All language subtitles for Ancient.Superstructures.S01E03.720p.HDTV.x264-CBFM

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