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NARRATOR: Notre Dame de Paris--

Official YIFY movies site: YTS.MX

a treasured icon of Gothic architecture

and medieval engineering,

built from glass,

stone,

and timber over the course

of two centuries.

For 850 years, this 226-foot-tall cathedral

has been an enduring symbol at the heart

of French culture, and more...

JOHN DICKAS: Notre Dame is one of humanity's greatest

artistic and architectural achievements.

PATRICK CHAUVET (translated): Notre Dame is not just Paris.

It's France.

And beyond France,

it's the world.

NARRATOR: But on April 15, 2019,

a disaster that threatens to destroy it all strikes.

A massive fire raging out of control...

(person gasping, cries out)

MAN: Oh, my God!

NARRATOR: ...leaves the cathedral in ruins.

♪ ♪

Now, an elite team of engineers,

scientists,

and master craftspeople,

battle to save this fragile structure

from a catastrophic collapse.

(alarm blaring) LISE LEROUX (in French):

♪ ♪

NARRATOR: Out of tragedy, an opportunity is born...

Oh!

This is a dating fossil.

NARRATOR: ...to solve archaeological mysteries

and understand the very fabric

of this medieval megastructure

like never before.

CLAUDINE LOISEL: We can identify each chemical element.

♪ ♪

NARRATOR: Can clues from the past

help save and rebuild this landmark?

And can pioneering technology

prevent another disaster?

(mechanism whirring)

What we are producing today

will be the information usable for the next generations.

NARRATOR: "Saving Notre Dame"--

right now, on "NOVA."

♪ ♪

ANNOUNCER: Major funding for "NOVA" is provided by the following:

♪ ♪

NARRATOR: The Cathedral of Notre Dame de Paris--

an 850-year-old Gothic wonder.

It's the heart of France.

The distance from Paris to all other places

is traditionally measured from this iconic structure.

SANDRON: There is a continuation,

a historical continuation,

from the Middle Ages to nowadays.

And it's very important to build a kind of identity.

Notre Dame is one of the monuments

which achieved this identity.

For Christians, it's a place of worship, right?

And, and for those of us with different beliefs,

it's one of just this incredible artistic and historic landmark.

You've had coronations there,

you've had the crowning of Napoleon and King Henry.

There's just so much attached

to the cathedral.

NARRATOR: But Notre Dame is much more than that.

It's also a pinnacle of medieval engineering.

The cathedral can hold 9,000 worshippers,

and its 100-foot tall walls

contain more than 32,000 square feet of stained glass.

The ceiling is a series of domed Gothic vaults

that hold up the cathedral from the inside.

A complex 550-ton web of timber

forms a cross-shaped roof,

topped with 1,300 lead tiles

and a 300-foot tall central spire.

Wrapped around the church are 28 flying buttresses,

limestone arches that brace the walls from the outside.

And at the front, two mighty towers,

with ten massive bronze bells inside,

soar over 226 feet into the sky over Paris.

SANDRON: The construction took many generations.

Architecture was not learned at the university,

so the architects and all workers learned mostly on site.

NARRATOR: Along the way, there were many setbacks.

In 1789, at the height of the French Revolution,

anti-Catholic forces

destroy parts of the Cathedral.

A newly secular France leaves Notre Dame

in a state of neglect.

But when Victor Hugo writes "The Hunchback of Notre Dame"

in 1831, it sparks a $60 million restoration,

that tops out the cathedral with a new roof

and a 750-ton timber and lead spire.

Periodic renovations continue to this day.

On April 15, 2019,

Notre Dame is wrapped in 550 tons of scaffolding,

as workers begin

a $6 million operation to shore up the cathedral's spire.

CHAUVET AND MADO:

CHAUVET:

NARRATOR: Notre Dame's rector,

Father Patrick Chauvet,

has finished evening worship.

His world is about to be turned upside down.

CHAUVET (translated): I stopped here because

I really like Mado.

She offered me a drink, and when she came back she said,

"Father, there's smoke above the spire of the cathedral."

So I left my drink and went back to check there was nobody

in the cathedral.

NARRATOR: At 6:18 p.m., a sensor detects smoke

in the medieval roof timbers.

The system sends a coded fire alert to the security team.

Instead of heading straight for the roof,

a guard is dispatched to the sacristy building nearby,

to check for a fire.

But he finds nothing.

He climbs up into the church attic.

But by the time he gets there, he's too late.

The fire has been burning for almost 30 minutes

and has spread across the roof.

(siren blaring)

DICKAS: And there was this horrifyingly

huge plume of smoke billowing up out of it.

It was surreal.

I'd never seen anything like that before.

(sirens blaring)

You saw the fire trucks come up

alongside the cathedral and ladders went up,

and the hoses came out, you could see that the ladders

were just too small for a building of this size

and the hoses were not nearly big enough

for this kind of blaze.

It was tragic; the resources

that were available were not going to be

what was needed to bring this thing under control.

♪ ♪

A lot of us realized

that this fire was just going to ravage the cathedral.

MIKA (translated): We saw what was happening,

but we were powerless,

we could do nothing.

It really looked like the end of the world.

It was so chaotic.

A delay in responding to a fire of this nature

is absolutely critical.

A small fire burning locally is a very different thing

than ten minutes later when all of the timber elements

are involved.

So in a situation like this,

five, ten, 30 minutes can make all the difference.

NARRATOR: This delay will have huge repercussions.

As firefighters arrive on scene,

so does one of France's chief architects

of historic monuments,

Rémi Fromont.

FROMONT (translated): I managed to pass the police checkpoint

and I joined the firefighters.

NARRATOR: As the inferno rages at the top of the cathedral,

Rémi risks his life to venture inside

with the firefighters.

(translated): We did a tour of the cathedral several times.

We checked the nave.

I saw the flames and saw the blaze.

I gave them all the advice that I could.

NARRATOR: Within minutes, the firefighters are pumping tons of water

into the roof space,

but to no avail.

To the horror of the growing crowd,

the fire engulfs the iconic spire.

♪ ♪

The world watches helplessly

as the 750 ton oak and lead masterpiece gives way.

(people gasping)

MAN: Oh, my God!

Oh, my God...

That is awful.

BISBY: When the spire

fell into the roof, additional ventilation

will have caused more oxygen-rich air

to be sucked in at the bottom of the compartment.

That influx of oxygen

could have caused an increase in the severity of the fire

within Notre Dame.

(translated): All of a sudden, there was a huge, huge ball of fire

rising out of the cathedral.

It was spitting ash and debris everywhere, so we took shelter.

DICKAS: It was just devastating to watch.

We were suddenly really aware

that of, of how easily this whole thing could come down.

♪ ♪

NARRATOR: 90 minutes after the fire begins,

the entire roof of the cathedral is ablaze.

Inside, it's become even more dangerous for Rémi

and the firefighters.

Getting this fire under control looks impossible.

FROMONT (translated): The fire on the ground,

smoke everywhere, a hole in the ceiling.

We were trying to understand what was going on,

where the problems where, check what had collapsed

and if there were other risks.

NARRATOR: A southeasterly wind picks up

and pushes the blaze towards the famous bell towers.

FROMONT (translated): If the bell towers catch fire,

and the bells fall,

then they will smash through everything below.

NARRATOR: Inside the ingeniously engineered

13th century north tower

a scaffold of wooden beams holds eight bells.

The biggest weighing more than four tons.

If the beams burn through,

they'll spark a fatal chain reaction,

causing the bells to fall like wrecking balls,

destroying the tower's wooden backbone.

If the tower falls, it could trigger a deadly domino effect

that brings down the entire cathedral.

(sirens blaring)

To avert this catastrophic collapse,

the firefighters have no option

but to venture deeper inside.

CHAUVET (translated): President Macron said: "No doubt,

we must send the firefighters in.

The cathedral must be saved."

♪ ♪

FROMONT (translated): We headed to the North tower

just when the flames had reached the belfry.

♪ ♪

The firefighters also knew it well.

We were guiding each other.

♪ ♪

NARRATOR: To douse the fire on the roof, firefighters pump water

from the River Seine and feed it to fire trucks

around the cathedral.

But to stop the towers collapsing they must send a team

into the burning structure.

Their mission: drop hoses in between the towers

and fight the fire spreading from the roof.

But the steady wind doesn't let up.

And despite their efforts, the timber frame holding the bells

has caught fire and could trigger the destruction

of the cathedral at any moment.

So the team must drag their hoses to the top of the tower

and soak the timber frame to prevent the unthinkable.

Throughout the night,

the fate of Notre Dame hangs in the balance.

Eventually the firefighters get the upper hand.

The flames have been beaten back and only glowing embers

light up the night sky.

Nobody knows how the fire started.

An investigation begins.

But for now, the urgent question:

how damaged is the structure

and can it ever be rebuilt?

President Macron pledges to restore the cathedral

in five years.

(translated): Tonight, I tell you very solemnly,

we will rebuild this cathedral together.

NARRATOR: Meanwhile, the world keeps vigil for Notre Dame.

(crowd singing in French)

NARRATOR: Daylight reveals the full extent

of the terrible destruction wrought by the fire.

♪ ♪

The oak roof and spire are completely destroyed.

Tons of toxic lead that covered the roof

have been sprayed into the air,

contaminating the site.

Burned roof timbers cover the vaulting.

Three gaping holes in the stone vaults

weaken the entire structure.

And the 550 ton scorched carcass of scaffolding

could collapse at any moment,

something unthinkable to those tasked with preserving

France's rich cultural heritage.

PHILLIPE VILLENEUVE (translated): I'm in front of my cathedral,

which is in this state.

I need to work.

NARRATOR: Phillipe Villeneuve is in charge

of historic monuments in France.

This is the cathedral that inspired him

to become an architect.

VILLENEUVE (translated): I must have been five

or six years old.

My parents brought me here one day,

like every child from Paris.

I was fascinated by the architecture.

It stayed with me since.

NARRATOR: Since 2013, Phillipe has been responsible

for conserving Notre Dame.

VILLENEUVE (translated): It was the culmination

of a dream.

A dream come true.

Today that dream has turned into a nightmare.

♪ ♪

NARRATOR: The stricken cathedral is a giant house of cards.

If the stone vaulting collapses

the weight of the buttresses will push in the 100-foot walls.

And Notre Dame will be no more.

♪ ♪

VILLENEUVE:

NARRATOR: So Phillipe heads up a rapid response team--

dozens of engineers,

architects, and scientists.

Their task is to prevent a total collapse of the cathedral.

VILLENEUVE (translated): From the bottom of my heart,

I want to thank you all for your dedication, your approach,

your passion.

You are doing a very difficult job, which is essential

for the cathedral.

NARRATOR: It's not only a difficult job,

it's also hazardous.

The crumbling stone vaults

and twisted scaffolding make any visit

inside to investigate the stability of the structure

extremely dangerous.

(translated): On the vaults we have the problem of the impact

of the fire, but we will also have to evaluate

the impact of the water used to put out the fire.

(translated): And we can see from here the inside of...

(alarm blaring)

LEROUX: Go out.

The scaffolding is moving.

Scaffolding!

NARRATOR: Motion sensors are installed in the melted jumble

of scaffolding overhead.

These can be triggered by gusts of wind--

a warning before a possible full-scale collapse.

(alarm blaring continues)

(translated): It's the alarm, because the scaffolding has moved.

We must leave.

NARRATOR: There are evacuations like this each week; necessary,

but an impediment to the urgent work

of stabilizing the structure.

♪ ♪

(translated): It's very difficult to juggle all these issues.

The problem is that we have to take action very quickly.

But we need to consider

the reality of this building.

It's still in danger of collapse.

We are still in the stabilization phase

of the cathedral.

NARRATOR: To avert a catastrophic collapse,

engineers could build a steel skeleton inside the nave

to brace the walls.

Then, even if the vaulting caves in,

the walls of Notre Dame would stay standing.

But it's far too dangerous for workers to erect steelwork

beneath the compromised structure.

We cannot go under the vaults because we don't know

whether they'll fall or not.

NARRATOR: So, instead of bracing the walls from the inside,

the team will build timber frames

under the buttresses outside.

Now, if the vaulting does fall in, the buttresses can't push

on the walls, and they won't come tumbling down.

PERSON 1 (speaking French):

PERSON 2 (speaking French):

VILLENEUVE (translated): They are very difficult because

no flying buttress is identical to another.

They are made to measure.

NARRATOR: Workers at this factory race to cut

and assemble around 250 tons of timber

to create the massive supports Philippe's team needs

to prop up the vaults.

It's critical each support fits perfectly

beneath each flying buttress

to hold its weight.

♪ ♪

Working around and inside this space

is a logistical nightmare.

210 tons of lead cladding covered the cathedral roof.

This was mostly melted during the fire,

and now toxic lead dust covers every surface.

The worksite is highly contaminated.

Until the site is cleaned,

team members must wear full protective clothing

to pass into the contaminated zone.

When leaving site,

they undress,

discard all clothing,

carefully wash equipment,

then shower themselves.

Only then can they go back to the clean area

even for a lunch break.

(Villeneuve speaking French)

VILLENEUVE (translated): It's very difficult to endure for the workers

who have had to deal with these procedures for months.

These regulations are not normal.

But this whole site is not normal.

NARRATOR: But, finally, five months later,

all 28 flying buttresses are locked in place

and the walls are safe.

Now they can turn to the next challenge--

secure the melted mass of scaffolding

that hangs precariously over the cathedral.

The scaffold weighs more than a jumbo jet,

and only rests on four spindly legs.

The team plans to wrap three massive steel lattice beams

around it to tie the fragile upper parts together.

Then they'll build more scaffolding either side

and lay steel beams across it.

That way workers can get inside the stricken scaffolding

to help cut off its 50,000 steel poles,

a truly Herculean task.

Only then can the team put up a temporary roof

to protect them from the elements

while they rebuild Notre Dame.

VILLENEUVE (translated): It's going to be an extremely dangerous operation.

The spire has disappeared,

but the scaffolding is still there.

It moves a bit, but it's still there.

NARRATOR: While engineers gear up to remove the scaffolding,

architect Rémi Fromont

and Livio De Luca

begin a groundbreaking project that will combine

the investigative work with new scientific analysis.

Their ambition is to create a data-rich model of Notre Dame--

a digital twin.

The digital twin will embed not only the geometric structure,

or the visual appearance of the cathedral,

but also all the scientific data coming from the studies.

For example, you can click on a stone in the vault

and access to all the information

about its physical properties such as the provenance,

but also the mechanical behavior

within the entire structure.

NARRATOR: Luckily for Livio, a series of highly detailed

laser scans of the cathedral have been conducted since 2006.

These are brought together in this priceless 3D dynamic map

to show every stone, timber, and iron nail in the structure,

across time, from the 12th century to the present day.

DE LUCA: This is an unprecedented project.

The ambition is to collect all the information from the past,

to pass it to the future.

NARRATOR: There's very little first-hand information

about the construction of Notre Dame,

or the craftspeople who built it.

In the wake of the fire,

new studies of the cathedral's materials

could unlock these secrets.

♪ ♪

This new data, once included in the digital twin,

will provide a blueprint for the restoration and rebuild.

♪ ♪

Inside Notre Dame,

scientists begin to gather data and investigate the damage

to treasured statues,

murals,

and windows.

The cathedral's most fragile wonder, its stained glass,

dates back to the 13th century.

36 windows circled the lower level,

42 around the middle level, and 43 around the upper level.

The three famous Rose windows

span up to 42 feet in diameter

and are made up of over 1,100 panels

of beautiful stained glass.

Miraculously, they survive the fire intact.

But the intense heat that melted

the cathedral's lead-covered roof

means that much of the glasswork

is now covered in a layer of toxic lead powder.

Removing it could damage the delicate glass

and be harmful to restorers.

CLAUDINE LOISEL: It was really painful to see the catastrophe on the TV.

I was looking to see what's happen around the windows

and it was, of course, totally difficult

to have a good idea of what's happened.

There is a before and after 15 April,

for historical monuments, that's for sure.

NARRATOR: Glass scientist Claudine Loisel

uses a handheld digital microscope

to investigate the levels of lead powder

on the stained glass.

She must then formulate a strategy

to clean every single panel; a vast decontamination program.

This window is in the back of the cathedral,

in the lower level, furthest from the inferno.

But it's still badly contaminated.

(speaking French)

NARRATOR: Fortunately, these windows have not been cleaned for 100 years,

so the lead has settled on top

of a dust layer, not on the glass itself.

The first thick layer of deposit was, we can say

has a small protection in one way.

So we have just to remove all the deposit,

to clean these windows from the 19th century.

NARRATOR: Claudine examines deposits from windows around the cathedral.

The samples reveal vital clues

about the spread of the lead contamination.

LOISEL: After the spire fell,

the cloud of dust, lead, and different particle,

push in the other direction,

so we are a little bit more protected in this area.

NARRATOR: The windows of the upper level, in the path of the lead cloud,

have been most contaminated.

The team takes out and transports these panels

to this special laboratory

where they experiment with ways to remove the lead.

First, Claudine uses a precision vacuum cleaner

to remove the hundred years of dust

and most of the lead powder along with it.

LOISEL: So this is a good way to protect the conservator.

You can control the action, the pressure on the glass

and also on the painting.

NARRATOR: Then she uses water and cotton balls

to remove the last of the lead.

LOISEL: Of course,

you need scientific evidence that it's working.

NARRATOR: Claudine uses x-ray spectroscopy

to determine exactly how many wipes it takes

to bring the lead down to normal levels.

LOISEL: So we can identify each chemical element we have in the material.

NARRATOR: Too few wipes and the lead will remain.

Too many wipes and restoration will take longer than necessary.

LOISEL: Okay,

now the analysis is finished.

NARRATOR: After five wipes, Claudine checks to see

if the glass is decontaminated.

♪ ♪

LOISEL: Okay, we have different chemical element-- calcium, iron,

and if we want to see the lead...

there is no lead! (laughs)

After nine months we can see

a good solution, a good way to clean and to preserve

the stained glass windows from Notre Dame.

♪ ♪

NARRATOR: The upper level windows were not only in the path

of the lead cloud,

but also closest to the inferno.

Claudine hunts for hairline cracks

caused by thermal shock,

the rapid heating and cooling of the glass.

LOISEL: These cracks is due to the fire.

This is a recent cracks

and this is typical thermal shock.

NARRATOR: It looks like the upper level stained glass

will need to be painstakingly glued back together.

But inside Notre Dame,

the lower level stained glass appears

to have survived unscathed.

LOISEL: And here we can see we have a good stability,

adherence of the painting,

so there is absolutely no thermal shock,

that's good news for us.

NARRATOR: On site,

the teams of scientists

meet the engineers and architects

to share their findings.

LOISEL (speaking French):

NARRATOR: Once Claudine's team has restored Notre Dame's glasswork

to its former glory, they may use

a radical new preservation technique

to safeguard it for future generations.

It's being used on a huge scale here, in northern England.

♪ ♪

This is York Minster,

an 800-year-old Gothic masterpiece

and home to the largest expanse of medieval stained glass

in the U.K.,

the Great East Window.

It is one of the largest windows ever made

anywhere in the medieval world.

We've got glass from the 12th right through

to the 18th century in quite significant quantities.

And it is really our national treasure house of stained glass.

♪ ♪

NARRATOR: Engineers here are completing a $12 million project

to protect York Minster's stained glass

from harmful UV rays and the corrosive effects of moisture.

In modern stained-glass conservation,

we're really doing as much as we can to keep

both surfaces of the historic stained glass dry and stable,

and that's where our ventilated,

environmental protective glazing comes into play.

♪ ♪

MATT NICKELS: You can see that I'm almost in.

(chuckles)

I think it's just this last bit here.

NARRATOR: Matt Nickels is installing this new conservation system.

He slots a protective clear glass exterior frame

into the window opening.

This goes into the original glazing groove,

where the medieval glass would have been.

NARRATOR: This protective glazing prevents corrosive condensation

from forming on the 800-year-old stained glass

that will sit behind it.

NICKELS: The gap created means that

there's air circulation running through.

And when you've got air circulation,

it's regulating the temperature,

which means that there's less moisture on the glass.

NARRATOR: Each frame is custom made and takes great skill to fit.

NICKELS: You don't want to make it too small

because it's going to obviously slide through.

No two windows are gonna be the same.

NARRATOR: With the outer panel installed,

they can reinstate the layer of medieval glass.

NICKELS: They're actually in fairly good condition

considering that they're early 13th century.

There's always the worry whenever

you're handling glass like this,

but you just got to make sure

that you're really, really careful.

There's nothing quite like

seeing it with sunlight behind it.

When you put it up like this, it's quite magical, isn't it?

♪ ♪

NARRATOR: Techniques like this

offer a glimpse of how scientists like Claudine

may eventually preserve Notre Dame's glass.

This is the best way to protect stained glass windows,

so it will be for sure an option

to protect the windows for Notre Dame.

♪ ♪

NARRATOR: Had the vaulting collapsed

next to the windows,

the glass could have been badly damaged.

But luckily, the stone vaulting,

which sits just under the timber and lead roof,

protected the windows from the inferno above.

VILLENEUVE (translated): When the architects of the Middle Ages

constructed this vaulting,

they used it to separate the timber frame of the roof

from the rest of the cathedral.

So the vaulting took the shock of the falling timber

and the fire and the firefighters' water.

♪ ♪

NARRATOR: The magnificent vaulting was built to be resilient,

thanks to precise medieval craftsmanship,

using over a thousand cubic yards of limestone.

♪ ♪

The arches work together to support the roof

and stabilize the outer walls.

But the intense heat from the fire

and the collapsing spire

took out 15% of the stone vaulting.

(spire crashes)

♪ ♪

Today, three 40-foot-wide holes

and several smaller gaps mean the vaults could collapse

at any moment.

♪ ♪

The team collects, stores and catalogues the fallen stone

in this tent, located alongside the cathedral.

They may be able to use some of this stone

to reconstruct the vaults.

♪ ♪

But it's clear they'll also need to source new stone.

Notre Dame is made up of many different types of limestone.

Medieval masons chose hard limestone for the towers,

pillars, and outer walls to build tall and hold up the roof.

For the sculptures, they chose dense, fine-grained limestone,

that can be carved with great detail.

And for the vaults they selected softer, more porous limestone

that's light but strong.

If the team rebuilding the vaults

pick a limestone that is too heavy,

the new vaults may not last as long as they should.

Geologist Lise Leroux

investigates what quarry this stone came from.

LISE LEROUX: We have some blocks coming from

the collapse of the vault for study.

NARRATOR: This detective work will help the team source

replacement stone that shares identical mechanical properties.

LEROUX: We have to verify.

NARRATOR: The fallen vaulting stone contains a rare micro-fossil

called orbitolites complanatus,

a kind of plankton.

Fossils like this are found in just one layer of rock.

This will make sourcing new stone

of the same type even trickier.

Can they use this geological fingerprint

to discover the original source of the vaulting stone?

LEROUX (speaking French):

NARRATOR: To find out, Lise and fellow Notre Dame scientist

Claudine Loisel venture deep beneath Paris.

Hidden under the city streets is a rich source of limestone,

a vast labyrinth of quarry tunnels.

Lise and Claudine enter this maze

two miles south of Notre Dame in the famous Catacombs.

Oh!

(speaking French)

LEROUX (speaking French):

LOISEL (speaking French):

LEROUX (speaking French):

DANY SANDRON: In the late 18th century,

the quarries were given a different purpose

and they housed bones from old cemeteries,

which were inside the towns.

Cemeteries which were closed at the end of the 18th century

for sanitary reasons.

NARRATOR: Among the bones,

Lise and Claudine find traces left by the medieval miners.

LEROUX (translated): Here, the block's been removed

and we're left with this trace.

They then square off the sides,

and use it to build Notre Dame.

And the strata height here, it dictates the height of the block

that can be extracted.

The blocks we see at Notre Dame have this height.

So the quarry itself puts a constraint

on the construction of Notre Dame.

LOISEL (translated): We have life and we have death.

LEROUX (translated): Well, yes.

NARRATOR: The upper level of the quarry

holds hard limestone with

large, well-preserved fossils.

LEROUX: These fossils are

more characteristic of the limestones

used for the pillars, the arch in Notre Dame.

But not for the vault.

NARRATOR: Lise and Claudine hope to find

a match for the soft vaulting stone

in the lower level of the quarry.

Now to look if we can find

the specific micro-fossils.

I'm not sure, because the surface is very rough

and it's not so clear because of all of the state of the surface.

NARRATOR: The limestone here is softer,

but Lise cannot see a match for the rare micro-fossil

found in the Notre Dame vaulting sample.

♪ ♪

So, back in the lab,

she takes a closer look at a sample of limestone

from the lower level of the quarry.

These little fossils--

this one, this one,

this one--

are, in fact some planktonic fossils,

which are called foraminifera.

NARRATOR: It's not the fossil signature she's looking for.

But then...

Oh!

This one here is orbitolites complanatus.

This little planktonic fossil is a dating fossil,

which match with the stone coming from the vault.

It's a stratigraphic indicator,

characteristic from the Middle Lutetian,

which is a geological age of deposit.

NARRATOR: Lise confirms the origin of the Notre Dame vaulting stone.

It's quarried from the deepest seams

of limestone beneath Paris.

Conclusive.

NARRATOR: But what about the harder limestone,

used by medieval masons to build

Notre Dame's load-bearing pillars and arches?

Another micro fossil signature confirms the origin

of this type as well.

LEROUX: The arches are built with a hard stone--

with a resistant stone, to support the vault.

And the vault itself

is logically constructed

with a lighter, more porous stone.

And in the quarry located in Paris,

we have this two kind of stone.

NARRATOR: Medieval masons knew exactly how to exploit

the varying mechanical properties

of the limestone for Notre Dame;

knowledge passed down through the generations.

Sourcing more of the correct stone won't be easy--

the old quarries are no longer active.

But engineers now know

what limestone to look for--

this will help them find a match in quarries outside Paris.

Stone is not the only raw material

that will need to be replaced

as engineers reconstruct Notre Dame.

The timber roof was also a medieval wonder.

It was constructed from 25,000 cubic feet of timber,

cut from 52 acres of oak--

that's approximately 1,300 trees.

For this reason, it was known as "the forest."

Every single oak in Notre Dame's forest

was handpicked for the physical properties needed

in the roof structure--

from dense straight oak for pillars,

to curved oak for support arches.

(wood cracking)

But the fire burned every beam in the forest.

Today, this intricate 550-ton timber jigsaw lies in ruins.

LAVIER (translated): We thought this sublime roof would be here forever.

It was a big puzzle with beams

from different periods,

all the way back to the 13th century.

And to see it suddenly

all burned, all mixed up...

Oh, it's very emotional.

It's very difficult.

NARRATOR: Almost 60 tons of the precious roof timber

still lie precariously on top of the vaults.

Despite the destruction,

every single beam holds the history of Notre Dame.

It has deep archaeological value.

It's vital that workers forensically record

the position where each beam fell,

before they remove them.

This helps them determine

where it originally sat in the roof structure.

Now, these highly trained rope access technicians

gear up to catalogue and clear

the charred timber on the vaults.

BOTH (speaking French):

It's not possible to walk on the vaults,

because the structure is very precarious.

They needed to

create a way to access with ropes.

We need to wear a special mask

because of the lead dust that we might inhale.

We label the timbers

and we mark them with a code

that the architects will

be able to identify.

(speaking French):

NARRATOR: The team has their work cut out--

there are thousands of separate pieces

of timber to catalog.

DE GUILLEBON: We are working day and night.

We have a lot of work to do.

NARRATOR: They've already extracted

around 4,000 pieces.

Timber scientist Catherine Lavier

begins painstaking detective work to reveal

how Notre Dame's vast forest

was originally assembled

and could be rebuilt today.

LAVIER: Some pieces were very well-preserved

because as you see here, with different faces

and another piece of wood is coming here,

with a wooden joint here to assemble them.

And it's rather typical from the medieval period.

And here,

you have a mark,

of carpenters.

So they are sure that this piece with this piece are together.

It's very important for carpenters.

They prepare the wood on the ground

and after that,

they go to the roof and reassemble again.

Every carpenter has his own way to mark,

but in general it's based on

the Roman numbers,

but we can find some differences between teams of carpenters.

We were very surprised to find that

because I thought everything will be destroyed.

And, finally, not.

NARRATOR: The tree rings of the timbers conceal further clues.

Each ring represents one year of growth;

a time capsule of information

about the life of the tree in that year.

Catherine analyzes core samples from Notre Dame's roof trusses.

She measures each ring to reveal the secret story

of some of the original oak trees

the structure was made from.

(translated): This screen shows the size

of each ring I measured.

At the start of its life,

you see it has very, very large rings,

which correspond to very rapid growth.

Next, it looks like it experienced

some more dramatic events,

some difficult years,

here, when the rings are very thin,

This could be because of too much rain,

not enough sun, and not enough nutrients.

And then, the life of the tree continues

until it's cut down, around its 96th year.

NARRATOR: Catherine is gaining new insight

into the types of trees best suited to rebuild

the complex forest of Notre Dame.

This extraordinary challenge will require

around 1,300 oak trees,

craftspeople versed in the lost art

of medieval carpentry practices,

and a blueprint for possibly the most geometrically complex

timber structures in Europe.

The one person who can unlock

the lost forest's geometrical secrets

is architect Rémi Fromont.

In 2014, Rémi spent the entire year

mapping every inch of the timber.

(translated): It was a magical place

to go in there; there was a smell.

There was a very special atmosphere of light.

We still had the traces of tools also on the woods.

It sometimes seemed like they only left yesterday.

We are collecting photographs,

3D point clouds,

and the physical and chemical characterization

of all the materials.

NARRATOR: The fire at Notre Dame

triggers a race across France

to 3D scan historical monuments, inside and out.

These represent a digital insurance policy

to preserve French heritage.

♪ ♪

The laser bounces off each contour in the room.

The machine then measures

the time it takes for the laser to return.

Millions of measurements

form a cloud of data

called a "point cloud."

♪ ♪

In 2016, researchers used this same technology

to create a full point cloud

of Notre Dame's lost timber roof structure.

This remarkable 3D scan will combine with Rémi's 2014 survey,

in Livio's digital twin for Notre Dame.

DE LUCA: What we are producing today

will be probably the information useful

for the next generations.

NARRATOR: The team now has the data they need

to rebuild the timber roof with the exact same geometry.

The new oak needed

could come from forests like this.

Almost a third of France

is covered with forest.

Oak is a vital strategic resource

throughout the Middle Ages and the Renaissance.

Vast forests are needed to build cities and expand navies.

This is the Château de Beaumesnil in Normandy.

It's a National Historic Monument,

built on the site of

an 1,100-year-old castle.

RÉMY DESMONTS (translated): It was built in seven years.

It's something extraordinary for just seven years' work.

NARRATOR: The château has seen better days.

The curved beams that hold up the roof

are close to collapse and must be replaced.

(translated): The wood grain has been cut through.

This weakens the support beam.

And then you see that the beam is completely eaten away.

The wood is degraded, eaten by the fungus.

NARRATOR: The restoration work here

requires much of the same skill and knowledge

it will take to rebuild Notre Dame's lost forest.

The timber has been chosen so the curve of the grain

perfectly matches the curve of the new beam.

LEO ROUSSEAU: If you get a straight tree,

which has a straight grain, and...

if you cut a curved piece, piece of wood inside of this,

so here is the fiber, so it can break, right there.

But if you take the tree that's curved,

the fiber is like this.

So it cannot break.

You keep all of the structural strength of the tree.

NARRATOR: The carpenters use

an original beam as a template

to mark out the new beam on the oak.

(speaking French):

NARRATOR: The carpenters who built Notre Dame

would be familiar with the tools this team uses

to hew the raw timber.

(chopping)

ROSSEAU: So after you split most of the wood,

you use a broad-axe.

They have a single bevel,

long cutting edge, and the handle is offset.

So...

if you're working,

as you go down, your hand here,

you see I'm not hitting this sharp edge.

(echoing chopping)

NARRATOR: For skilled carpenters,

cutting Notre Dame's roof timbers with axes,

compared to a modern sawmill,

will take roughly twice the time;

possibly too long.

This curved oak will be

one of ten the team needs to install

as part of the château roof restoration.

It sits alongside this 400-year-old original beam.

DESMONTS (translated): This one was cut

probably 1635, '37,

and then this one 2020.

I hope our ancestors are happy with this.

NARRATOR: Just like the Notre Dame beams,

the Château's original beam holds

messages from the old carpenters.

DESMONTS (translated): It's extraordinary to find all these marks.

It's very old and at the same time,

it looks like it was done yesterday.

NARRATOR: French craftspeople

have the oak,

they have the skills, and they have the plans required

to reconstruct Notre Dame's vast forest of roof timbers.

It's over a year since the fire ravaged Notre Dame cathedral,

and the investigators have not pinpointed

the cause of the blaze.

Immense challenges and uncertainties still lie ahead.

The building is not yet out of danger.

Over the next 12 months,

engineers must remove the melted scaffolding

and seal the cathedral roof to make it watertight,

then stabilize the weakened vaulting.

It's a monumental task.

And rebuilding the entire cathedral

could take much longer than

the five years decreed by President Macron.

(translated): Faced with such a drama,

thankfully there's hope.

FROMONT (translated): We need faith for this project.

It's this building itself that generates this faith--

even for atheists-- and that's something magical.

♪ ♪

NARRATOR: Architects around the world

have unleashed their imaginations

to submit grand plans for what the new spire above Notre Dame

could look like--

from mirrored roofs with kaleidoscopic pinnacles,

and vast solar panels powering nearby buildings,

to stained glass edifices that will light up the Paris skyline.

However Notre Dame is rebuilt,

the unique collaboration of architects and scientists

is rewriting how we understand

the very fabric of this magnificent cathedral.

DICKAS: I think the fire in some ways

helped remind a lot of people

what an important part of our sort of shared history

and shared culture this is.

NARRATOR: Soon, a complete digital twin of Notre Dame

should allow future generations of craftspeople

to maintain, protect, and faithfully rebuild Notre Dame,

preserving this world treasure for all time.

(translated): I have only one obsession--

save the cathedral, resurrect it,

and reopen it to the public.

♪ ♪