Unearthed

The great wall of China.

The longest, most expensive fortification ever built.

No matter what height you climb,

how good your eyes are,

no matter how good your binoculars are,

you can't see it all.

It really is a geographical wonder.

It's recognized all over the world,

but its construction is still shrouded in mystery.

Every time I come to a place like this on the great wall

where it stretches for miles and miles,

I'm just blown away every single time.

What did it take to build it?

Who was it built to keep out?

Although they were few in number,

they were the most feared warriors in the world.

The only way to find out is to blow this wall apart.

Dismantling its defenses,

and pulling apart its towers brick

by brick will help us

unearth the astonishing engineering secrets

at the heart of this ancient wonder.

captions paid for by discovery communications

the great wall of China.

A vast fortification that, in total,

stretches over 13,000 miles across the continent.

This immense mega-structure was a defensive shield

for the world's most populous nation.

It stretches from snow-covered

deserts all the way to the yellow sea

and was the largest construction project

the world had ever seen.

This wall is the greatest man- made structure on our planet.

Over a billion tough clay bricks helped it

survive for centuries.

The wall rises about 23 feet high

with tall battlements to shield defenders...

And built-in arrow loops for shooting at invaders.

This monumental brick structure is thousands of miles

long and stretches over 16 feet wide...

...with tens of thousands of watch towers standing guard.

For centuries, it was China's first line of defense,

but the construction of this iconic

brick wall wasn't the first time that China's emperors

had built a barricade across the country.

Their predecessors erected primitive earth walls

over 1,500 years before the famous

brick wall was even started.

The big mystery... what drove China's rulers

to build such mammoth structures?

Quite beautiful mountains, these,

and making the scenery more magnificent

is the wall right here.

This was the ancient frontier.

William lindesay is one of the world's

foremost great wall experts.

He has walked the wall's length

to investigate its many secrets.

The thousands of great wall miles

under William's belt

have given him a unique understanding

of its history.

This way is China.

This way is Mongolia, the nomadic lands.

The Chinese had a good deal. They could farm.

They could produce a lot of food,

more than they needed.

People in the north, bad deal.

It's impossible to farm there.

The growing season is too short.

The nomads lived to the north of China

and moved their herds with the seasons.

They raided China on horseback to get a hold of the goods

and foods they couldn't produce themselves.

The drought or a terrible winter

could destroy everything the people had,

and then they had a survival challenge.

Where to go for relief.

They didn't go north to Siberia.

They naturally came south and trespassed on Chinese land.

The nomads were naturally a crack army.

From childhood, they're riding horses...

even without saddles and stirrups.

This kind of schooling made the nomads

the most feared army in the world.

To make things worse for the Chinese,

the nomads weren't just excellent horsemen.

They armed themselves with an incredibly

sophisticated weapon...

the composite bow.

Yang fuxi is one of the world's best bowmakers.

For ten generations, his family has practiced this ancient,

noble art, which was the key

to making the nomad horde so devastating.

Master Yung's task is to create a bow

that stores enough energy to kill a man.

His first step is to carve out and glue

together the bow's wooden core.

But wood on its own doesn't have the flex

and strength to propel an arrow to a lethal speed.

The secret that stops the wooden bow

from snapping is its clever construction.

Master Yung glues a layer of tough

buffalo horn onto the bow

to make the wooden core stronger.

To give the bow a bigger punch,

he glues tendons from a bull's neck onto the other side.

Upon release, the arrow flies away

at an astounding 186 miles per hour.

How could the Chinese defend thousands of miles of territory

against these vicious nomad attacks?

In order to protect the ancient Chinese

from invading Mongolians,

an elaborate barrier thousands of miles

long was constructed.

Beneath the brick we see today

lie the remains of an earlier earthen wall.

Layers of compacted soil created a rock-hard barricade.

Thin Reed mats separated the layers

for stability and drainage.

These multi-layered earth walls rose up nearly 20 feet tall.

Satellite imagery reveals that these first walls

stretched from the yellow sea

all the way to the desert of western China.

But how did the Chinese build a wall like this

in a barren desert

with no obvious building materials around?

Every time I come to a place like this on the great wall

where it stretches for miles and miles,

I'm just blown away every single time.

Great wall explorer Richard fairbrother

is in China's western desert.

He's searching for secrets of the earliest great wall,

which wasn't made of bricks, but out of soil.

It's amazing to think about the number of people

involved and the amount of labor

it took to build something like this.

To investigate how the walls

have stood up to these harsh desert winds...

...Richard is going to reconstruct part of the wall.

Builder Mr. Yung and his team

are using original methods for this project.

The finished structure is known as a rammed earth wall.

You build these walls by layers of soil,

and then layers of these reeds,

and you put them on top there,

and then spread them around.

One another advantage of these reeds

is that they provide a drainage mechanism for the wall.

So the rain comes down through the soil,

and then is passed out by the structure of these reeds.

Even with reeds for drainage, violent storms

and scouring winds eventually wear everything

to dust in this unforgiving landscape.

So, how are parts of the wall

over 2,000 years old still standing?

Lab tests reveal that the desert soil known as "less"

has a special property thanks in part

to the crushed shells of long-dead snails.

Less is rich in calcium carbonate,

and when that's exposed to water,

it gives the wall or the soil a self-cementing characteristic.

The ancient builders

unknowingly made use of this clever chemistry

to build a wall that stood for millennia.

The purpose of the ramming

is to compress the layers together and...

...give it that tremendous strength

that these rammed-earth walls are known for.

If you have to do it all day long in the blazing desert sun,

I think you'd be pretty knackered by the end.

A team of 80 men could build over a half-mile of wall

in just 7 days.

It towered almost 20 feet tall

and kept enemies out of China for centuries.

And the only way through was to climb

the wall under a hail of lethal arrows.

By the 16th century, things had changed.

China's enemies had more ferocious weapons.

And the old walls were lacking.

They already had a lot of wall, so what was the next option?

More wall? Better wall?

That was the question to be answered.

The only option was to undergo

the most ambitious construction project

the world had ever seen.

The original great wall of China

was built from mud over 2,000 years ago...

And constructed to defend the Chinese empire

against nomad attack.

But over the centuries, as warfare developed,

this earth wall became a less fearsome obstacle.

Military advanced forced the Chinese

to strengthen their line of defense.

Historian William lindesay is walking the great wall

at jiankou in the mountains north of Beijing.

He's trying to understand what prompted the Chinese

to upgrade this vast defensive barrier.

Just 50 miles over there is Beijing.

That was not a very safe position.