever more complex.

Damascus, sitting at the crossroads of Africa and Asia.

Uruk, ancient city of Mesopotamia

and at the time largest in the world.

Babylon, the sinful city of the Bible,

famed for its huge walls, hanging gardens

and mythical tower of Babel.

And Jerusalem, the Holy City,

lying just 19 miles east of Jericho.

Methods may have changed,

but the construction of defensive walls in this region

continues to the present day.

Standing between Jerusalem and the ancient side of Jericho,

a very modern defensive wall:

the Israeli West Bank wall.

The Israeli government call it a security barrier.

It divides their population from the Palestinians.

Walls are to a certain extent a testament of fear,

because they are about building something

to stop someone or something from getting in.

Cutting through 440 miles of undulating land,

the West Bank wall is a feat of modern engineering,

like it or love it.

It's made of concrete slabs up to 26 feet tall,

bristling with watchtowers and cameras.

At other points, giant electric fences

supplemented by patrol roads, razor wire, and trenches.

Modern walls can be built fast,

but for thousands of years the defensive walls purpose

has remained the same: keeping other people out.

Just a few hundred miles away from this modern border

lies the home of a very different civilization.

One that began its own period of wall building

and construction 5,000 years ago.

(dramatic music)

Ancient Egypt.

When it came to building walls,

the Eguptians were brilliant.

Egyptian civilization took building in stone

to another level:

jaw dropping palaces, soaring pyramids,

and temples featuring beautiful decorated columns.

When it came to that temples,

in which gods were worshiped,

the Egyptians tended to surround these temples,

which were built from stone,

with thick huge walls of mud brick.

Solid walls fortified the temples,

defining the land within as sacred

and protecting the complex.

But in some cases,

there was something highly unusual about them.

Bizarre undulating forms,

no other Egyptian walls looked like this.

So was this pragmatic engineering

or did it have a deeper purpose?

It's one of the great mysteries of ancient Egypt.

In search of answers, archeologists and engineers

make detailed structural examinations.

Finding no engineering explanation

for the undulating design, historians had a brainwave.

And there is the school of thought

that this is meant to replicate the waters of chaos.

Ancient Egyptians believed

that the world was born from the sea.

And archeologists now think

the wavy walls reflect that myth.

But could there be another reason why curvy walls

appealed to the engineers of Egypt?

Neuroscientists at the University of Toronto

may have come up with an explanation.

After testing volunteers in a brain imaging machine.

Participants were shown pictures of internal architecture,

both curvy and straight edged.

Their brain scans revealed a fascinating truth.

So, as humans, when we're viewing structures

that have very sharp lines or sharp edges or corners,

it almost conveys threat.

Whereas curved lines, soft edges

much more comfortable and welcoming.

This insight could help explain

why most defensive walls aren't curvy.

So defensive walls are there

to not only physically keep people out

but also to look intimidating and domineering.

This may also explain

why many modern architects are creating buildings

with curved walls.

The Guggenheim in Bilbao

with its dazzling curves of titanium, limestone and glass

creating surfaces that ripple in the change in light.

The Metropol Parasol, largest wooden structure in the world,

like a giant mushroom springing up in the heart of Seville.

And the National Museum of Qatar

with its interlocking discs

inspired by the desert rose crystal.

Across the globe, a new generation of iconic buildings,

abandoning the tyranny of straight edges.

Their designs too mathematically complex

to draw on paper,

created instead with cutting edge 3D technology.

What the computer enables you to do

is to model not just the design, not just the building,

but also how you're going to construct it.

Working at the limits of the possible,

this is high risk engineering.

If you get it wrong,

it's not just that these buildings will look bad,

it's that they simply won't stand up.

These mind-bending modern structures

are changing the language of architecture.

but they're follow in the footsteps

of an earlier architectural revolution.

One that 2,000 years ago to wall building

to a whole new level and set out to build the impossible.

(dramatic music)

Ancient Rome.

Roman roads, bridges, monuments and public buildings

were constructed on an unprecedented scale.

And one leader would push their engineering prowess

to the very limit, creating one of the most extraordinary

defensive walls in history.

The emperor Hadrian.

Hadrian, above all, was a wall builder.

He was someone who wanted to set the boundaries

of the Roman empire,

to divide what was Roman from what was not Roman.

Hadrian came to power in 117 AD,

inheriting the Roman empire at its height.

It's stretched from the sands of the Sahara desert

in North Africa, to the rivers of Mesopotamia in Iraq

and northward to the Rhine and Danube in central Europe.

But on the remote fringe of this vast empire

laid one troublesome province: ancient Britain.

For the average Roman infantryman,

the prospect of traveling to Britain

would have been absolutely terrifying.

The tribes of ancient Britain

were not taking occupation lightly.

One of the defensive tactics was to create displays

of hysteria, supposedly, to really play up

the role of the barbarian,

and the Roman soldiers were petrified.

One part of the country

proved particularly nerve wracking: Caledonia,

modern day Scotland.

Faced with unruly locals

and the inhospitable Caledonian landscape,

Hadrian came up with a unique solution:

a gigantic wall like no other.

Hadrian decided to build what we now know

as Hadrian's wall,

which was to be a boundary, but, but also a checkpoint,

a place where the Romans could control movement

between what was their territory

and what was controlled by the tribes north of the wall.

His plan would require a massive defensive wall

studded with forts and towers.

It would have to be manned by thousands of troops,

built across mountains, valleys and rivers

running the width of Britain, dividing the land.

The wall would begin at the banks of the river Tyne

near the North Sea and continue all the way

to the Solway Firth on the Irish Sea,

cutting across the country for over 70 miles.

For the Roman occupiers far from home,

it was an incredibly ambitious goal.

Anyone who's visited Cumbria or Northern England

will note it is a spectacular landscape

but not the easiest to traverse.

Engineers would have to deal

with fast flowing rivers, hard rock

and mile upon mile of rolling hills.

But where would they find thousands of laborers?

The hard graft of the wall building

was all done by Roman soldiers from three different legions.

30,000 men were available

for wall building duties.

The Roman army spent very much more of its time building

and making and putting infrastructure in place.

In the army itself, we don't just have skilled fighters,

but also engineers, surveyors, doctors,

chefs, people who made trumpets,

people who looked after the animals.

It was a whole community.

Construction began in 122 AD.

Soon, men from every legion were hard at work on the wall.

Over 75,000 cubic yards of stone was torn from the ground

and taken where needed.

Stonemasons dressed each stone with hammers and chisels.

They were then set in place

by the hands of the legionnaires.

Two columns were built and the space between them

filled with a core of rubble, earth and mortar,

and for added strength Roman engineers had invented a way

to set their mortar rock hard.

What you do is you take a limestone

and you roast it at a temperature

anything between 900 to 12,000 degrees centigrade.

This drives out all the moisture, creating quicklime.

And what you can do with quicklime

is you grind it into a powder, you can slake it with water,

and then what you had in essence is a form of concrete.

The Romans used this super tough concrete

to lock stones together.

And it's still in evidence in some parts of the wall

nearly 2,000 years later.

(sheep bleating)

(gentle music)

And the wall today holds other secrets,

carved into its fabric.

We know that Roman soldiers built the wall,

because they left their signatures on some of the stones.

We have the names of the legions who were stationed there.

Sometimes we have the emblem

for the legion as well, their identity.

But how did the Romans ensure

their mighty fortification

would succeed in its intended job?

Only monumental engineering could hold

the so-called barbarians.

The main stone wall stood almost 10 feet wide

and nearly 20 feet high.

Small forts built into the walls a Roman mile apart

housed 32 soldiers.

And two stone lookout turrets were placed between each fort.

Any sign of trouble

and reinforcements could be deployed rapidly along the wall.

14 much larger forts were also built next to the wall.

A purpose was to fortify the wall.

Soldiers would be stationed in these forts

to keep control over who was passing through the wall

to keep watch at who might be approaching.

And so that they could send signals

to assemble a field army if necessary.

Hadrian's wall was a huge statement of intent.

To the local tribes, this would have been

a powerful expression of Roman dominance.

This was a structure which sent the message:

We're here to stay.

This is something alien.

This is something brutal.

And this is something that's been designed to control us.

But massive walls weren't the only deterrent

in Rome's arsenal.

The occupiers also engineered small-scale defensive weapons

that were surprisingly effective.

And in the soil beneath the imposing battlements,

archeologists discovered a piece of Roman engineering

so simple, yet so devastating

that it's design is still in use today.

The caltrop scattered on the ground.

Its sharpened edges can lame a soldier or a horse.

Modern equivalents are now used

to take on horsepower of a different nature.

Deployed by armies and police forces to disable vehicles.

(helicopter buzzing)

For the Roman military forces,

the ability to stop the enemy was equally important.

Thanks to their incredible manpower

and engineering prowess,

Hadrian's wall was completed in 128 AD.

It had taken just six years to construct.

Hadrian's wall served as the Northwest frontier

of the Roman empire for 300 years.

At its peak, more than 10,000 men lived

and worked here as border guards.

Today the wall is a world heritage site,

recognized as a landmark in military engineering.

It's rumored that Hadrian's masterpiece

was inspired by word of an even bigger defensive wall.

One being built in a mysterious kingdom

thousands of miles away from ancient Rome.

China.

Today at the forefront of engineering,

over the last 30 years China has transformed itself,

rebuilding infrastructure on a massive scale.

Modern day China is a huge engineering project.

60% of the population now live in urban areas,

including six of the world's mega cities.

The Three Gorges Dam is the world's largest

and possibly the greatest single engineering project

of the 20th century.

This manmade mountain took wall building

to record breaking heights.

Standing nearly 600 feet tall and 1.4 miles long,

it required 988 million cubic feet of concrete

and 463,000 tons of steel,

enough to build 63 Eiffel towers.

Other gigantic engineering projects

include the fastest supercomputer, highest bridge,

and the world's largest high-speed rail network.

China is a civilization on the move.

They find out how to remove the barriers.

They don't let things hold them back.

They will advance and keep developing

and keep pushing the boundaries.

Groundbreaking engineering

is in the country's DNA.

China first became a unified nation 2,000 years ago

and almost immediately embarked

on the legendary engineering project.

One that defines the nation to this day.

The Great wall of China, the most astonishing fortification

ever seen on earth.

It's so long.

The guards at the eastern end of the wall

would witness the sunrise one hour and 20 minutes

before guards at the western end.

This gigantic stone dragon is an iconic image

recognized through the world,

and even from space.

The great wall of China is undoubtedly

one of the great sights of the whole world.

As a work of architecture, it's extraordinary

and a work of engineering, it's extraordinary.

The story of wall begins in 221 BC.

China was a newly unified country and under attack.

To the north of China were a nomadic group of people

known as the Xiongnu.

The so called barbarians lived in the mountains

and the plains of what is today Mongolia.

They regularly crossed into China, raiding farms

taking food and goods by force.

(riders yelling)

These nomadic warriors work famous for their use

of bow and arrow and also for their horsemanship.

So they were renowned for their lightning fast attacks.

It would have been absolutely terrifying

being attacked by them.

With a huge border to defend

and unable to mount a military response,

emperor Qin came up with an audacious plan:

to keep the enemy out with a wall,

a wall thousands of miles long

to be built across extreme terrain, deserts, high mountains,

plunging valleys and rivers.

Only a madman would contemplate such a feat of engineering.

But the emperor Qin had the resources of an empire

at his disposal.

He was one of the most powerful autocrats in history.

He was the most ruthless of the most ruthless.

(dramatic music)

Nothing would stand in the way

of the emperor's dream of the biggest wall of all time.

Especially, when it came to assembling a huge workforce.

Well, the most forced labor, ordinary people,

were taken from the heart of China,

and criminals also sent off to the great wall.

More than 200 crimes were punishable

with laboring on the wall.

And if someone died during their sentence,

one of their family members had to take their place.

For the next 10 years,

more than 3 million soldiers, peasants,

prisoners, and laborers were forced to build the wall.

They started in the barren lands of the Gobi desert.

Here, there was little stone to be found

for hundreds of miles.

So engineers had to find

a different material for construction.

For centuries Chinese farmers have been building walls

out of rammed earth to protect their homes.

Working in large groups, they create a wooden frame

and then fill the space with a mixture of earth and reeds.

They then ram the earth down using tools or bare feet.

Engineers call this downward force compression.

Pressure packs the earth together, making it very strong.

Workers then added another layer and continued the process,

building ever higher until the wall stood 20 feet tall.

The finished earth and structures were so tough,

that section still stand in the desert today

more than 2,000 years later.

Now, if you came galloping in across the Gobi,

you would have seen a wall

such as no one had seen the history of Asia,

and perhaps in human history.

Beyond the desert sections,

walls erected by farmers already stood

along some of the route.

So the Qin emperor, he took these sections of old wall

which already existed, and he built new sections as well

to create a single unified barrier.

As the wall snaked on through valleys

and across mountains,

Qin's engineers used whatever materials

they could get their hands on: stone, earth or wood.

The Chinese people paid a terrible price

for their emperor's vision.

It's been estimated that more than 400,000 died of disease,

malnutrition or exhaustion.

The wall stretch from Lintao

into deserts of Northwest China,

to the banks of the Yellow River

and onto the steps of inner Mongolia,

ending in the Northeastern region of Liaodong.

A truly formidable defense.

Emperor Qin's wall was known as the Long Wall.

It wasn't finished in his lifetime.

It would take hundreds of years and a twist of history

for it to become the Great Wall we know today.

Emperor Qin died in 210 BC.

In life he ruled with an iron fist.

And it turned out he planned to do the same in death.

In 1974, workers unearthed

another incredible construction project:

thousands of life-sized warriors made of clay

ready to fight for the emperor in the afterlife.

The terracotta army.

Like most walls, emperor Qin's Long Wall

was built to keep people out.

Throughout history walls have been built

as a defining point to say: on this side, we are civilized,

but on that side, you are not.

But perhaps the most famous modern wall

was created to keep people in.

The Berlin wall.

It's purpose to stop East Germans escaping to the West.

In the early hours of August the 13th, 1961,

East German construction workers

backed up by soldiers and police

began erecting barriers throughout Berlin,

dividing the city street by street.

What began as barbed wire and fences

evolved into a sophisticated reinforced concrete wall

with watchtowers, kill zones and minefields.

Guards were ordered to shoot anybody who tried to escape.

For 28 years, the wall stood

as a symbol of the Cold War, dividing Europe.

Until the 9th of November, 1989,

when thousands of demonstrators marched on the checkpoints,

forcing overwhelmed guards to open the gates.

Rejoicing crowds poured into West Berlin.

Revelers danced on top of the wall and demolished it.

The years of separation were over.

The Berlin wall had fallen.

A reminder that throughout history regimes come and go.

And in the year 1368,

a new regime was taking power in China, the Ming dynasty.

And they would push Qin's architectural legacy

to the next level.

China really changed under the Ming dynasty.

The Ming were a modern, outward looking dynasty

sending trade missions abroad and selling their silk,

brocade and porcelain.

This trade brought them great wealth.

Before they tackled the Great Wall,

the Ming first chose to stamp their identity on China

with another extraordinary construction project:

the Forbidden City, the emperor's new palace.

10 years in the planning and built

with some of the most ingenious engineering techniques

of all time.

It was a palace unlike any other

that had ever been built in China.

And it was a real statement

that the man living in the middle of this palace,

the emperor, was the son of heaven.

But like many cities around the world,

the Forbidden City was built on a geological fault line.

Earthquakes are a really devastating force

that gets applied to our structures.

It's a large force.

It's chaotic, that often very unpredictable.

And while modern engineers seek ways

to defend their buildings against earthquakes,

it seems they have much to learn

from their ancient Chinese equivalents.

The Forbidden City has been hit by over 200 quakes

and survived them all.

Present day architects wanted to learn how.

Researchers examined the wooden buildings

and discovered the answer lay with their complex carpentry.

Wooden beams and columns formed the structural core

of all the forbidden cities buildings,

and between their crossbeams and roofs are ornate brackets.

They're made of interwoven beams and blocks.

None of them fixed.

They support the eaves and roof,

and sit on a large beam supported by tall columns.

But the columns are free standing.

All these structural parts bear the roof's immense weight,

but nothing is fixed glued or nailed.

So when quake strikes, all parts are free to move.

They figured out how to allow the buildings

to rock in an earthquake without breaking.

We're now fully understanding how that works.

The forbidden city was built

to survive natural disasters,

but there's another secret hiding within its walls.

The flashy show palace doubles up as a fortress

defended by soldiers.

33 feet high defensive walls, a moat, and watchtowers.

Walled cities may no longer have a role in the modern world,

but even today the most important buildings

are still designed with security in mind.

One of the most recent is the U.S. embassy in London.

It's been designed to look open, transparent

and be a symbol of equality.

$1 billion buys a lot of subtlety.

It looks lightweight.

It looks very soft and almost delicate.

But don't be fooled.

The glass is blast proof.

The interior is solid concrete.

And the embassy boasts all the latest defense systems,

including a 100 foot blast zone, defensive ditch,

hidden steel and concrete bollards, even a moat.

Inside, there's a barracks for soldiers,

anti surveillance technology, and a fully staffed hospital.

The pursuit of security would also lead the Ming dynasty

to a groundbreaking engineering solution.

One that would go down in history

as among the most amazing feats of construction of all time.

Although the Ming dynasty was looking outward

as much as possible,

there was still a problem with the Northern border.

In the 15th century,

China's old barbarian enemies

were threatening the nation once again.

And sections of the emperor Qin's ancient walls

were now crumbling.

The Ming dynasty decided to rebuild the wall

to firm up that boundary between China and the North.

The Ming aimed to create a huge new wall

thousands of miles long.

It would be manned by guards

and dotted with towers and fortresses.

This time their engineers would use

the same basic building block,

first created thousands of years earlier:

in the wall cities of the ancient Middle East.

The wall was built out of bricks

and they created these enormous kilns

to create those many, many thousands of bricks.

But no brick is effective

without a good mortar.

And for this, the Chinese had a secret weapon.

So I've read that they use sticky rice in there,

which is not something you would see

on a materials list today.

And the reason we believe that that was done

is because the starch and the sticky rice

gave the mortar a little bit of flexibility,

so that when the wall was experiencing

the temperature variations it did,

it just allowed that little bit of flex,

so that you wouldn't get any major cracking.

This mixture of clay, lime and rice

was so strong, it's still bonding many of the bricks today.

The wall was an epic production line of quarrying,

stone cutting, brick making and construction.

And it was brutal.

Some sections ran across mountain ranges,

angles as steep as 70 degrees,

before plunging down into deep valleys.

It's incredible.

You can hardly walk or climb up there.

You'd need ropes to hang on.

And yet they've built a wall

on a knife edge on the top of a mountain.

So the people who did this, they were incredibly brave.

And it's an amazing piece of engineering.

Solutions to these physical challenges

came from an expert in an entirely different field of study.

Liu Hui was one of the greatest mathematicians

of ancient China.

He wrote a book that solved complex engineering problems,

such as how to calculate the depth of a ravine,

the width of a river or the height of a building.

Using these instructions,

an army of engineers plotted the course

and construction of the Great Wall

and one simple but effective bit of Chinese engineering

helped get their equipment up the steep and narrow paths.

It is believed the Chinese invented the wheelbarrow

somewhere around 200 AD, and it remains in use today,

a staple of the modern building site.

But other aspects of wall construction

were far removed from the modern world.

The Ming weren't big on health and safety.

The building work would have been incredibly difficult.

We know that they suffered badly from lack of clothes.

And we know they also suffered badly from a lack of food.

And yet, the wall still managed to be built.

Thousands of people died building the Ming wall

and the horror of the work gave rise to superstitions

that persist to this day.

It's no surprise that myths and legends of grownups

suggesting that the ball may have actually been packed

with the bodies of dead laborers.

Some stories, even claim

the famous white rice mortar

was actually made from the bones of workers.

Generations lived and died on the wall.

It was a building project that took 200 years to complete.

So the Wall still stands today

as a magnificent engineering achievement

but there's a shadow here,

because it could never have been constructed

without the deaths of thousands of Chinese people.

The world's greatest defensive wall

begins in the East, on the edge of the Gobi desert

at Jiayu Pass.

From here it runs through mountains and across rivers

to the Shanhai Pass and the coast of the Yellow Sea.

When finished, it was patrolled by a huge army

occupying 25,000 watchtowers and garrisons.

If attacked, they send smoke signals by day

or fire by night.

And the wall would become a highway

bringing reinforcements into battle.

But despite this extraordinary engineering triumph,

the Ming dynasty fell in 1644.

The cost of building and maintaining the wall

contributed to an economic crisis.

The rebel army of peasants angered by poverty and starvation

stormed the capital.

Historians believe that the wall was actually crucial

to the collapse of the Ming dynasty.

So expensive was it to build and then to maintain

and to man with soldiers,

that it led to financial and strategic collapse.

Shortly afterwards,

an army of Mongols from the north invaded

and took over China.

The very people the wall had been built to keep out.

It may not ultimately have succeeded,

but the Great Wall of China is a landmark in human history,

almost incomprehensible in scale.

But it was never one wall:

different emperors added to it across 2,000 years.

And in fact, it's precise length

has been something of a mystery until now.

After a five-year survey measuring

all known sections of the wall,

the Chinese government announced that the official length

of the Great Wall of China is just over 13,170 miles.

The Great Wall of China stands as testament

to those extraordinary engineers

but also the people who had the vision

of creating a wall to defend an entire empire.

(dramatic music)

The need for protection

is as old as civilization itself.

Defensive walls first appeared 10,000 years ago,

built around the earliest cities.

The Romans went on to supersize them.

And they reached their absolute peak

with the Great Wall of China.

Many still stand today,

their construction involved thousands of workers,

overcoming enormous challenges,

learning to build ever more complex structures,

ancient engineers paving the way for the modern world.

(dramatic music)