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

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traversing an entire country
from east to west,

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the Great Wall of China.

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

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this remarkable structure
is the largest ever built by man

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in terms of length,
mass and surface area.

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It's majestic, it is imposing.

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

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It is incredible.

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This marvel of the ancient world

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was built
from nearly 4 billion bricks.

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To the 16 million tourists
who visit each year,

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

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

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also known as 'the dragon',

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crosses no fewer
than 15 Chinese provinces.

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

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So that just gives
you an idea of the scale.

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

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While the human eye can't detect

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the full extent
of this incredible construction,

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the latest technology allows experts

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to calculate
the true length of the wall

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by detecting hidden portions of it.

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

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and new technology
is exposing a construction technique

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that shows
the genius of ancient builders.

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By examining the wall

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from a satellite scale
down to a molecular level,

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

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

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

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It became virtually impenetrable,
even to cannonballs.

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Thanks to the latest technology,

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we can lift the veil on the
extraordinary construction methods

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of this UNESCO World Heritage site

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

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What revolutionary techniques

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allowed the Chinese to construct
thousands of kilometres of wall

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in under 70 years,
and on such steep terrain?

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And what secret ingredient,
recently revealed by science,

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explains its unbelievable longevity?

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A brick is a brick, or so I thought,

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but, no,
these bricks are exceptional.

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History, science, construction.

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Examined like never before,

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the Great Wall of China
reveals its secrets...

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on every scale.

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

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Snaking through
the mountains of northern China,

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this incredible structure was built
to protect the Chinese Middle Kingdom

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from Mongolian attacks.

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But what visitors
can't see at first glance,

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is the true extent
of this engineering feat

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constructed over 2000 years.

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It is the longest man-made
monument on Earth.

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And it is colossal.

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It almost transcends nature,

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because it crosses
very inhospitable terrain,

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

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For more than a century,

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archaeologists and explorers
have travelled across most of Asia,

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following the wall by foot

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in order to chart
this jewel of humanity.

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

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to observe the extent
of this Chinese monument,

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

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It crosses the wild plains
of the western Gobi desert,

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snakes through
the northern mountains,

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

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The wall is so vast
that at its eastern end,

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the sun rises a full one hour
and twenty minutes later

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than at its western extremity.

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The length of this wall
was long calculated

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using observations
taken by the naked eye.

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In 2012, it was estimated
at 6700 kilometres long,

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a little more
than the length of China.

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

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on a satellite scale,

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archaeologists have the means
to explore a new hypothesis.

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That the wall's true length
is much greater.

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

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are invisible to the naked eye,

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that sections
have become hidden underground

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or destroyed over the centuries,

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

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

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

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The Chinese refer to the Great Wall

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

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It's not an absolute measurement.

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When the Chinese speak
in terms of numbers,

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they signify certain things.

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For example,
one hundred means many.

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One thousand means
a great multitude.

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Ten thousand
is almost an eternal figure.

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

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

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But beyond the myth,

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how many kilometres of
this endless wall will be revealed?

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What is the real length
of this incredible fortification,

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

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Determined to use
all of the latest technology

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to reassess
the length of the monument,

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

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For five long years,
Xu Huijun led a team

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that investigated the entire
territory in search of an answer.

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

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has regularly been repaired
or rebuilt over the centuries.

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

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Xu Huijun focussed
on tracking down missing portions,

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hunting for sections
now destroyed or underground.

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This herculean task to find
areas invisible to the human eye

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was facilitated
by the latest digital technology.

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

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By flying over areas
determined by the archaeologists,

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radars swept the ground
to establish a precise location

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for every centimetre of the wall
it found.

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The technology emits electromagnetic
waves towards the Earth,

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which are reflected
back to the transmitter,

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revealing remains
that were invisible to the naked eye

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in remote areas of the Gobi desert.

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Now, satellite imagery
allows for the visualisation

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of the results of
this huge mapping project.

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When seen from the sky,
the discovery is astonishing.

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

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There are many sections of the wall,

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

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and this is due
to its historical composition.

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Portions from different eras,

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

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Over 21,000 kilometres,

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

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An immense number,
which makes the Great Wall of China

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easily the longest monument
in the world.

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

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is something
that no other country can match.

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

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from one end of the wall to another.

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So that just gives
you an idea of the scale.

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To put that in perspective,

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

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so that would stretch
halfway around the world.

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That is amazing.

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As well as revealing the
extraordinary length of the wall,

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the radar measurements allowed
the discovery of portions of the wall

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built no less than 1600 years
before the parts near Beijing

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best known to the general public.

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But what did these thousand-year-old,

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totally unknown portions
of the Great Wall look like?

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And what materials and techniques

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did the first builders use
to make them?

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To discover these mysterious remains,

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

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to the gates of Mongolia
and the Gobi desert.

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

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rediscovered the oldest remains
of the Great Wall of China.

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Fortifications which measured
up to 9 metres in height.

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To better understand
why this result is surprising,

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we have to shift
to a microscopic scale

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to better understand
the composition of these walls.

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They are made of
compacted earth and sand.

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So, what is the first
secret construction technique

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that allowed these
2200-year-old structures to withstand

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

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First of all,
it is traditional excavation work,

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which allowed archaeologists
to analyse these ancient walls.

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But now,
thanks to 3D modelling techniques,

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it is possible to visualise
the wall like never before.

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The wall was composed
of different layers,

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formed of materials from the region,

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layers of compacted earth
interspersed with dried reeds.

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Computer modelling allows scientists

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to better understand the first
builders' construction methods.

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But to confirm or disprove
their hypothesis,

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they made giant models for real.

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At this construction site,

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

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It's a very, very
simple technology.

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

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To be able to frame the wall...

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

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

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

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3D modelling techniques
allow the visualisation

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of the compression
effect on the earth.

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On a microscopic scale,
it removes pockets of air,

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allowing the material
to become more compact and solid.

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But, for scientists,
this ancient construction method,

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known as 'rammed earth',

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

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

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capable of standing
for over two millennia?

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

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At this scale,

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remains can be located 100 kilometres
from the Loess Plateau,

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

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

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On a microscopic scale,

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

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But there was more.

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On a molecular scale,

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they identified a very particular
molecule, calcium carbonate.

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Calcium carbonate,
which exists on the Earth's surface

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as limestone, as chalk, as seashells

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and also as snail shells.

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

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

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which were crushed and broken down
and added to the mortar

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to give it this additional strength,
this reinforcing

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it otherwise would not have.

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The unique composition
of the Loess Plateau,

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

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

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a hydraulic binder which, once dried,

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acquires an extraordinary resistance
comparable to that of cement.

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Then it dries out, and

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

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

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to be able to withstand the elements
or withstand attack.

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The discovery
at the microscopic scale

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that loess sand contains
molecules of calcium carbonate,

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

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

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

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

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

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But why did the Chinese choose
this particular plant material?

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And what is its role?

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

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

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These earth walls have a weakness,
they absorb humidity.

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By carrying out simulations
of the reeds' function,

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3D imagery allows researchers
to confirm their hypothesis,

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and reveal that the reeds
had a double role.

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

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And alternate layers of these reeds

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would create
this sort of tensile strength,

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which made the wall very strong.

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But because the reeds were hollow,

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

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because the air would circulate
and dry the reeds.

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Otherwise, they would rot.

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These observations reveal
the genius of the ancient builders.

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00:17:22,400 --> 00:17:26,800
Without modern technology,
they located clay sand in the desert

246
00:17:26,960 --> 00:17:29,600
to successfully build a solid,
reinforced structure,

247
00:17:29,760 --> 00:17:32,560
capable of self-ventilation.

248
00:17:33,240 --> 00:17:35,840
An incredibly resistant wall

249
00:17:36,000 --> 00:17:38,680
that would withstand wars
and harsh weather.

250
00:17:38,840 --> 00:17:42,240
One of the enduring qualities
of the Great Wall

251
00:17:42,400 --> 00:17:47,440
is that whilst it uses
very resilient materials,

252
00:17:47,600 --> 00:17:49,600
it also is flexible

253
00:17:49,760 --> 00:17:50,920
and it's permeable.

254
00:17:51,080 --> 00:17:57,240
And it can accommodate ground
movements, seasonal movements.

255
00:17:57,400 --> 00:18:03,560
And it's far more resilient than
a rigid structure would be today.

256
00:18:03,720 --> 00:18:07,200
And what you're left with
is this huge wall that is so strong

257
00:18:07,360 --> 00:18:10,720
that you could get a bulldozer
and take a run up,

258
00:18:10,880 --> 00:18:12,080
but it won't go anywhere,

259
00:18:12,240 --> 00:18:14,480
you're just going to
hit a brick wall,

260
00:18:14,640 --> 00:18:16,520
except it's a rammed earth wall.

261
00:18:27,200 --> 00:18:29,320
The latest technology
has shed new light

262
00:18:29,480 --> 00:18:32,520
on the hidden face of the Great Wall.

263
00:18:33,320 --> 00:18:36,520
In total,
over 10,000 kilometres of wall

264
00:18:36,680 --> 00:18:40,160
and earthen fortifications
of extraordinary strength.

265
00:18:42,440 --> 00:18:46,440
A strength that proved essential
when the Chinese Middle Kingdom

266
00:18:46,600 --> 00:18:49,560
was faced with repeated attacks
from the north.

267
00:18:54,760 --> 00:18:58,840
Whether the territory crossed
deserts, plains or mountains,

268
00:18:59,000 --> 00:19:03,640
the Chinese needed a speed of
construction that was unprecedented.

269
00:19:06,920 --> 00:19:11,520
The most striking example
can be found near Beijing.

270
00:19:11,680 --> 00:19:15,920
Here, the Ming dynasty built,
from the 15th century,

271
00:19:16,080 --> 00:19:19,040
no fewer than 8500 kilometres of wall

272
00:19:19,200 --> 00:19:23,440
in a record time of only 70 years.

273
00:19:23,600 --> 00:19:26,160
At the time,
the Middle Kingdom had to face

274
00:19:26,320 --> 00:19:30,440
the worst assailants
it had ever known, the Mongolians.

275
00:19:30,960 --> 00:19:31,960
As a consequence,

276
00:19:32,120 --> 00:19:36,640
builders were obliged to urgently
find revolutionary techniques

277
00:19:36,800 --> 00:19:41,760
to construct thousands of kilometres
of wall at lightning speed.

278
00:19:41,920 --> 00:19:45,600
During the Ming dynasty,
there was this impatience,

279
00:19:45,760 --> 00:19:48,360
this urgency
to build the wall very quickly.

280
00:19:55,160 --> 00:19:58,920
So, how did Ming builders
succeed in constructing their wall

281
00:19:59,080 --> 00:20:00,280
in just a few decades?

282
00:20:00,440 --> 00:20:03,640
What secrets
is this fortification hiding,

283
00:20:03,800 --> 00:20:06,680
that explains the incredible speed
of its construction?

284
00:20:13,920 --> 00:20:18,120
Today, the latest technology
proves invaluable for research.

285
00:20:22,080 --> 00:20:27,840
Like many archaeologists, Xu Huijun's
team usually works on the ground.

286
00:20:29,040 --> 00:20:32,440
But since 2018,
the use of drones has allowed her

287
00:20:32,600 --> 00:20:35,640
to better understand the structure
of the Great Wall of China,

288
00:20:35,800 --> 00:20:39,720
and help solve the mystery of the
incredible speed of its construction.

289
00:20:57,880 --> 00:20:59,240
Covered by vegetation

290
00:20:59,400 --> 00:21:02,240
and difficult to access
because of the dizzying heights,

291
00:21:02,400 --> 00:21:04,000
many collapsed portions of the wall

292
00:21:04,160 --> 00:21:08,040
are almost impossible to explore
through traditional means.

293
00:21:09,160 --> 00:21:13,160
Now, thanks to this drone
equipped with sophisticated cameras,

294
00:21:13,320 --> 00:21:16,200
researchers can learn more
about these dangerous areas

295
00:21:16,360 --> 00:21:20,360
and capture tiny details
on an aerial scale.

296
00:21:29,480 --> 00:21:31,320
In a few places,

297
00:21:31,480 --> 00:21:36,280
which were either not rebuilt at
all, or which were simply abandoned,

298
00:21:36,440 --> 00:21:39,280
in those parts,
archaeologists can very clearly

299
00:21:39,440 --> 00:21:41,040
see the outline of the wall,

300
00:21:41,200 --> 00:21:44,720
so they can reconstruct how high
it must have been, how wide.

301
00:21:45,880 --> 00:21:50,000
The drone is capable of covering
the full site on an aerial scale.

302
00:21:50,160 --> 00:21:54,560
It's equipped with multiple cameras
that record a huge amount of data.

303
00:21:56,280 --> 00:22:00,280
By processing the data with powerful
artificial intelligence programs,

304
00:22:00,440 --> 00:22:03,840
researchers can recreate
the crumbled walls in 3D,

305
00:22:04,000 --> 00:22:06,800
and analyse
the structure of each portion.

306
00:22:24,480 --> 00:22:29,120
Today, 3D imagery lets researchers
detect precious clues,

307
00:22:29,280 --> 00:22:33,200
which could explain the rapid speed
of these walls' construction.

308
00:22:33,360 --> 00:22:34,520
To the naked eye,

309
00:22:34,680 --> 00:22:37,560
this monumental structure
seems to be cut into the rock

310
00:22:37,720 --> 00:22:39,720
because of its grey-blue colour.

311
00:22:39,880 --> 00:22:43,600
But analysing scans of the wall
shows its true nature,

312
00:22:43,760 --> 00:22:45,640
and the materials
from which it is made.

313
00:22:47,560 --> 00:22:52,600
The Ming walls looked from afar
as if they were built with rocks.

314
00:22:53,440 --> 00:22:55,120
When you look closely,

315
00:22:55,280 --> 00:22:58,960
you see that it's broken down
into component parts,

316
00:22:59,120 --> 00:23:00,800
which are human scale.

317
00:23:11,960 --> 00:23:16,280
3D images reveal the structure
of this incredible fortification.

318
00:23:18,640 --> 00:23:20,600
The Ming builders
chose to use granite,

319
00:23:20,760 --> 00:23:22,800
but only for the foundations.

320
00:23:24,000 --> 00:23:27,200
On top, to accelerate
the process of construction,

321
00:23:27,360 --> 00:23:31,120
they decided to use bricks
to build the gigantic facade.

322
00:23:33,000 --> 00:23:34,920
This was their primary technique

323
00:23:35,080 --> 00:23:37,920
to quickly build
enormous parts of the wall.

324
00:23:38,960 --> 00:23:41,920
There is no shortage of rocks,
natural rocks,

325
00:23:42,080 --> 00:23:45,680
because most of the territory
that is being used is mountainous.

326
00:23:46,800 --> 00:23:50,280
So they could have chiselled out
large stone blocks.

327
00:23:51,800 --> 00:23:56,960
However, to get them into a square
size, into a rectangular size,

328
00:23:57,120 --> 00:23:58,800
would have taken a lot of time.

329
00:23:58,960 --> 00:24:00,720
These bricks were preferred to stone

330
00:24:00,880 --> 00:24:05,160
because they were easier to build
with, and had another advantage.

331
00:24:05,320 --> 00:24:07,320
Ming builders
could create large bricks

332
00:24:07,480 --> 00:24:10,320
that helped accelerate
the speed of construction.

333
00:24:11,040 --> 00:24:14,720
By using 3D modelling to compare
the size of a classic brick

334
00:24:14,880 --> 00:24:17,840
to the size of a brick
made for this Chinese monster,

335
00:24:18,000 --> 00:24:20,680
the difference becomes clear.

336
00:24:20,840 --> 00:24:24,080
They made their bricks four times
the size of normal bricks.

337
00:24:24,800 --> 00:24:29,520
So you are increasing your output
of material by four times.

338
00:24:41,720 --> 00:24:44,560
This ingenious trick
helped builders to construct

339
00:24:44,720 --> 00:24:49,800
an impressive 8500 kilometres
of wall in only 70 years.

340
00:24:53,000 --> 00:24:55,280
Today, scientists are investigating

341
00:24:55,440 --> 00:24:58,480
the Ming engineers'
extraordinary exploits.

342
00:24:58,640 --> 00:25:01,160
Thanks to drones
and artificial intelligence,

343
00:25:01,320 --> 00:25:03,680
they can deduce
the incredible number of bricks

344
00:25:03,840 --> 00:25:07,160
assembled over
the entire length of the wall.

345
00:25:07,600 --> 00:25:08,840
It's been calculated

346
00:25:09,000 --> 00:25:12,080
that the bricks that have been used
to build the Great Wall of China,

347
00:25:12,240 --> 00:25:15,720
total, it's just shy of 4 billion.

348
00:25:17,360 --> 00:25:20,240
That is an incredible
amount of bricks.

349
00:25:28,680 --> 00:25:31,720
Billions of bricks,
which, on a microscopic scale,

350
00:25:31,880 --> 00:25:35,560
reveal that
they are 75% composed of clay.

351
00:25:35,720 --> 00:25:39,640
A material which is malleable
and easy to work with.

352
00:25:50,480 --> 00:25:53,720
So where did the builders obtain
the billions of tons of clay

353
00:25:53,880 --> 00:25:57,440
necessary to rapidly construct
so many kilometres of wall?

354
00:25:57,600 --> 00:26:00,880
To solve this mystery,
a bird's-eye view is necessary

355
00:26:01,040 --> 00:26:04,360
to observe the Chinese landscape
on a satellite scale.

356
00:26:04,520 --> 00:26:07,320
The Ming wall,
fixed atop rocky ridges,

357
00:26:07,480 --> 00:26:11,640
is situated only a few kilometres
from steep and narrow valleys.

358
00:26:12,440 --> 00:26:13,800
3D modelling techniques

359
00:26:13,960 --> 00:26:17,280
allow the visualisation
of the territory's geology

360
00:26:17,440 --> 00:26:19,920
and make it possible
to detect information,

361
00:26:20,080 --> 00:26:23,000
which is invisible to the naked eye.

362
00:26:23,560 --> 00:26:25,080
At the foot of the mountains,

363
00:26:25,240 --> 00:26:28,760
a cross-section of ground
reveals large reserves of clay,

364
00:26:28,920 --> 00:26:32,680
situated between earth and rock,
at a depth of one metre.

365
00:26:47,240 --> 00:26:49,280
In the interests of efficiency,

366
00:26:49,440 --> 00:26:53,440
Ming builders formed bricks
using the raw material at the site,

367
00:26:53,600 --> 00:26:56,440
the clay
from the foot of the mountains.

368
00:26:56,600 --> 00:27:00,440
But how did they cook
billions of bricks in record time?

369
00:27:03,920 --> 00:27:05,280
Part of the answer is found

370
00:27:05,440 --> 00:27:09,160
in the small town of Banchangyu,
300 kilometres from Beijing.

371
00:27:12,640 --> 00:27:17,080
Guohua Xu is a descendant of builders
who came to the south of China

372
00:27:17,240 --> 00:27:20,080
to construct
this large part of the Great Wall.

373
00:27:21,320 --> 00:27:25,040
A wall which, today,
has mostly been knocked down.

374
00:27:36,160 --> 00:27:38,560
To honour
the memory of his ancestors,

375
00:27:38,720 --> 00:27:41,720
Guohua Xu directs
an astounding restoration project

376
00:27:41,880 --> 00:27:44,680
on behalf of the Chinese government.

377
00:27:44,840 --> 00:27:48,400
His worksite is located in a valley
at the foot of the Great Wall,

378
00:27:48,560 --> 00:27:52,480
where reserves of clay
used by the Ming can still be found.

379
00:27:52,640 --> 00:27:54,600
During preparatory excavations,

380
00:27:54,760 --> 00:27:57,760
his team of experts
made a remarkable discovery,

381
00:27:57,920 --> 00:28:00,440
countless ovens
hidden under the earth.

382
00:28:18,960 --> 00:28:20,560
On a satellite scale,

383
00:28:20,720 --> 00:28:23,840
we can visualise
the full extent of this discovery.

384
00:28:24,000 --> 00:28:26,600
Hundreds of ovens
all situated in the valley,

385
00:28:26,760 --> 00:28:28,960
directly next to the source of clay.

386
00:28:29,120 --> 00:28:31,600
The proximity
reduced the time of production

387
00:28:31,760 --> 00:28:34,000
and sped up
their transport to the wall.

388
00:28:41,960 --> 00:28:47,040
The kilns were firing
5 to 10,000 bricks at a time,

389
00:28:47,200 --> 00:28:48,920
so you can just imagine

390
00:28:49,080 --> 00:28:52,640
that the whole area
where they were firing bricks

391
00:28:52,800 --> 00:28:57,000
would be
like the Industrial Revolution.

392
00:28:57,160 --> 00:29:00,480
By setting up their production site
at the foot of the mountain,

393
00:29:00,640 --> 00:29:03,880
the Chinese were able to build
thousands of kilometres of wall

394
00:29:04,040 --> 00:29:05,680
in only a few decades.

395
00:29:06,360 --> 00:29:07,640
But they also developed

396
00:29:07,800 --> 00:29:10,080
a revolutionary method
for baking bricks

397
00:29:10,240 --> 00:29:13,720
that allowed them to supply
the construction site faster.

398
00:29:15,960 --> 00:29:19,640
Today, at the restoration site
near Banchangyu,

399
00:29:19,800 --> 00:29:22,080
the techniques developed
by the Ming are recreated

400
00:29:22,240 --> 00:29:23,960
to mould and bake giant bricks

401
00:29:24,120 --> 00:29:27,200
identical to those
used in the Great Wall of China.

402
00:29:28,240 --> 00:29:29,680
The technique takes time.

403
00:29:29,840 --> 00:29:34,280
Two weeks of drying in the sun
and no fewer than 48 hours of baking.

404
00:29:34,840 --> 00:29:36,080
To speed up production,

405
00:29:36,240 --> 00:29:40,320
builders found an ingenious way
to save time during the last stage,

406
00:29:40,480 --> 00:29:43,320
when the bricks
needed to be cooled after baking.

407
00:29:44,760 --> 00:29:48,200
When you fire a brick kiln,
when you shut the fires out,

408
00:29:48,360 --> 00:29:50,800
there's two or three days
where you wait

409
00:29:50,960 --> 00:29:54,280
until the bricks are cool enough
to be able to handle.

410
00:29:54,440 --> 00:29:59,600
So what the engineers discovered
was that, on the top of the kiln,

411
00:29:59,760 --> 00:30:04,720
if you dig this little moat,
fill it with water,

412
00:30:04,880 --> 00:30:08,680
it allows them to cool down
much more quickly without cracking.

413
00:30:10,120 --> 00:30:12,520
The efficient cooling technique
developed by the Ming

414
00:30:12,680 --> 00:30:15,240
had an unexpected effect
on the colour of the bricks.

415
00:30:16,920 --> 00:30:19,200
The bricks on the Great Wall

416
00:30:19,360 --> 00:30:23,080
are a uniform grey colour,

417
00:30:23,240 --> 00:30:24,400
and that is unusual

418
00:30:24,560 --> 00:30:28,920
in an area where the clay
fires at a terracotta-red colour.

419
00:30:29,640 --> 00:30:32,240
The unusual colour
is a result of the cold water

420
00:30:32,400 --> 00:30:35,400
poured onto the hot ovens
to cool them down more quickly.

421
00:30:35,560 --> 00:30:37,920
The other thing it does
is it creates steam.

422
00:30:38,080 --> 00:30:41,920
it creates steam in the kiln to the
point where fresh air can't get in.

423
00:30:42,080 --> 00:30:46,400
So that means that the iron
that is contained within the clay

424
00:30:46,560 --> 00:30:47,720
can't oxidise.

425
00:30:55,200 --> 00:30:58,000
Through observations
on a microscopic scale,

426
00:30:58,160 --> 00:31:01,800
scientists discovered an explanation
to the unique colour of the bricks.

427
00:31:02,880 --> 00:31:07,880
The red clay contains iron oxide,
which turns red on contact with air.

428
00:31:11,080 --> 00:31:13,840
But reduced oxygen levels
inside the oven

429
00:31:14,000 --> 00:31:16,920
resulted in the bricks
staying grey, like stone.

430
00:31:18,480 --> 00:31:23,160
It stays this greyish-blue colour,
and it's quite distinctive.

431
00:31:23,320 --> 00:31:25,680
The Chinese weren't trying
to create grey bricks,

432
00:31:25,840 --> 00:31:27,640
they were trying
to get them out quicker,

433
00:31:27,800 --> 00:31:29,480
but they accidentally discovered

434
00:31:29,640 --> 00:31:33,080
that you can change the colour of
the clay by taking the oxygen out.

435
00:31:34,640 --> 00:31:38,640
Through a physical reaction, caused
by the accelerated cooling of ovens,

436
00:31:38,800 --> 00:31:43,840
Ming builders changed the face
of the Great Wall of China forever.

437
00:31:46,000 --> 00:31:48,320
Thanks to
the incredible speed of production,

438
00:31:48,480 --> 00:31:50,560
they successfully
made billions of bricks

439
00:31:50,720 --> 00:31:55,280
to construct thousands of kilometres
of wall in only a few centuries.

440
00:31:56,520 --> 00:32:00,000
But once the wall had been built,
engineers faced a new challenge...

441
00:32:00,840 --> 00:32:03,680
to resist repeated assaults
by the Mongolians,

442
00:32:03,840 --> 00:32:06,400
who were increasingly well armed.

443
00:32:15,080 --> 00:32:16,960
It became virtually impenetrable.

444
00:32:17,120 --> 00:32:20,960
Even to firearms, to cannonballs,

445
00:32:21,120 --> 00:32:24,800
which were being used
from the 16th century onwards.

446
00:32:25,920 --> 00:32:28,520
How did the Ming
give their fortification

447
00:32:28,680 --> 00:32:31,120
its extraordinary resistance?

448
00:32:31,280 --> 00:32:34,240
And what secret ingredients did
they hide in the heart of the walls

449
00:32:34,400 --> 00:32:36,960
to allow its incredible longevity?

450
00:32:45,280 --> 00:32:49,280
According to specialists, the secret
to the strength of China's Great Wall

451
00:32:49,440 --> 00:32:51,760
can be found within its bricks.

452
00:32:51,920 --> 00:32:54,280
They were very, very high
technology.

453
00:32:54,440 --> 00:32:59,040
Even by today's standards, I mean,
a brick is a brick, or so I thought,

454
00:32:59,200 --> 00:33:01,400
but, no,
these bricks are exceptional.

455
00:33:01,560 --> 00:33:03,440
And they are extremely hard.

456
00:33:04,440 --> 00:33:06,600
The analysis of ancient texts

457
00:33:06,760 --> 00:33:09,200
coupled with the recent discoveries
of hundreds of ovens

458
00:33:09,360 --> 00:33:11,560
have brought to light
the technical revolution

459
00:33:11,720 --> 00:33:14,480
developed here in the 16th century.

460
00:33:16,200 --> 00:33:17,800
This traditional research work

461
00:33:17,960 --> 00:33:19,800
allowed
the investigators of the past

462
00:33:19,960 --> 00:33:23,840
to analyse the revolutionary
ovens used by Ming builders.

463
00:33:24,000 --> 00:33:26,880
But today,
the latest 3D modelling technology

464
00:33:27,040 --> 00:33:30,960
lets experts visualise
the inside of the ovens, too.

465
00:33:31,120 --> 00:33:33,120
Buried at a depth of several metres,

466
00:33:33,280 --> 00:33:37,280
they were covered with a round roof
under which the bricks were placed.

467
00:33:40,000 --> 00:33:44,200
By reproducing the exceptional
mastery of fire from the 16th century

468
00:33:44,360 --> 00:33:46,440
archaeologists
have discovered the recipe

469
00:33:46,600 --> 00:33:49,720
that allows these bricks
to be particularly resistant.

470
00:34:00,600 --> 00:34:04,280
Managing the fires,
managing the increase in temperature

471
00:34:04,440 --> 00:34:07,560
is perhaps the most difficult thing
in firing a brick kiln.

472
00:34:08,200 --> 00:34:11,560
Baking bricks directly in the fire
could make them burst.

473
00:34:12,200 --> 00:34:13,760
So the goal of ancient builders

474
00:34:13,920 --> 00:34:17,600
was to produce constant
and durable heat in their ovens.

475
00:34:17,760 --> 00:34:19,560
After being dried in the sun,

476
00:34:19,720 --> 00:34:21,800
the bricks were placed
one on top of the other,

477
00:34:21,960 --> 00:34:25,200
with enough space between them
to let the heat through.

478
00:34:25,360 --> 00:34:28,560
But how were they baked
without being burned?

479
00:34:28,720 --> 00:34:31,720
Archaeologists' research,
coupled with 3D imagery,

480
00:34:31,880 --> 00:34:34,360
allows them to better understand
heat management

481
00:34:34,520 --> 00:34:36,280
in this horseshoe-shaped oven,

482
00:34:36,440 --> 00:34:40,000
as well as the role
of the strangely-rounded earth roof.

483
00:34:40,160 --> 00:34:43,800
In fact, the fire wasn't underneath
the bricks, but next to it,

484
00:34:43,960 --> 00:34:47,840
in a tunnel,
which served as an inlet for hot air.

485
00:34:48,520 --> 00:34:51,240
The shape of the oven
forced the heat between the bricks

486
00:34:51,400 --> 00:34:52,880
and out through the chimney.

487
00:34:53,040 --> 00:34:58,680
So the baking, which approached
1000 degrees Celsius, was very even.

488
00:34:59,840 --> 00:35:04,320
They elevated brickmaking technology
to a level you don't see today.

489
00:35:04,480 --> 00:35:08,600
These are very hard bricks,
which can be reused many times.

490
00:35:08,760 --> 00:35:09,840
So, in other words,

491
00:35:10,000 --> 00:35:15,560
the Ming had actually perfected
the art of the burning of bricks,

492
00:35:15,720 --> 00:35:18,360
to the extent that
it became a military secret.

493
00:35:19,400 --> 00:35:23,000
The brickworks were guarded
24 hours a day by the Ming army,

494
00:35:23,160 --> 00:35:24,920
and worked constantly to produce

495
00:35:25,080 --> 00:35:29,200
thousands of extremely resistant
bricks, weighing up to 10 kilos.

496
00:35:33,280 --> 00:35:34,920
Nearly 4 billion of these bricks

497
00:35:35,080 --> 00:35:37,680
were carried up the mountain
to build the wall,

498
00:35:37,840 --> 00:35:40,840
but, inside, workers
had hidden a secret ingredient,

499
00:35:41,000 --> 00:35:42,400
invisible to the naked eye.

500
00:35:45,040 --> 00:35:46,480
An essential element

501
00:35:46,640 --> 00:35:50,200
that gives the entire structure
its incredible strength.

502
00:35:50,360 --> 00:35:54,920
These bricks are there
not only to provide the structure,

503
00:35:55,080 --> 00:35:58,280
but to contain the core of the wall.

504
00:35:58,440 --> 00:36:02,800
So, what is hidden
inside the Great Wall of China?

505
00:36:02,960 --> 00:36:06,720
Imagery taken by drones
reveals precious clues.

506
00:36:06,880 --> 00:36:08,400
In the middle of the mountains,

507
00:36:08,560 --> 00:36:10,560
open parts of the wall
make it possible

508
00:36:10,720 --> 00:36:14,920
to analyse the composition of the
heart of this World Heritage site.

509
00:36:15,080 --> 00:36:17,760
On a macroscopic scale,
we discover rock,

510
00:36:17,920 --> 00:36:22,120
and in particular granite,
a very dense and hard material.

511
00:36:37,800 --> 00:36:39,200
But why didn't the builders

512
00:36:39,360 --> 00:36:43,360
choose to construct the entire
width of the wall in brick?

513
00:36:44,080 --> 00:36:49,200
You wouldn't use premium materials
in locations you didn't need to.

514
00:36:49,360 --> 00:36:51,320
So, the bricks and the stone

515
00:36:51,480 --> 00:36:53,600
create the face
or the skin of the wall,

516
00:36:53,760 --> 00:36:57,080
and inside is the rubble,

517
00:36:57,240 --> 00:37:01,040
which is carved out of the rock

518
00:37:01,200 --> 00:37:03,200
in preparing the foundation.

519
00:37:03,360 --> 00:37:06,560
There's earth, which is displaced

520
00:37:06,720 --> 00:37:09,520
when you're digging down
into the clay for the bricks.

521
00:37:09,680 --> 00:37:11,920
So all the material is used.

522
00:37:13,160 --> 00:37:15,000
Today, by modelling the monument,

523
00:37:15,160 --> 00:37:17,760
investigators
can detect the invisible

524
00:37:17,920 --> 00:37:21,800
and expose the inside
of this gigantic fortification.

525
00:37:21,960 --> 00:37:23,560
Above the granite slabs,

526
00:37:23,720 --> 00:37:27,800
the Ming packed rubble
made up of granite and earth.

527
00:37:27,960 --> 00:37:31,640
Then, stone slabs were used
to seal the structure.

528
00:37:43,920 --> 00:37:48,640
Once this had been attained,
it became almost impenetrable.

529
00:37:49,280 --> 00:37:53,960
This ingenious technique was used to
build a wall of immense dimensions,

530
00:37:54,120 --> 00:37:57,360
over 7 metres high,
and even reaching 12 metres

531
00:37:57,520 --> 00:38:02,080
at the lookout towers,
which punctuate the fortification.

532
00:38:02,240 --> 00:38:05,320
Steep-sided walls
of breathtaking longevity

533
00:38:05,480 --> 00:38:09,840
whose incredible resistance
is due to a final secret ingredient.

534
00:38:10,920 --> 00:38:14,200
An ingredient
recently revealed by science.

535
00:38:16,600 --> 00:38:21,800
As well as gathering structural data
about this gigantic fortification,

536
00:38:21,960 --> 00:38:26,280
drone technology allows visualisation
of the wall like never before,

537
00:38:26,440 --> 00:38:28,160
thanks to new sensors.

538
00:38:28,320 --> 00:38:30,600
These sensors allow investigators

539
00:38:30,760 --> 00:38:33,160
to determine every material
used in the wall

540
00:38:33,320 --> 00:38:36,320
and unravel the mystery
of its strength.

541
00:38:42,320 --> 00:38:45,040
The first innovative technology
fixed to the drone

542
00:38:45,200 --> 00:38:47,680
is infra-red thermography.

543
00:38:47,840 --> 00:38:50,800
This equipment
can see through cloud and fog

544
00:38:50,960 --> 00:38:54,280
to highlight the changes in
temperature of different materials,

545
00:38:54,440 --> 00:38:57,200
revealing it
through a special colour scheme.

546
00:38:59,720 --> 00:39:02,080
Once analysed by a heat technician,

547
00:39:02,240 --> 00:39:06,200
this imagery gives precious
information about the materials used.

548
00:39:23,920 --> 00:39:25,680
Attached to the drone,

549
00:39:25,840 --> 00:39:29,800
the hyperspectral sensor records
rays invisible to the naked eye,

550
00:39:29,960 --> 00:39:32,520
by interpreting
the intensity of infra-red signals

551
00:39:32,680 --> 00:39:34,040
that reach the camera,

552
00:39:34,200 --> 00:39:37,840
on a level
which has never been attained before.

553
00:39:39,640 --> 00:39:42,240
To the naked eye,
or with a normal camera,

554
00:39:42,400 --> 00:39:44,680
we can make out
the colours of the material.

555
00:39:44,840 --> 00:39:46,600
But with the hyperspectral sensor,

556
00:39:46,760 --> 00:39:50,280
scientists can see
the chemical components of the wall.

557
00:40:12,160 --> 00:40:13,920
Thanks to this data,

558
00:40:14,080 --> 00:40:18,960
scientists can compare the chemical
composition of each part of the wall,

559
00:40:19,120 --> 00:40:21,840
and it has allowed them
to make a startling revelation

560
00:40:22,000 --> 00:40:25,280
about the ancient builders'
biggest construction secret.

561
00:40:26,040 --> 00:40:28,280
Researchers have observed
that mortar,

562
00:40:28,440 --> 00:40:32,080
which was used by the Ming
to cover bricks over 600 years ago,

563
00:40:32,240 --> 00:40:35,960
seems to have an extraordinary
chemical composition.

564
00:40:36,960 --> 00:40:40,680
This unique mortar
blends both non-organic compounds

565
00:40:40,840 --> 00:40:45,400
like rock or sand
with a mysterious organic compound.

566
00:40:46,200 --> 00:40:47,560
The Ming dynasty,

567
00:40:47,720 --> 00:40:52,600
not only were they building
with these beautiful blue bricks,

568
00:40:52,760 --> 00:40:54,840
but they also
had a secret ingredient

569
00:40:55,000 --> 00:40:58,160
used in the
mortar between those bricks.

570
00:40:58,320 --> 00:41:01,360
They were actually sealed
by a kind of concrete,

571
00:41:01,520 --> 00:41:05,480
which makes them
impenetrable against human attacks,

572
00:41:05,640 --> 00:41:07,360
but also against the elements.

573
00:41:07,520 --> 00:41:09,120
In the mountains,

574
00:41:09,280 --> 00:41:15,040
you get temperatures, which
drop down -30, -40 degrees Celsius,

575
00:41:15,200 --> 00:41:17,560
and it needs
a much more resilient mortar.

576
00:41:22,360 --> 00:41:25,520
So, what secret ingredient
did the builders use

577
00:41:25,680 --> 00:41:28,360
to give the wall
its incredible longevity?

578
00:41:28,520 --> 00:41:29,880
In the field,

579
00:41:30,040 --> 00:41:33,800
archaeologists discovered
a material that was strangely white.

580
00:41:33,960 --> 00:41:37,960
The colour, associated with the
mortar's indestructible reputation,

581
00:41:38,120 --> 00:41:40,960
has spawned a macabre legend.

582
00:41:49,000 --> 00:41:52,520
Contemporary sources...

583
00:41:52,680 --> 00:41:54,400
demonstrate that there was a belief

584
00:41:54,560 --> 00:41:56,680
that people were actually
buried in the wall,

585
00:41:56,840 --> 00:41:59,120
or the bones were ground up
to make mortar.

586
00:41:59,280 --> 00:42:02,480
So far, no bone fragments
have actually been found

587
00:42:02,640 --> 00:42:03,840
within the wall itself.

588
00:42:23,200 --> 00:42:26,320
So, if human bones
weren't used to build the wall,

589
00:42:26,480 --> 00:42:29,440
what secret
ingredient made it so robust?

590
00:42:29,920 --> 00:42:32,880
The analysis of samples
on a microscopic scale

591
00:42:33,040 --> 00:42:37,120
told experts that the mortar
was made out of sand, rock and lime,

592
00:42:37,280 --> 00:42:39,920
a material made of calcium carbonate.

593
00:42:40,600 --> 00:42:41,840
But, at first glance,

594
00:42:42,000 --> 00:42:46,160
nothing reveals the organic component
detected by infra-red.

595
00:42:47,160 --> 00:42:49,400
In order to reproduce
ancient techniques,

596
00:42:49,560 --> 00:42:51,840
during his restoration
of the Great Wall,

597
00:42:52,000 --> 00:42:54,280
Guohua Xu took a particular interest

598
00:42:54,440 --> 00:42:57,240
in his builder ancestors'
secret recipe.

599
00:43:13,280 --> 00:43:16,440
After centuries of legend,
science now has the means

600
00:43:16,600 --> 00:43:19,480
to unravel the secrets
of this wall's construction,

601
00:43:19,640 --> 00:43:23,960
by analysing material
on a molecular scale.

602
00:43:24,120 --> 00:43:27,200
In the south of Shanghai,
at Zhejiang University,

603
00:43:27,360 --> 00:43:31,240
Dr Zhang's team patiently studied
the samples of mortar

604
00:43:31,400 --> 00:43:33,480
in their laboratory.

605
00:43:36,560 --> 00:43:38,640
The mission
of this materials' specialist

606
00:43:38,800 --> 00:43:42,400
is to find the origin of the organic
molecule contained in the mortar

607
00:43:42,560 --> 00:43:45,320
using an iodine-based chemical test.

608
00:43:54,160 --> 00:43:56,360
This scientific test
proves that starch,

609
00:43:56,520 --> 00:43:59,080
a molecule that can generally
be found in plants,

610
00:43:59,240 --> 00:44:01,520
is present in this sample.

611
00:44:02,640 --> 00:44:06,160
But where does this mysterious
ingredient come from?

612
00:44:11,760 --> 00:44:14,680
Dr Zhang's research revealed
the Chinese builders' secret.

613
00:44:16,600 --> 00:44:19,400
They used an ingredient
from their daily diet

614
00:44:19,560 --> 00:44:23,040
to create one of the world's
most unusual mortars.

615
00:44:23,200 --> 00:44:24,800
And that was sticky rice.

616
00:44:25,800 --> 00:44:28,920
And the ingredient,
what makes the rice sticky,

617
00:44:29,080 --> 00:44:31,200
this was used in the wall.

618
00:44:46,120 --> 00:44:50,160
The latest 3D modelling technology
makes it possible to examine

619
00:44:50,320 --> 00:44:54,960
the exceptional properties of sticky
rice mortar on a molecular scale.

620
00:44:55,120 --> 00:44:58,440
Starch is composed of
a molecule called amylopectin,

621
00:44:58,600 --> 00:45:01,920
which has long branches that
trap molecules of calcium carbonate

622
00:45:02,080 --> 00:45:05,720
contained in lime,
preventing them from crystallising.

623
00:45:05,880 --> 00:45:09,000
Because the crystals are much smaller
than in ordinary mortar,

624
00:45:09,160 --> 00:45:12,800
they are more strongly bonded
and incredibly resistant.

625
00:45:36,200 --> 00:45:39,720
Scientists also discovered
that the Great Wall of China's mortar

626
00:45:39,880 --> 00:45:44,280
is sturdy on the outside
and soft and supple on the inside.

627
00:45:44,440 --> 00:45:46,400
An extraordinary characteristic,

628
00:45:46,560 --> 00:45:49,200
which has allowed the wall
to resist extreme temperatures

629
00:45:49,360 --> 00:45:50,520
over the centuries,

630
00:45:50,680 --> 00:45:54,600
as well as earthquakes
and even cannon fire.

631
00:45:56,320 --> 00:45:58,520
In addition to
making the mortar resistant,

632
00:45:58,680 --> 00:46:01,240
starch protects
the molecules of lime,

633
00:46:01,400 --> 00:46:04,600
restricting their contact
with any air inside the mortar.

634
00:46:05,680 --> 00:46:08,320
When the hard lime
erodes onto the surface,

635
00:46:08,480 --> 00:46:10,520
the soft lime behind it solidifies,

636
00:46:10,680 --> 00:46:13,600
adding to the wall's
exceptional longevity

637
00:46:37,840 --> 00:46:39,520
According to scientists,

638
00:46:39,680 --> 00:46:44,320
sticky rice mortar is one of Chinese
civilization's major inventions.

639
00:46:44,480 --> 00:46:49,480
This is possibly
the first known composite mortar.

640
00:46:49,640 --> 00:46:51,120
So it's the first known mortar

641
00:46:51,280 --> 00:46:55,680
that uses both inorganic compounds
like rocks or sand

642
00:46:55,840 --> 00:46:58,320
with organic compounds, rice flour.

643
00:47:18,000 --> 00:47:19,360
Decade after decade,

644
00:47:19,520 --> 00:47:23,880
the latest technology has unravelled
many of the Great Wall's mysteries.

645
00:47:24,680 --> 00:47:29,800
Like its true length,
at 21,196 kilometres,

646
00:47:29,960 --> 00:47:32,680
a large part of which
has lasted millennia,

647
00:47:32,840 --> 00:47:36,600
thanks to a structure made
of rammed earth and layers of reeds.

648
00:47:36,760 --> 00:47:41,360
And scientific analysis has shown
how clay bricks and composite mortar

649
00:47:41,520 --> 00:47:43,240
is the explanation for the resistance

650
00:47:43,400 --> 00:47:46,040
of the most well-known
sections of wall.

651
00:47:46,200 --> 00:47:50,160
But researchers think
there are still secrets to uncover.

652
00:47:50,320 --> 00:47:53,360
It's only recently
that we are really starting

653
00:47:53,520 --> 00:47:58,280
to look at exactly
how the Great Wall has been made up,

654
00:47:58,440 --> 00:48:01,840
and I think
there is so much more to discover.

655
00:48:02,000 --> 00:48:03,640
Archaeologists are still hunting

656
00:48:03,800 --> 00:48:08,080
for missing sections of the Great
Wall that remain lost underground.

657
00:48:09,000 --> 00:48:11,560
And the search
is already provoking debate

658
00:48:11,720 --> 00:48:15,200
about its official
21,000-kilometre length.

659
00:48:27,160 --> 00:48:31,200
It is as great as the greatest
natural phenomenon on Earth.

660
00:48:31,360 --> 00:48:34,360
It is like the roof of the world,
it's like the Himalayas.

661
00:48:34,520 --> 00:48:37,560
Here is something
that humans created.

662
00:48:38,480 --> 00:48:41,280
This is a monument
in harmony with nature,

663
00:48:41,440 --> 00:48:43,160
snaking across steep terrain,

664
00:48:43,320 --> 00:48:44,960
and still making specialists

665
00:48:45,120 --> 00:48:48,800
wonder about the technical
and human means used to build it.

666
00:48:56,800 --> 00:49:02,400
How the logistics were coordinated
over thousands of years,

667
00:49:02,560 --> 00:49:06,000
to maintain this momentum
building the wall.

668
00:49:06,160 --> 00:49:10,400
To me, that is a very great mystery.

669
00:49:40,320 --> 00:49:43,200
Captions C SBS Australia 2020