♪ ♪ NARRATOR: Notre Dame de Paris-- 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."

♪ ♪ ♪ ♪ 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.

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