Nova (1974–…): Season 45, Episode 5 - Secrets of the Forbidden City - full transcript

The architectural design of Beijing's ancient complex of palaces and temples enabled it to survive centuries of earthquakes.

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It's one of the great
architectural wonders

of the world...

China's Forbidden City,

a vast collection of buildings,
all made mainly of wood.

It's the largest complex
of palace buildings

anywhere in the world.

Said to have had
more than 9,000 rooms

it was a place
of staggering power.

Home to China's rulers
for 500 years,

it was filled
with dazzling treasures.

Planned over a decade,



major construction took
just four years.

But it raises many questions.

How did they build it?

Where did
the resources come from

to create something so vast?

Behind this grand edifice
was a great superpower...

Ming Dynasty China.

They had
the most powerful economy

and technology much more
in advance of the West.

But can scientists decode
this ancient wonder,

unravel its engineering
secrets...

And discover how it withstood

one of nature's great forces...

Earthquakes.



Earthquakes are frequent here,

so they pose a huge threat
to our heritage.

What's it going to take
to knock this thing down?

Closed to the world
for centuries...

Now open to reveal

the "Secrets of
the Forbidden City."

Right now on "NOVA."

The Forbidden City,
in the heart of Beijing.

It's about the size
of about 140 football fields

and has at least 800 buildings.

Completed in year 1420,
it was a pleasure palace,

an impregnable fortress,
and a technological marvel.

With the top craftsmen, top
carpenters, and top materials,

so it's the best example
of the technology at that time.

A seat of immense power...

And mystery.

Economically important,

politically important,
and militarily important.

The most important site
in the whole of Asia

for the last 500 years.

That power was embodied

in the Forbidden City's
awe-inspiring grandeur.

This is the largest

palace complex in the world,

and not just the largest palace
complex that survives,

but the largest palace complex
built at any time in history.

The scale of what we see here
is unparalleled

in the world at the time.

And everyone was struck by this

because they would never, ever
see anything like this

in their lives.

And they knew this.

They knew that
this was unique in the world.

Built primarily of wood,
the complex follows

a clearly laid-out plan,
aligned on a north-south axis.

It was home to vast halls...

...the private quarters
of the imperial family;

temples reflecting the many
forms of worship observed there,

including Buddhism, Daoism,
and Confucian philosophy.

And it was built
at an incredible pace.

Major construction

of the original Forbidden City
buildings took just four years.

In comparison,
the great cathedrals of Europe

took decades, even centuries,
to complete.

There are two pressing questions

at the heart
of the Forbidden City.

How was it built on such
a massive scale so quickly?

And how has it has survived
the powerful earthquakes

that have repeatedly struck
the region over the centuries?

China's extensive imperial
documents go back

thousands of years,

recording everything
from battles to earthquakes,

they enable researchers
to build up a detailed picture

of China's seismic history.

This is an earthquake fault map

of northern China.

And Beijing is located here,
right by this fault.

All the red stars are
earthquakes

that occurred
in the past 3,000 years or so,

recorded in
the historical books.

So, since the construction
of the Forbidden City,

we can see four large
earthquakes.

They really jump around
from fault to fault.

In 1976, a massive earthquake
shook Beijing.

At magnitude 7.8,

it was centered
in Tangshan City,

around 90 miles east
of the capital.

In just 15 seconds,

it killed at least
a quarter of a million people...

...the highest death toll
of any earthquake

in the 20th century.

But in this
earthquake-prone region,

the Forbidden City
has never been brought down.

What is the secret
of its resilience?

No Ming dynasty plans

for the original great structure
have survived.

Builders and engineers have
gained a wealth of knowledge

as they have restored
this aging complex.

All of the main buildings

have a supporting wooden
structural frame at their heart

with walls made of brick
and great tiled roofs.

But why and how
was the Forbidden City built?

It was the idea of one man...

A military commander, emperor,
and visionary known as Yongle.

Without the Yongle Emperor,
you simply would not have had

a Forbidden City
that we see today.

Chinese history would have taken
a completely different path

and China would not have
a civilization that lasted

down to the present day.

Ruling from 1402 to 1424,

he was the third emperor
of the Ming dynasty.

The Ming drew on earlier
technologies unique to China.

Most importantly was
printing, paper...

gunpowder,

and also nautical technology.

These were all things
that were unknown in the West.

Even lavatory paper.

The vast Ming empire stretched

from what is now Vietnam
almost to Mongolia.

Both a scholar,
and a military commander,

Emperor Yongle built
the Forbidden City

as the center
of his new capital at Beijing.

It was an historic decision,

built to shift control
into his own northern powerbase

and to be close to the
troublesome border plains

occupied by the Mongols.

But unbeknownst to him, Beijing
is in an earthquake zone.

Not only are earthquakes
here frequent,

their causes are
not clearly understood.

By contrast, a massive
20th-century earthquake

took place in 1906
in San Francisco,

followed by a deadly fire.

The city was virtually
flattened,

and the reason is clear.

San Francisco sits beside
the San Andreas fault,

which penetrates
deep into the earth

between two tectonic plates:

the North American and Pacific.

The two plates are moving
against each other,

in particular the Pacific Plate
is moving northward

relative to North America.

These plates move
past each other,

but when they become stuck,
tension builds up.

When it's released,
the result is an earthquake.

If you look

around the world,
90% of the earthquakes

occurred along boundaries
between plates.

So North China, which sits
in the middle of a plate,

should be stable, but it's not.

You have to keep in mind
that this area is far away

from plate boundaries.

So this is a very peculiar place

to have earthquakes
inside the plate.

In North China,

the earth's crust is unusually
thin and weak,

giving rise to faults
within the plate itself.

These are known
as intra-plate faults.

The North China plate
is cut into three pieces

by three long faults.

So sometimes the energy
is released on this fault.

Sometimes on that one,
sometimes on this one.

Over the centuries,
the Beijing region has been hit

by several quakes of similar
size to the San Francisco killer

and scores of smaller ones.

Dr. Zhou, the Forbidden City's
resident seismologist,

believes the Forbidden City

was designed
with earthquakes in mind.

They had a lot of experience
with earthquakes,

which shaped the building
of the Forbidden City.

In the centuries before
the Forbidden City was built,

the Chinese had learned
to create resilient structures

using wood.

American woodworker
Richard Wiborg...

This is incredible.

...has long made a study

of China's ancient
wooden structures.

I saw a photograph of a man
sitting on the roof of a pagoda,

and, in comparison to the size
of the timbers, he looked tiny.

And just the massive quality
of the wood,

the intricacy, the number
of pieces connected together.

A light went on and I said,

"I've gotta find out more
about this."

A resident of San Francisco,
Richard wants to understand

how the buildings
of the Forbidden City

have withstood earthquakes.

Today, he's visiting
Tai Miao Temple,

the temple of ancestral worship,
built by Emperor Yongle in 1420.

This huge hall was where
sacrifices were made

to honor the emperor's ancestors

in the hope of bringing
good fortune.

Richard climbs up
to the ceiling to investigate

some of the key structural
elements of Ming architecture.

Here we are in a place
I never imagined I'd be,

20 feet off ground
in a Ming Imperial building.

High up, Richard gets right next
to a set of brackets

that link the timber frame
to the roof.

These are called dougong,

and they support the roof
and stabilize the building.

A bracket set
is a complicated puzzle,

many different parts

that fit together
to make a whole,

(and what we're hoping to
discover is A) how they're made

(and B) why they were made.

Simple dougong brackets
go back 3,000 years.

They gradually became more
and more complex.

By the time
the Forbidden City was built,

they had become essential
structural elements.

Every building
has a forest of dougongs,

linking the columns and beams
to the roof.

In Beijing's
architecture museum,

Richard Wiborg takes one
of these brackets apart.

Now, after taking a deep breath,

we'll try and put this
back together again.

The only things
holding it together

are precise carpentry and
layers of interlocking pieces.

You can see this isn't...

Even for somebody who's done it
a few times...

It's complicated.

One piece goes
in the front here.

A bracket set.

So, you can see
it's a little bit flexible.

It's very strong...
Pressing down,

it can take a tremendous
amount of weight.

It ties together with other
parts of the building

and is a beautiful, creative
feature of Chinese architecture.

A key function of
the dougong brackets

is to distribute the heavy
weight of the tile roof.

They do that by channeling
the load evenly

onto horizontal beams,

and down through the narrow
columns supporting the roof.

The dougong has the role

of connecting the columns

to the purlin and the rafter
that forms the roof.

The dougong is like an element
that funnels in all the loads

and redistributes it
on the top of the column

in a very uniform way.

But what role do the dougong
play in protecting the buildings

of the Forbidden City from the
region's powerful earthquakes?

A unique scale model

of a classic Forbidden City
structure,

one-fifth the original size,
will reveal the answer

when tested under simulated
earthquake conditions.

The model has four key
structural elements:

vertical columns
and horizontal beams

that make a timber frame.

So we're seeing a column and
a horizontal beam go together.

Well done!

The frame holds the dougong

that will support
the large tiled roof.

This is the base block
of a bracket set.

And like the brackets
in the Forbidden City itself,

this set is assembled
without any fasteners.

There are no nails,
screws, or glue.

It's sort of like
a wooden puzzle.

It's so fabulously complex.

This is a wonderful combination
of form and function.

Very artistic, very imaginative,
and very strong.

It's a very sophisticated piece
of woodworking,

and we're going to see
what will happen to it

when we subject it
to the extreme forces

of the earthquake test.

This model is patterned on
a building in the private area

of the Forbidden City:

the Palace
of Longevity and Health.

It's a typical
Forbidden City structure.

What you immediately notice

is this huge golden roof
that curves outwards

away from the building in a way

that you just don't see
in European architecture.

Looking up here,

what we have is these
elegantly colored crossbeams

decorated with imperial dragons
and phoenixes,

and they're there to support
these huge red columns

that bear the structural load
of that massive roof.

And if you look up here
in between the crossbeams

and the roof,

underneath that mesh,

there's these
flower-shaped dougong.

Together, these four
structural components...

Columns, beams,
dougongs, and roof...

Somehow protect these buildings
from earthquakes.

But how do they work together?

To find the answer,
the scale model is re-assembled

in a seismic lab...

...ready for a series
of shake table tests

that will subject it to dozens
of simulated earthquakes.

These will push the columns,
beams, brackets, and roof

to their limits.

Wow, the roof is so big,
it's amazing.

Yes, it is.

This is a very authentic,

traditional timber frame
building.

The model has traditional,
Ming-style brick walls,

two layers of brickwork
with rubble between.

Yes, we are ready to start.

Okay, let's go.

Altogether,
it weighs over three tons;

and could endanger
anyone standing nearby.

So, a safe zone is created
around the shake table

in all directions.

Test one, start!

As the test begins,

the shake table moves
horizontally,

replicating the energy
and impact of an earthquake.

Gradually,
the acceleration increases.

Finally, three tons
of Ming-style architecture

starts to move.

The engineers have never tested
a model like this before.

No one knows
how long it will last.

An earthquake's size
is defined by its magnitude:

the amount of energy released.

Magnitude 4 is generally
considered a weak earthquake

that most modern structures
can survive.

But what about the model,
based on an ancient design?

The magnitude rises gradually.

The model survives a 4,
and then a 4.5.

The sound was like
the creaking of a ship,

the wood compressing,
twisting, groaning,

but no obvious damage.

Next is magnitude 5,

a massive increase, with over
30 times the energy of a 4.

The movement piles pressure onto
all the structural components.

As the magnitude
progresses beyond 5,

the sideways movement
proves too much

for one of the brick walls.

Maybe we'll take one step back.

Yes, we are worried.

It's unpredictable.

While the timber frame flexes,
the walls are just too brittle.

The second wall topples.

There it goes.

In traditional Chinese building,

the walls have only the role of
defining and dividing the space.

They don't really have
a structural role.

Because they are not supporting
the roof,

their failure does not threaten
the overall stability

of the structure.

That was pretty exciting.

We can see the walls

have completely collapsed.

But the traditional timber frame
is still standing strong.

As the test proceeds
to magnitude 7,

the model is shaking,
but it remains intact.

What role do the dougong
brackets play in this.

The timber frames move
during the earthquake

because of the dougong,

and because of this very
particular connection.

Through their ingenious design,

the dougong disperse
the energy of the tremors.

So as the vibration
comes through the columns,

and into the dougong,

each of the dougong parts
move relative to each other.

So, through friction,

this movement
will dissipate the energy,

reducing the overall effects
of the earthquake

on the integrity
of the structure.

In addition
to structural elements,

to build the Forbidden City,

Ming architects and engineers
had to come up with ideas

and innovations
on an enormous scale.

Before the main building phase
could begin,

a decade of preparation
was needed

to set up a vast
and complex system of logistics,

to conscript a huge labor force,

and create a massive supply
chain across China.

Because Beijing
lacks natural resources,

virtually everything
had to be brought here,

including much of the wood.

The Forbidden City's
palaces, temples,

and the Emperor Yongle's
great halls are dominated

by tall wooden columns.

In the ancestral hall
at Tai Miao temple,

42-foot-tall columns sparkle
as if speckled with gold.

Look at this magnificent column,

so round and so smooth.

Imagine how big the tree was
that this came from.

In fact, we have 68 columns
in total for this hall,

and they're supporting
the structure.

And they look beautiful as well.

This is a single log
of a nanmu tree.

Today, one of the rarest
Chinese woods,

it's straight, dense,
and resistant to rot.

Just one of these great logs
weighed over 40 tons.

But nanmu trees grow more than
a thousand miles from Beijing,

in Sichuan to the southwest,

hidden deep
in mountainous forests.

Felling the trees,
and then dragging the heavy logs

to the Yangtze River for
transportation, was treacherous.

According to a local saying:

"1,000 people
go into the mountains,

and only 500 come out."

The mortality rate was terrible.

Despite the cost and risks,

the emperor demanded nearly
100,000 logs to be imported

from the southwest.

These alone could take
four years to reach Beijing.

The Forbidden City also needed
golden-colored floor tiles,

made with special clay in
Jiangsu Province 600 miles away.

Perfectly polished,

each tile took
more than a year to make.

Eighty million bricks
were produced in kilns

on an industrial scale
outside Beijing.

And piles of gold leaf,
hammered by hand...

...to one-five-hundredth
the thickness of a human hair,

produced in specialized
workshops in distant Nanjing.

But how could all these supplies
reach Beijing at the right time?

To accomplish this,

the emperor's engineers extended
China's ancient Grand Canal.

A simpler version
was constructed

over a thousand years earlier.

But Yongle revitalized
and expanded it

into a water super-highway
able to carry everything needed

to Beijing.

The Grand Canal

is both longer and older

than the Panama Canal
and the Suez Canal.

They really solved a very large
problem of canal engineering

at the time.

The Grand Canal ran over 1,000
miles from commercial Hangzhou

across the Yangtze and Yellow
rivers before reaching Beijing.

This is the main channel
of the Grand Canal.

To the south is Hangzhou;
to the north is Beijing.

Today, this section
is mostly dry,

but it's still possible
to see the remains

of the Ming's water engineering.

This is what it would have
looked like 600 years ago.

It would have been full
of barges carrying workers,

soldiers, materials
and grain supplies north,

to feed the huge Beijing
building site.

But before this could happen,

the Ming faced an enormous
engineering challenge.

Standing in the way of the canal
was a series of hills.

The high point of the canal
was at Nanwang,

300 miles south of Beijing.

This point here is approximately

the watershed between
the north and south section

of the Grand Canal.

So if we were to drop water
here,

water on this side of the line
would flow north,

water on this side of the line
would flow south.

To get the canal traffic
over the high ground at Nanwang

meant that millions of gallons
of water had to be diverted

from nearby rivers.

Here we are

at the center of the
Nanwang water diversion project,

as part of the Grand Canal
system.

This Ming dynasty map
shows how rivers were dammed

to feed water
into huge locks at Nanwang.

These large enclosures
could be flooded and drained

to raise and lower barges,

allowing them to get over
the high ground.

To fill the locks
and raise the barges,

water had to be diverted
from the Wen River

and stored in three huge
man-made lakes.

Barges travelling up the canal
to Nanwang

entered a series of locks
over three miles long.

The river water poured in
to raise the barges,

so they could proceed,
step by step,

over the high ground
toward Beijing.

As a modern-day engineer,

I think the main reason why
this is so impressive

is partly because of the size,

but also because
of the complexity

of trying to manage the water
over such a large area.

That's a huge achievement,

even in modern-day
engineering terms.

For just this section
of the canal

there was 300,000 laborers
conscripted,

working for a period
of 200 days constantly.

And they created this with only
the most rudimentary hand tools.

But how could the Ming
afford this?

The cost was borne
by the Emperor's subjects

in money,

materials,

and lives.

Tens of thousands of people

starved to death.

He worked convicts,

he worked the military,
he worked ordinary peasants,

he squeezed as much as he could
out of the taxes,

in order to build the capital.

The result was
an engineering masterpiece.

Much of the Grand Canal
is still in use today.

This massive infrastructure
project changed China forever.

Just as the Forbidden City

altered the landscape
of Beijing itself.

This hill gives us
an amazing aerial view

of the Forbidden City.

What's more incredible is that
prior to the construction,

this hill didn't even exist.

It was actually built

using the earth
excavated from the moat.

All this was produced through
the meticulous planning

Emperor Yongle demanded.

He insisted on controlling
each step

of the vast project himself.

But there were some forces

neither he nor his successors
could control.

Back at the seismic lab,
Dr. Zhou and Richard Wiborg

are ready to continue
the shake table tests

on the scale model
of a Forbidden City building.

They're still there, too.

Having reached magnitude 7 with
only the brick walls falling,

they now want to see if the
model can withstand the force

of an even greater earthquake...

Magnitude 7.8.

This matches the size of the
1976 Tangshan City earthquake

that killed a quarter of a
million people east of Beijing.

It's a massive human disaster.

And many, many,
the whole families

not a single person
survived this event.

In just 15 seconds,
85% percent of the structures

in the city were destroyed.

The Tangshan area

has this traditional stone
type of buildings.

So basically all the walls,
all the houses,

they were all constructed
by very heavy rocks.

Good for insulating
heat and cold,

but they're not very good
at resisting earthquakes.

If the heavy stone buildings
of Tangshan failed to resist

a magnitude 7.8 earthquake...

how will the wooden model
perform under a similar assault?

Get ready to jump.

Whoa!

It scared me, I'll tell you.

Will the columns and beams,

the dougong brackets,

and the heavy tile roof

stand up to such
powerful shaking?

Amazingly, even after a period
of 30 seconds...

Longer than many real-life
earthquakes... they do.

What's it going to take
to knock this thing down?

When Dr. Zhou and Richard
examine the model,

they find one more secret to its
resilience... the columns.

This red line shows
the column's position

before the earthquake.

We can see that
after the earthquake

the column has moved over here.

The model replicates the way

the columns stand
in the Forbidden City itself,

resting freely
on stone foundation plinths.

If they were pounded
deep into the stone,

they would break if overwhelmed
by the shaking of an earthquake.

Instead,
by being left free to move,

they can ride out the tremor.

The model shows
how the columns move,

dispersing energy,
and helping the whole building

withstand the shaking.

What we see here is the effect
of the shaking on the columns.

We can see that all the columns
rotate slightly.

This helps to dissipate energy,

and to isolate the movement
to the roof.

In that way the structure
is still surviving.

But what about
the large, heavy roof?

Its curved shape is a tradition

that dates back 2,000 years.

To build the roof, timber boards
are laid onto the rafters.

On the outside,
layers of red clay

are used to waterproof it

before glazed tiles
are pushed into the clay

and overlapped.

The colorful roof tiles

and mythical beasts
that adorn them

are still produced in kilns
outside Beijing

using traditional methods.

The area's unusual clay
produces long-lasting tiles.

But they are heavy.

The roof of a big hall
in the Forbidden City

is estimated to weigh
2,000 tons...

all that weight resting on
the dougong brackets

connecting it
to the columns below.

Exactly the same tiles

and construction technique

have been used on the
one-fifth scale test model.

Look at the roof moving

It's swings back into place,
helping it to recover.

Because of the interlocking
construction,

the dougong, roof, and frame
are connected

but can move independently,
isolated from each other.

The timber frame
is moving at a higher speed,

while the roof is moving
at a much lower speed.

Slow motion reveals
how the movement of the roof

lags behind the timber frame.

And while the bottom of the
structure shifts

in one direction,

the roof moves the opposite way,

countering the effect
of the shaking.

There is almost
like a re-centering action here

because as the timber frame

tends to move out
in one direction,

and the roof is still moving
in the opposite direction,

it pulls it back towards
the resting position.

The roof acts
as a counterweight,

balancing the structure,

and saving the building
from collapse.

But state of the art
engineering, carpentry,

and logistics alone
did not satisfy the ambitions

of the Ming Emperor Yongle.

He also demanded artistry.

The centerpiece of the
Hall of Preserving Harmony

is a magnificent
grand staircase.

It's the largest stone sculpture
in the Forbidden City,

showing dragons
playing in the clouds.

Hand-carved from
a single vast block of marble

more than 50 feet long,
it weighs over 300 tons.

More than two jet airliners.

This marble came from quarries
40 miles from Beijing.

But getting the raw stone from
the quarry to the Forbidden City

was a major challenge.

After the stones were quarried,

they were too large
for human labor to move.

So, what did they do?

Beijing's climate is harsh,

and in the winter,
the area freezes over.

Ming records describe
an ice road,

shaped by engineers
to cross any obstacle.

But how did the Ming
move the massive marble blocks

along the ice roads?

Rob Dwyer-Joyce is an engineer
who researches friction.

He has designed an experiment
to answer this question.

What we have is a wooden sled.

And we've loaded it with stone
weighing about a ton,

1,000 kilograms or 2,200 pounds.

And we're going to do
an experiment

to try to pull this along
by hand on the ice.

This is going to be
quite difficult.

Because it's...

No, can't get it shifted.

As they struggle,

a whole ice hockey team
comes to the rescue.

But even these strong athletes
have a hard time

getting it moving.

Whoa!

So how much force does it take

to get a one-ton sled
moving across the ice?

So now we put a load cell

in front of the sled,

and we're going to measure
the force on this load cell

as the guys start pulling.

And now let's have
some more skaters.

Again.

What we can see here is
the force versus time graph.

As we added a player you can see
it increases in steps.

And then it gets
to a certain force

and then drops.

And this drops because
the sledge starts to slide.

And when we do that,
the force required

to pull the sledge along
plateaus.

It takes a team of players
to get the sled moving.

But only one to keep it moving.

So the challenge
is to get it started.

We know from historical sources
that the Chinese

had water supplies
along the route

that they were moving
the stones.

And it's not quite clear
what they used that water for.

Ancient documents describe wells
dug at 550-yard intervals

along the ice road.

Rob has a theory
that the wells provided water

to help get the huge marble
blocks moving along the road.

I'm going to put
some water down there.

And see how many we need when
we've slightly wetted the ice.

Go on, pull it, pull it, guys.

The water acts as a lubricant,

between the ice and the sled,

making it easier
to get it to start.

And once it's moving,

continuing pressure
melts the ice,

keeping the sled going.

The Chinese moved
these large lumps of stone

on a much bigger scale
than what we've done today.

I rather admire them
for having done that

several hundred years ago
to realize that

once the sled was moving
it generated friction,

but they probably needed water

to form that film
between the sled and the ice

to get started
in the first place.

This level of engineering skill
made it possible for artists

to carve the magnificent
marble staircase

at the Hall
of Preserving Harmony.

But similar artistry is found
throughout the Forbidden City.

This is epitomized by the use
of a vast array of colors...

Turquoise, blue and green...

Both inside and outside
the buildings.

New discoveries
about the Forbidden City's paint

are now coming to light.

The Hall of Mental Cultivation

was built in a quiet, private
area of the Forbidden City,

which is being restored.

Dr. Lei and his team are working
to identify the precise pigments

used in the paints.

The paint is falling off.

So first of all we need to find
out what type of paint it is.

The scanner emits x-rays,
which are reflected back

at a unique frequency
for each element

in the pigment.

Back in his lab,

Dr. Lei uses both the scan data

and the microscopic examination
of paint fragments

to identify
the the precise pigments used.

It looks like
a man-made pigment.

It's called smalt.

Blue smalt is not
a naturally occurring pigment.

And it wasn't made in China.

It's a synthetic color,

made by adding cobalt oxide
to molten glass.

It was made in Europe
after China's Ming period.

So these layers of paint
were likely applied

during a later restoration,

and may be evidence

of international trade
and exchange.

It also suggests
that no expense was spared

to decorate the Forbidden City

with the world's
finest materials.

Today, the Forbidden City
is a World Heritage Site

that attracts 16 million
visitors a year.

But 600 years ago
these halls and courtyards

were reserved for the elite.

The name itself,
the Forbidden City,

indicates that ordinary people
were not welcome here.

It took over a million laborers

to build the expansive complex.

In December 1420,
it was complete.

Diplomats and dignitaries
arrived from across Asia

for its grand opening ceremony.

Visiting ambassadors
describe how elephants

flanked the main gate.

A hundred thousand people
from the Ming empire and beyond

filled the great square.

They have never seen
this level of craftsmanship,

they've never the scale of
Beijing or the Forbidden City.

They're overwhelmed
by just the physical space.

Of course that is the intention
of building the Forbidden City.

It's this statement
of political power...

The power of the dynasty but
also the power of the emperor,

the man who sits on the throne.

And for most people of the time,
they were persuaded.

Yeah, that's right,

this is the most powerful man
in the world.

The Emperor's day
typically began

at the Palace
of Heavenly Purity.

This courtyard,
and the palace behind me,

is the most sacred location
in the entire Forbidden City.

This was Yongle's residence.

And every morning he would rise,
at about 4:00.

And he would come down
this central pathway,

only he could pass along
the central line

for the dawn audience.

At daybreak,

hundreds of officials
would gather in silence

to kowtow before his majesty...

Filling the courtyard in front

of the city's grandest
structure,

the Hall of Supreme Harmony.

In the very center
is the throne.

And above you have
the symbol of the heavens,

and the throne is directly
underneath the heaven.

Why?

Because the emperor
is the son of heaven.

China's traditional
belief system

demanded that the emperor
constantly prove

he had the mandate of heaven.

And he had to demonstrate as
well that he was worthy to rule

by maintaining a careful balance

between the earthly
and cosmic orders.

But that harmony
could be disrupted by nature.

Fire, caused by accidents
or lightning strikes,

was a constant threat to these
mainly wooden structures.

The Hall of Supreme Harmony
itself burned down many times.

But another great fear
was earthquakes.

On the morning
of September 2, 1679,

Beijing was struck by disaster.

The city was hit by
one of the biggest earthquakes

in its history.

Imperial records suggest it
killed around 15,000 people.

China's detailed
historic records

hold valuable clues,

which Professor An Yin
uses to work out the magnitude

and nature
of ancient earthquakes,

including the one in 1679.

There is some kind of tombstone,
which is supposed

to face to one direction,
usually faced to the south.

But that tombstone rotate.

So that is very important
to geologists

because we can use
those historical records

to determine the way
the earthquake was generated...

Whether it's moving
up and down or sideways.

So for this one it has to be
moved by the sideways.

Imperial Records suggest
the 1679 quake

was a magnitude 8.

It destroyed thousands
of stone structures,

including the original
white pagoda, or stupa...

For Buddhists,
a place of meditation.

So here we see the white stupa,
constructed originally in 1651

and then destroyed
in a massive earthquake,

but then almost immediately
rebuilt.

The restored stupa still towers

over the Imperial Park
and Beijing.

But while the original stupa
collapsed in 1679,

records indicate that
the Forbidden City,

just 500 yards away,

suffered only minor damage,
mainly to the brick walls.

Most descriptions focus on the
city walls and the city gates,

it seems like the major palaces'
architecture are not damaged.

But can the model,
in its final seismic test,

withstand a force even greater
than the 1679 earthquake?

The scientists will now expose
the model to magnitude 8.2,

the power of the largest
earthquake ever to hit

the Beijing region.

And stronger
than the 1906 earthquake

that destroyed San Francisco.

Magnitude 8.2, begin!

Now the quake
is getting stronger.

The frame is shaking badly.

Pretty darned strong.

The test moves to magnitude 8.6,

the strongest earthquake
to hit this part of Asia.

It shook Tibet in 1950,
destroying over 70 villages

and causing massive landslides.

As the magnitude increases,
and the shake table

delivers more energy,

the structure sways
dramatically.

But it remains standing.

This is incredible.

After a series
of escalating shocks,

the test now moves beyond 9.5...

The largest quake
in recorded history.

This devastated Chile in 1960,

causing a tsunami
as far away as Hawaii.

Magnitude 10.1, begin!

In this final phase,

the shake table peaks
at magnitude 10.1,

as high as the technology
can go.

The test must come to an end.

Test finished!

We wanted to break this building

and it wouldn't break,
so, wonderful test.

What was most surprising to me

was how much the building
can move,

how flexible it was.

This is a fabulous proof

of the genius of Chinese
traditional architecture.

To build the Forbidden City
600 years ago,

the Ming Chinese had to meet
a vast array of challenges...

Mastering logistics on land,

water, and ice...

Applying engineering ingenuity

and carpentry skill,

and demonstrating artistry
of color and design.

It cost thousands of lives
and the toil of a nation,

but they built a city
unique in all the world,

at least 800 buildings,
mainly all out of wood.

It was the Ming dynasty's
most enduring creation,

the beating heart
of their empire,

the Forbidden City.

This "NOVA" program
is available on DVD.

"NOVA" is also available
for download on iTunes.