Original subtitles

In Asia, in the heart of the biggest continent on the planet,

traversing an entire country from east to west,

the Great Wall of China.

Constructed on rocky ridges at an altitude of more than 1000 metres,

this remarkable structure is the largest ever built by man

in terms of length, mass and surface area.

It's majestic, it is imposing.

It's the largest man-made structure on the planet.

It is incredible.

This marvel of the ancient world

was built from nearly 4 billion bricks.

To the 16 million tourists who visit each year,

only a short, 80-kilometre section near Beijing is well known.

But, when observed by satellite, we discover that this wall,

also known as 'the dragon',

crosses no fewer than 15 Chinese provinces.

It would take two years, walking from one end of the wall to another.

So that just gives you an idea of the scale.

It's the endless wall, the wall without end.

While the human eye can't detect

the full extent of this incredible construction,

the latest technology allows experts

to calculate the true length of the wall

by detecting hidden portions of it.

Now, intact walls dating back 2000 years have been discovered,

and new technology is exposing a construction technique

that shows the genius of ancient builders.

By examining the wall

from a satellite scale down to a molecular level,

today's experts seek to dissect the techniques of yesterday's builders.

They aim to unravel the mysteries of this jewel of humanity,

and use the newest science to find the truth behind its construction.

It became virtually impenetrable, even to cannonballs.

Thanks to the latest technology,

we can lift the veil on the extraordinary construction methods

of this UNESCO World Heritage site

by scanning the walls, surveying its depth, and taking to the sky.

What revolutionary techniques

allowed the Chinese to construct thousands of kilometres of wall

in under 70 years, and on such steep terrain?

And what secret ingredient, recently revealed by science,

explains its unbelievable longevity?

A brick is a brick, or so I thought,

but, no, these bricks are exceptional.

History, science, construction.

Examined like never before,

the Great Wall of China reveals its secrets...

on every scale.

The Great Wall of China's world fame owes a lot to its unique form.

Snaking through the mountains of northern China,

this incredible structure was built to protect the Chinese Middle Kingdom

from Mongolian attacks.

But what visitors can't see at first glance,

is the true extent of this engineering feat

constructed over 2000 years.

It is the longest man-made monument on Earth.

And it is colossal.

It almost transcends nature,

because it crosses very inhospitable terrain,

it goes across deserts, over mountains, to the sea.

For more than a century,

archaeologists and explorers have travelled across most of Asia,

following the wall by foot

in order to chart this jewel of humanity.

Now, experts can use the latest technology, like spatial imaging,

to observe the extent of this Chinese monument,

and visualise it like never before, on a satellite scale.

It crosses the wild plains of the western Gobi desert,

snakes through the northern mountains,

and finally plunges into the Yellow Sea in north-east China.

The wall is so vast that at its eastern end,

the sun rises a full one hour and twenty minutes later

than at its western extremity.

The length of this wall was long calculated

using observations taken by the naked eye.

In 2012, it was estimated at 6700 kilometres long,

a little more than the length of China.

But now, thanks to new technology like drones and radar systems,

on a satellite scale,

archaeologists have the means to explore a new hypothesis.

That the wall's true length is much greater.

Today, investigators think that many parts of the Great Wall of China

are invisible to the naked eye,

that sections have become hidden underground

or destroyed over the centuries,

and have not been added to the total length of the wall.

It's a hypothesis backed up by the study of ancient texts,

and also by a very particular expression from the Chinese language.

The Chinese refer to the Great Wall

as, in Chinese, 'Wan Li Chang Cheng'.

It's not an absolute measurement.

When the Chinese speak in terms of numbers,

they signify certain things.

For example, one hundred means many.

One thousand means a great multitude.

Ten thousand is almost an eternal figure.

So 'wan li', the ten thousand miles, means

it's the endless wall, the wall without end.

But beyond the myth,

how many kilometres of this endless wall will be revealed?

What is the real length of this incredible fortification,

this wall, which is also known as 'the dragon'?

Determined to use all of the latest technology

to reassess the length of the monument,

in 2007, China appealed to the country's experts.

For five long years, Xu Huijun led a team

that investigated the entire territory in search of an answer.

This military fortification, whose history started over 2000 years ago,

has regularly been repaired or rebuilt over the centuries.

So, to accurately determine the total length of the wall,

Xu Huijun focussed on tracking down missing portions,

hunting for sections now destroyed or underground.

This herculean task to find areas invisible to the human eye

was facilitated by the latest digital technology.

Then, on an airborne scale, the use of remote sensing took over.

By flying over areas determined by the archaeologists,

radars swept the ground to establish a precise location

for every centimetre of the wall it found.

The technology emits electromagnetic waves towards the Earth,

which are reflected back to the transmitter,

revealing remains that were invisible to the naked eye

in remote areas of the Gobi desert.

Now, satellite imagery allows for the visualisation

of the results of this huge mapping project.

When seen from the sky, the discovery is astonishing.

It is not one single wall, but many spread across the whole of China.

There are many sections of the wall,

which are to the north and south of the Ming wall, as we know it,

and this is due to its historical composition.

Portions from different eras,

which, added together in 2012, revealed the true length of the wall.

Over 21,000 kilometres,

that's four times more than the 6700 km previously calculated.

An immense number, which makes the Great Wall of China

easily the longest monument in the world.

To actually arrive at a figure of 21,000 kilometres,

is something that no other country can match.

A fit person, it would take them two years, walking 8 hours a day,

from one end of the wall to another.

So that just gives you an idea of the scale.

To put that in perspective,

the circumference of the globe is around 40,000 kilometres,

so that would stretch halfway around the world.

That is amazing.

As well as revealing the extraordinary length of the wall,

the radar measurements allowed the discovery of portions of the wall

built no less than 1600 years before the parts near Beijing

best known to the general public.

But what did these thousand-year-old,

totally unknown portions of the Great Wall look like?

And what materials and techniques

did the first builders use to make them?

To discover these mysterious remains,

we have to head 2500 kilometres north-west of Beijing,

to the gates of Mongolia and the Gobi desert.

It is here that archaeologists, with the help of satellite radars,

rediscovered the oldest remains of the Great Wall of China.

Fortifications which measured up to 9 metres in height.

To better understand why this result is surprising,

we have to shift to a microscopic scale

to better understand the composition of these walls.

They are made of compacted earth and sand.

So, what is the first secret construction technique

that allowed these 2200-year-old structures to withstand

over 22 centuries of war, wind and this desert's extreme temperatures?

First of all, it is traditional excavation work,

which allowed archaeologists to analyse these ancient walls.

But now, thanks to 3D modelling techniques,

it is possible to visualise the wall like never before.

The wall was composed of different layers,

formed of materials from the region,

layers of compacted earth interspersed with dried reeds.

Computer modelling allows scientists

to better understand the first builders' construction methods.

But to confirm or disprove their hypothesis,

they made giant models for real.

At this construction site,

investigators want to reveal the secret of the wall's longevity.

It's a very, very simple technology.

You use a wooden former that's fixed in place.

To be able to frame the wall...

You mix up the earth, the sand, the gravel with some water, in situ,

and each layer has to be pounded down to compress it,

and it's that compression that gives it its strength.

3D modelling techniques allow the visualisation

of the compression effect on the earth.

On a microscopic scale, it removes pockets of air,

allowing the material to become more compact and solid.

But, for scientists, this ancient construction method,

known as 'rammed earth',

doesn't fully explain how these walls were so robust.

So how was this earth transformed into a material as hard as stone,

capable of standing for over two millennia?

The answer to this mystery is revealed on a satellite scale.

At this scale,

remains can be located 100 kilometres from the Loess Plateau,

a plateau which bears the name of the very fine sand which covers it.

Sand is scattered by the winds to the level of the wall.

On a microscopic scale,

researchers discovered that 20% of it was composed of clay.

But there was more.

On a molecular scale,

they identified a very particular molecule, calcium carbonate.

Calcium carbonate, which exists on the Earth's surface

as limestone, as chalk, as seashells

and also as snail shells.

When it breaks down, it acts as a cement in the formation of mortar.

And in the Gobi desert, you actually see snail shells,

which were crushed and broken down and added to the mortar

to give it this additional strength, this reinforcing

it otherwise would not have.

The unique composition of the Loess Plateau,

with its clay soil and small shells, is one of the builders' secrets.

Once ground up by hammer, the shells let out calcium carbonate,

a hydraulic binder which, once dried,

acquires an extraordinary resistance comparable to that of cement.

Then it dries out, and

...it's almost as solid as concrete.

But, on its own, it wouldn't have sufficient strength

to be able to withstand the elements or withstand attack.

The discovery at the microscopic scale

that loess sand contains molecules of calcium carbonate,

exposes one of the secrets to the wall's extraordinary resistance.

But it is on a human scale, by carrying out field excavations,

that experts discovered another clue to the wall's incredible longevity.

Today, it's possible to visualise the results in 3D.

The layers of earth were poured like concrete around long rows of reeds.

But why did the Chinese choose this particular plant material?

And what is its role?

So, the Chinese used water reeds to reinforce the wall,

and this acted like the steel mesh that we use in concrete today.

These earth walls have a weakness, they absorb humidity.

By carrying out simulations of the reeds' function,

3D imagery allows researchers to confirm their hypothesis,

and reveal that the reeds had a double role.

This hollow plant material reinforces the walls, but also ventilates them.

And alternate layers of these reeds

would create this sort of tensile strength,

which made the wall very strong.

But because the reeds were hollow,

if the wall got wet, it would not stay wet,

because the air would circulate and dry the reeds.

Otherwise, they would rot.

These observations reveal the genius of the ancient builders.

Without modern technology, they located clay sand in the desert

to successfully build a solid, reinforced structure,

capable of self-ventilation.

An incredibly resistant wall

that would withstand wars and harsh weather.

One of the enduring qualities of the Great Wall

is that whilst it uses very resilient materials,

it also is flexible

and it's permeable.

And it can accommodate ground movements, seasonal movements.

And it's far more resilient than a rigid structure would be today.

And what you're left with is this huge wall that is so strong

that you could get a bulldozer and take a run up,

but it won't go anywhere,

you're just going to hit a brick wall,

except it's a rammed earth wall.

The latest technology has shed new light

on the hidden face of the Great Wall.

In total, over 10,000 kilometres of wall

and earthen fortifications of extraordinary strength.

A strength that proved essential when the Chinese Middle Kingdom

was faced with repeated attacks from the north.

Whether the territory crossed deserts, plains or mountains,

the Chinese needed a speed of construction that was unprecedented.

The most striking example can be found near Beijing.

Here, the Ming dynasty built, from the 15th century,

no fewer than 8500 kilometres of wall

in a record time of only 70 years.

At the time, the Middle Kingdom had to face

the worst assailants it had ever known, the Mongolians.

As a consequence,

builders were obliged to urgently find revolutionary techniques

to construct thousands of kilometres of wall at lightning speed.

During the Ming dynasty, there was this impatience,

this urgency to build the wall very quickly.

So, how did Ming builders succeed in constructing their wall

in just a few decades?

What secrets is this fortification hiding,

that explains the incredible speed of its construction?

Today, the latest technology proves invaluable for research.

Like many archaeologists, Xu Huijun's team usually works on the ground.

But since 2018, the use of drones has allowed her

to better understand the structure of the Great Wall of China,

and help solve the mystery of the incredible speed of its construction.

Covered by vegetation

and difficult to access because of the dizzying heights,

many collapsed portions of the wall

are almost impossible to explore through traditional means.

Now, thanks to this drone equipped with sophisticated cameras,

researchers can learn more about these dangerous areas

and capture tiny details on an aerial scale.

In a few places,

which were either not rebuilt at all, or which were simply abandoned,

in those parts, archaeologists can very clearly

see the outline of the wall,

so they can reconstruct how high it must have been, how wide.

The drone is capable of covering the full site on an aerial scale.

It's equipped with multiple cameras that record a huge amount of data.

By processing the data with powerful artificial intelligence programs,

researchers can recreate the crumbled walls in 3D,

and analyse the structure of each portion.

Today, 3D imagery lets researchers detect precious clues,

which could explain the rapid speed of these walls' construction.

To the naked eye,

this monumental structure seems to be cut into the rock

because of its grey-blue colour.

But analysing scans of the wall shows its true nature,

and the materials from which it is made.

The Ming walls looked from afar as if they were built with rocks.

When you look closely,

you see that it's broken down into component parts,

which are human scale.

3D images reveal the structure of this incredible fortification.

The Ming builders chose to use granite,

but only for the foundations.

On top, to accelerate the process of construction,

they decided to use bricks to build the gigantic facade.

This was their primary technique

to quickly build enormous parts of the wall.

There is no shortage of rocks, natural rocks,

because most of the territory that is being used is mountainous.

So they could have chiselled out large stone blocks.

However, to get them into a square size, into a rectangular size,

would have taken a lot of time.

These bricks were preferred to stone

because they were easier to build with, and had another advantage.

Ming builders could create large bricks

that helped accelerate the speed of construction.

By using 3D modelling to compare the size of a classic brick

to the size of a brick made for this Chinese monster,

the difference becomes clear.

They made their bricks four times the size of normal bricks.

So you are increasing your output of material by four times.

This ingenious trick helped builders to construct

an impressive 8500 kilometres of wall in only 70 years.

Today, scientists are investigating

the Ming engineers' extraordinary exploits.

Thanks to drones and artificial intelligence,

they can deduce the incredible number of bricks

assembled over the entire length of the wall.

It's been calculated

that the bricks that have been used to build the Great Wall of China,

total, it's just shy of 4 billion.

That is an incredible amount of bricks.

Billions of bricks, which, on a microscopic scale,

reveal that they are 75% composed of clay.

A material which is malleable and easy to work with.

So where did the builders obtain the billions of tons of clay

necessary to rapidly construct so many kilometres of wall?

To solve this mystery, a bird's-eye view is necessary

to observe the Chinese landscape on a satellite scale.

The Ming wall, fixed atop rocky ridges,

is situated only a few kilometres from steep and narrow valleys.

3D modelling techniques

allow the visualisation of the territory's geology

and make it possible to detect information,

which is invisible to the naked eye.

At the foot of the mountains,

a cross-section of ground reveals large reserves of clay,

situated between earth and rock, at a depth of one metre.

In the interests of efficiency,

Ming builders formed bricks using the raw material at the site,

the clay from the foot of the mountains.

But how did they cook billions of bricks in record time?

Part of the answer is found

in the small town of Banchangyu, 300 kilometres from Beijing.

Guohua Xu is a descendant of builders who came to the south of China

to construct this large part of the Great Wall.

A wall which, today, has mostly been knocked down.

To honour the memory of his ancestors,

Guohua Xu directs an astounding restoration project

on behalf of the Chinese government.

His worksite is located in a valley at the foot of the Great Wall,

where reserves of clay used by the Ming can still be found.

During preparatory excavations,

his team of experts made a remarkable discovery,

countless ovens hidden under the earth.

On a satellite scale,

we can visualise the full extent of this discovery.

Hundreds of ovens all situated in the valley,

directly next to the source of clay.

The proximity reduced the time of production

and sped up their transport to the wall.

The kilns were firing 5 to 10,000 bricks at a time,

so you can just imagine

that the whole area where they were firing bricks

would be like the Industrial Revolution.

By setting up their production site at the foot of the mountain,

the Chinese were able to build thousands of kilometres of wall

in only a few decades.

But they also developed

a revolutionary method for baking bricks

that allowed them to supply the construction site faster.

Today, at the restoration site near Banchangyu,

the techniques developed by the Ming are recreated

to mould and bake giant bricks

identical to those used in the Great Wall of China.

The technique takes time.

Two weeks of drying in the sun and no fewer than 48 hours of baking.

To speed up production,

builders found an ingenious way to save time during the last stage,

when the bricks needed to be cooled after baking.

When you fire a brick kiln, when you shut the fires out,

there's two or three days where you wait

until the bricks are cool enough to be able to handle.

So what the engineers discovered was that, on the top of the kiln,

if you dig this little moat, fill it with water,

it allows them to cool down much more quickly without cracking.

The efficient cooling technique developed by the Ming

had an unexpected effect on the colour of the bricks.

The bricks on the Great Wall

are a uniform grey colour,

and that is unusual

in an area where the clay fires at a terracotta-red colour.

The unusual colour is a result of the cold water

poured onto the hot ovens to cool them down more quickly.

The other thing it does is it creates steam.

it creates steam in the kiln to the point where fresh air can't get in.

So that means that the iron that is contained within the clay

can't oxidise.

Through observations on a microscopic scale,

scientists discovered an explanation to the unique colour of the bricks.

The red clay contains iron oxide, which turns red on contact with air.

But reduced oxygen levels inside the oven

resulted in the bricks staying grey, like stone.

It stays this greyish-blue colour, and it's quite distinctive.

The Chinese weren't trying to create grey bricks,

they were trying to get them out quicker,

but they accidentally discovered

that you can change the colour of the clay by taking the oxygen out.

Through a physical reaction, caused by the accelerated cooling of ovens,

Ming builders changed the face of the Great Wall of China forever.

Thanks to the incredible speed of production,

they successfully made billions of bricks

to construct thousands of kilometres of wall in only a few centuries.

But once the wall had been built, engineers faced a new challenge...

to resist repeated assaults by the Mongolians,

who were increasingly well armed.

It became virtually impenetrable.

Even to firearms, to cannonballs,

which were being used from the 16th century onwards.

How did the Ming give their fortification

its extraordinary resistance?

And what secret ingredients did they hide in the heart of the walls

to allow its incredible longevity?

According to specialists, the secret to the strength of China's Great Wall

can be found within its bricks.

They were very, very high technology.

Even by today's standards, I mean, a brick is a brick, or so I thought,

but, no, these bricks are exceptional.

And they are extremely hard.

The analysis of ancient texts

coupled with the recent discoveries of hundreds of ovens

have brought to light the technical revolution

developed here in the 16th century.

This traditional research work

allowed the investigators of the past

to analyse the revolutionary ovens used by Ming builders.

But today, the latest 3D modelling technology

lets experts visualise the inside of the ovens, too.

Buried at a depth of several metres,

they were covered with a round roof under which the bricks were placed.

By reproducing the exceptional mastery of fire from the 16th century

archaeologists have discovered the recipe

that allows these bricks to be particularly resistant.

Managing the fires, managing the increase in temperature

is perhaps the most difficult thing in firing a brick kiln.

Baking bricks directly in the fire could make them burst.

So the goal of ancient builders

was to produce constant and durable heat in their ovens.

After being dried in the sun,

the bricks were placed one on top of the other,

with enough space between them to let the heat through.

But how were they baked without being burned?

Archaeologists' research, coupled with 3D imagery,

allows them to better understand heat management

in this horseshoe-shaped oven,

as well as the role of the strangely-rounded earth roof.

In fact, the fire wasn't underneath the bricks, but next to it,

in a tunnel, which served as an inlet for hot air.

The shape of the oven forced the heat between the bricks

and out through the chimney.

So the baking, which approached 1000 degrees Celsius, was very even.

They elevated brickmaking technology to a level you don't see today.

These are very hard bricks, which can be reused many times.

So, in other words,

the Ming had actually perfected the art of the burning of bricks,

to the extent that it became a military secret.

The brickworks were guarded 24 hours a day by the Ming army,

and worked constantly to produce

thousands of extremely resistant bricks, weighing up to 10 kilos.

Nearly 4 billion of these bricks

were carried up the mountain to build the wall,

but, inside, workers had hidden a secret ingredient,

invisible to the naked eye.

An essential element

that gives the entire structure its incredible strength.

These bricks are there not only to provide the structure,

but to contain the core of the wall.

So, what is hidden inside the Great Wall of China?

Imagery taken by drones reveals precious clues.

In the middle of the mountains,

open parts of the wall make it possible

to analyse the composition of the heart of this World Heritage site.

On a macroscopic scale, we discover rock,

and in particular granite, a very dense and hard material.

But why didn't the builders

choose to construct the entire width of the wall in brick?

You wouldn't use premium materials in locations you didn't need to.

So, the bricks and the stone

create the face or the skin of the wall,

and inside is the rubble,

which is carved out of the rock

in preparing the foundation.

There's earth, which is displaced

when you're digging down into the clay for the bricks.

So all the material is used.

Today, by modelling the monument,

investigators can detect the invisible

and expose the inside of this gigantic fortification.

Above the granite slabs,

the Ming packed rubble made up of granite and earth.

Then, stone slabs were used to seal the structure.

Once this had been attained, it became almost impenetrable.

This ingenious technique was used to build a wall of immense dimensions,

over 7 metres high, and even reaching 12 metres

at the lookout towers, which punctuate the fortification.

Steep-sided walls of breathtaking longevity

whose incredible resistance is due to a final secret ingredient.

An ingredient recently revealed by science.

As well as gathering structural data about this gigantic fortification,

drone technology allows visualisation of the wall like never before,

thanks to new sensors.

These sensors allow investigators

to determine every material used in the wall

and unravel the mystery of its strength.

The first innovative technology fixed to the drone

is infra-red thermography.

This equipment can see through cloud and fog

to highlight the changes in temperature of different materials,

revealing it through a special colour scheme.

Once analysed by a heat technician,

this imagery gives precious information about the materials used.

Attached to the drone,

the hyperspectral sensor records rays invisible to the naked eye,

by interpreting the intensity of infra-red signals

that reach the camera,

on a level which has never been attained before.

To the naked eye, or with a normal camera,

we can make out the colours of the material.

But with the hyperspectral sensor,

scientists can see the chemical components of the wall.

Thanks to this data,

scientists can compare the chemical composition of each part of the wall,

and it has allowed them to make a startling revelation

about the ancient builders' biggest construction secret.

Researchers have observed that mortar,

which was used by the Ming to cover bricks over 600 years ago,

seems to have an extraordinary chemical composition.

This unique mortar blends both non-organic compounds

like rock or sand with a mysterious organic compound.

The Ming dynasty,

not only were they building with these beautiful blue bricks,

but they also had a secret ingredient

used in the mortar between those bricks.

They were actually sealed by a kind of concrete,

which makes them impenetrable against human attacks,

but also against the elements.

In the mountains,

you get temperatures, which drop down -30, -40 degrees Celsius,

and it needs a much more resilient mortar.

So, what secret ingredient did the builders use

to give the wall its incredible longevity?

In the field,

archaeologists discovered a material that was strangely white.

The colour, associated with the mortar's indestructible reputation,

has spawned a macabre legend.

Contemporary sources...

demonstrate that there was a belief

that people were actually buried in the wall,

or the bones were ground up to make mortar.

So far, no bone fragments have actually been found

within the wall itself.

So, if human bones weren't used to build the wall,

what secret ingredient made it so robust?

The analysis of samples on a microscopic scale

told experts that the mortar was made out of sand, rock and lime,

a material made of calcium carbonate.

But, at first glance,

nothing reveals the organic component detected by infra-red.

In order to reproduce ancient techniques,

during his restoration of the Great Wall,

Guohua Xu took a particular interest

in his builder ancestors' secret recipe.

After centuries of legend, science now has the means

to unravel the secrets of this wall's construction,

by analysing material on a molecular scale.

In the south of Shanghai, at Zhejiang University,

Dr Zhang's team patiently studied the samples of mortar

in their laboratory.

The mission of this materials' specialist

is to find the origin of the organic molecule contained in the mortar

using an iodine-based chemical test.

This scientific test proves that starch,

a molecule that can generally be found in plants,

is present in this sample.

But where does this mysterious ingredient come from?

Dr Zhang's research revealed the Chinese builders' secret.

They used an ingredient from their daily diet

to create one of the world's most unusual mortars.

And that was sticky rice.

And the ingredient, what makes the rice sticky,

this was used in the wall.

The latest 3D modelling technology makes it possible to examine

the exceptional properties of sticky rice mortar on a molecular scale.

Starch is composed of a molecule called amylopectin,

which has long branches that trap molecules of calcium carbonate

contained in lime, preventing them from crystallising.

Because the crystals are much smaller than in ordinary mortar,

they are more strongly bonded and incredibly resistant.

Scientists also discovered that the Great Wall of China's mortar

is sturdy on the outside and soft and supple on the inside.

An extraordinary characteristic,

which has allowed the wall to resist extreme temperatures

over the centuries,

as well as earthquakes and even cannon fire.

In addition to making the mortar resistant,

starch protects the molecules of lime,

restricting their contact with any air inside the mortar.

When the hard lime erodes onto the surface,

the soft lime behind it solidifies,

adding to the wall's exceptional longevity

According to scientists,

sticky rice mortar is one of Chinese civilization's major inventions.

This is possibly the first known composite mortar.

So it's the first known mortar

that uses both inorganic compounds like rocks or sand

with organic compounds, rice flour.

Decade after decade,

the latest technology has unravelled many of the Great Wall's mysteries.

Like its true length, at 21,196 kilometres,

a large part of which has lasted millennia,

thanks to a structure made of rammed earth and layers of reeds.

And scientific analysis has shown how clay bricks and composite mortar

is the explanation for the resistance

of the most well-known sections of wall.

But researchers think there are still secrets to uncover.

It's only recently that we are really starting

to look at exactly how the Great Wall has been made up,

and I think there is so much more to discover.

Archaeologists are still hunting

for missing sections of the Great Wall that remain lost underground.

And the search is already provoking debate

about its official 21,000-kilometre length.

It is as great as the greatest natural phenomenon on Earth.

It is like the roof of the world, it's like the Himalayas.

Here is something that humans created.

This is a monument in harmony with nature,

snaking across steep terrain,

and still making specialists

wonder about the technical and human means used to build it.

How the logistics were coordinated over thousands of years,

to maintain this momentum building the wall.

To me, that is a very great mystery.

Captions C SBS Australia 2020