All language subtitles for rebuilding.notre.dame.s01e01.1080p.webrip.x264-cbfm[EZTVx.to]

(gentle music)
(air whooshing)

(bird squawking)
(gentle music continues)

On April 15th, 2019,

the world's most famous cathedral was ravaged by fire.

The roof went up in flames,

and the lofty spire collapsed,

crashing down through the vaulted ceiling.

That day, Notre-Dame de Paris became a ruin.

(gentle music continues)
(air whooshing)

Once the fragile building was stabilized,

the decision was made to rebuild

the cathedral exactly as it was before.

But where to start?

How do you reconstruct today an 800-year-old monument

when no one knows how it was originally built?

(air whooshing)

So began a remarkable challenge

in the very heart of Paris.

For the first time,

archaeologists, historians, geologists,

and specialists in ancient materials,

(grinder whirring)

as well as structural engineers

and acoustic and digital specialists,

explore the cathedral's entrails,

scan its vaults, and probe its foundations.

In the process, they reached spaces

that had been inaccessible for centuries,

rediscovering all the skills and techniques

of Notre-Dame's original builders,

for they hold the key to its reconstruction.

(dramatic music)

In their efforts to balance both the earthly and the sacred,

the actual and the virtual,

the scientists and the architects both share

a single obsession,

to bring beloved Notre-Dame back to life.

(moves to brooding music)

(wind howling)

(brooding music)

18 months after the fire,

the tragic wounds were finally exposed for all to see.

(brooding music continues)

The roof was a twisted tangle of scorched wood and metal.

Daylight flooded the huge nave in three ruin spots.

(brooding music continues)

The once majestic cathedral

was a fragile ghost of its former glory.

Its symphonic organ was silent.

Its ancient statues were now blind.

(brooding music continues)

The walls and stained glass windows were covered

by thick soot, and even seemed to be in mourning.

Hundreds of tons of damaged stone

had crashed onto the shattered floor.

(brooding music continues)

Iron ornaments, along with beams

that had supported the roof,

had plummeted more than 30 meters.

The entire disastrous mess

had to be carefully removed, piece by piece.

(construction vehicles whirring)

Several days after the fire,

archaeologist Marc Vire filmed inside the cathedral.

(construction droning)

(construction vehicle engine roaring)

When we arrived on the site,

the regional curator of archaeology, Stephane Duchaux,

said to me, "I've just straightened things out.

Up until now, everyone has been talking about rubble.

It isn't rubble.

These are pieces of French heritage.

Henceforth, we will be treating them

as archaeological vestiges."

That changed everything.

(soft dramatic music)

Majestic Notre-Dame was suddenly transformed

into a busy scientific work site.

Tents erected on the forecourt served as research stations.

(brooding music)
(construction vehicle beeping)

Picking through the vestiges,

the experts carefully took stock of what remained

after the terrible fire.

Each fragment and every nail was covered with toxic dust.

(brooding music continues)

The cathedral's lead roof had melted

into minuscule particles,

which had found their way into every nook and cranny.

Chief heritage curator Dorothee Chaoui-Derieux supervised

the project since the day after the fire.

No one wanted this fire, this tragedy,

but today we have to try and make the best things.

When we started sifting through the vestiges

and removing them, there was the physical aspect

of the work, that's undeniable.

We spent hours, entire days,

wearing that protective equipment and shoes,

the helmet, but also the mask with the battery on your back,

the sound of the breathing apparatus.

So physically, the conditions weren't easy at all.

Standing all day with your hands in the soot,

sorting for hours.

(rubble clattering)

By preserving all the charred pieces of the framework,

all of the fallen blocks of stone,

today we have a huge amount of data

that had previously been inaccessible.

Thousands of stone blocks,

which had formed the 100-foot-high vaulting,

were classified according to their original position.

The researchers now need to study every detail

to understand the complex process developed

by Notre-Dame's original builders.

Those early architects had managed to raise

the highest vaults throughout Christendom

during the 12th century.

(dramatic music)

The cathedral's medieval vaults must be rebuilt first.

(group speaking French)

Pascal Prunet is one of the chief architects

in charge of the restoration.

And for him, it is an unprecedented challenge,

a first since the Second World War.

I've never rebuilt vaults before.

It's actually been 60 or 70 years

since anyone's repaired a cathedral's roof.

There aren't any craftsmen or architects

with experience of that.

So we had to understand why there was

that irregularity in the shapes.

Why was there that more complex geometry

than the more simple one we'd had gone for?

What route did we need to take in order to rebuild all that?

Anticipating the deformations.

There are so many questions.

(soft dramatic music)

To answer these questions,

the architects need the expertise of geologists,

archaeologists, and materials specialists.

Art historian Yves Gallet oversees

the multidisciplinary group.

(soft dramatic music continues)

The millions of visitors to the cathedral

over the centuries before the fire

had made it difficult to study.

Studying Notre-Dame before the fire of 2019,

even just getting into Notre-Dame,

was really difficult.

So, after the fire,

we had a unique opportunity,

for a limited amount of time, unfortunately.

But a unique opportunity to get a really close look

at the cathedral.

(metal clanking)
(people speaking French)

Notre-Dame is a cathedral

which has survived through the centuries.

And since we want to study it as it was conceived

in the 12th century,

we're working our way back from the most recent

to the most ancient parts.

(soft ethereal music)

By the 18th century,

the vaulted transept ceiling was already

in danger of collapsing,

and it was new compared to most of Notre-Dame.

It was rebuilt using new stones

by architect Germain Boffrand in 1728.

(soft ethereal music continues)

Because it was redone once again

during a 19th-century restoration,

this part of the cathedral will be simpler to rebuild.

But the vaulting which collapsed in the nave

was built early in the 13th century

and had survived for over 800 years,

only to be destroyed on April the 15th, 2019.

The stone experts are fascinated by this architectural feat.

(soft ethereal music continues)

Notre-Dame had the highest vaulted roof

of its age.

This is a unique opportunity,

because here we have the elements of the construction,

but we have them disassembled.

(soft ethereal music continues)

A medieval vault,

as a puzzle made of heavy blocks of stone.

The cross-stone arches originally divided

the ceiling into six parts.

(soft ethereal music continues)

Smaller stones filled in spaces between the arches

to form the sexpartite vault.

The stone-working group

has this opportunity of coming to grips

with the very skeleton of the edifice.

So we study the stones,

their geological origin,

the way they'd been cut,

the way they'd been put in place.

Historian Bruno Phalip researched

the techniques and tools used

for Notre-Dame's original construction.

We can try to measure the size

of the tool's blade, its number of teeth,

the space between the teeth,

to work out if it was a big, heavy tool

or something lighter.

The approach we see here was quite physical,

with a heavy tool called a boucharde,

a sort of toothed ax, used at an angle like this.

But you can see that different methods were used

on these other sides.

Here, they wouldn't have used those tools

with the huge impacts.

Here, it was lots and lots of individual blows

to the surface.

(soft ethereal music)
(metal clanks)

What you had here were workers who were learning

what to do in situations that were quite exceptional.

You had the immense width of the nave and its height,

which was up to 30, 35 meters.

They were having to confront methods

which required workers who were keen to learn,

who wanted to surpass themselves,

who wanted to tackle this reality,

which they'd never before experienced.

(soft music)

In 1160, Notre-Dame's builders

were competing with the past

and with Noyon Cathedral in northern France,

which had begun construction 10 years earlier.

(soft ethereal music)

According to art historian Arnaud Timbert,

an architectural revolution was born.

Notre-Dame de Paris

shifted Gothic architecture up to gigantic dimensions,

both as regards surface area as to height.

It was a completely different world,

perhaps not in the conception of space,

because space remained that of a basilica,

but in the way you experienced it.

Your body was in a different movement,

a different place than in previous cathedrals.

(soft ethereal music continues)

Ambitious medieval builders

were determined to reach new heights

in their cathedrals' ascent towards heaven.

(ethereal choral singing)

This quest for greater height

is always associated with Gothic architecture.

It is locked into this constant striving for height,

which culminated in Beauvais Cathedral with its 48 meters

to the vaulting.

With Notre-Dame de Paris,

the builders achieved 33 meters up to the vaulting,

which was a colossal leap of the times,

because nowhere had previously got any higher

than 22, 24, or 25 meters.

So this was like a sudden leap of 10 more meters.

(soft ethereal music)

Notre-Dame's architects raised

their vaults 1.5 times higher than those of Noyon.

With this victory,

Notre-Dame de Paris had pioneered

a new trend in architecture.

(soft ethereal music continues)

It was the culmination of 30 years of experimentation

that began at Sens Cathedral in eastern France.

(soft ethereal music continues)

This Gothic structure boasted the first use

of ribbed vaults.

(soft ethereal music)
(air whooshing)

Notre-Dame didn't just spring up

from nothing, of course,

it was the fruit of lots of experimenting,

of dozens of years of experimenting beforehand.

From what we can consider the first Gothic cathedral,

Saint-Etienne de Sens,

the construction of which began

in about 1135 to 1140,

to Notre-Dame, which got started in about 1155 to 1160.

So we historians believe the link

is the sexpartite ribbed vault.

(soft ethereal music)

Builders continue to raise

the ribbed vaults higher and higher.

They were able to train men in this new art,

this new style, this new know-how.

(soft ethereal music continues)

Ironically, the fire that consumed Notre-Dame

on April the 15th revealed a hidden expertise

of its stonemasons.

Because there were holes,

the falling spire had smashed through the vaulting,

we discovered that the vaulting was,

in fact, extremely thin,

only about 12 to 15 centimeters thick,

which is nothing at all.

It was a stone veil 14 meters wide.

(soft ethereal music)

Could this be the secret

behind Notre-Dame's extremely high vaulted ceilings?

(soft ethereal music continues)

To answer this question,

historian Yves Gallet needed to examine

the vaulting in Sens Cathedral,

to compare it with that of Notre-Dame.

(soft ethereal music)
(birds chirping)

(air whooshing)

At first, the vaulted ceilings of the first Gothic cathedral

resemble upturned boats,

and they're also round like those in Notre-Dame.

But are they similar in other ways?

(soft dramatic music)

So we came here to Sens Cathedral

to carry out measurements using a geo-radar machine.

By sending passes of energy

through the stonework,

the geo-radar machine records data

about its density and thickness.

These measurements had never been taken before.

Here's the surface,

and here's a first reflection,

and here's a second.

So the thickness is about 35 centimeters,

with a 10% margin of error.

We've learned two things from this.

First, the vaulting is between 30 and 35 centimeters thick.

And second, there aren't two layers of masonry

to create that thickness,

it really does consist of blocks

that are 30 centimeters thick.

(soft dramatic music continues)

The vaults were scanned as well,

first from under the roof,

and then from inside the nave,

to confirm the geo-radar's measurements.

The goal of the operation

was to find out if we can measure directly,

if the geo-radar machine works

and gives us the same results as those we get

by indirect measuring via the superimposition

of point clouds.

(soft dramatic music continues)

We have the volume below and the volume above,

and by superimposing the two,

we can calculate in an indirect way

the space between the two.

In other words, the thickness of the vaulting

but also the thickness of the walls,

the thickness of everything that's in between

the inner and the outer.

And so, according to the points collected here,

we got a measurement of about 30 to 35 centimeters,

which is exactly the measurement we got using

the geo-radar machine.

The vaults in Sens

are 30 to 35 centimeters thick,

almost 20 centimeters more than those in Notre-Dame.

The vaulting in Paris is higher up

and it's thinner.

The vaulting of Sens Cathedral, at 24 meters,

is lower and it's thicker.

So there's probably some cause and effect going on here.

You have to take other parameters into account.

It's not just the thickness of the vaulting,

there's also the quality of the stone,

its density, the quality of the mortar used,

the techniques used for the flying buttresses.

But it definitely was a technical challenge,

and one of the major feats

of Notre-Dame's anonymous architect

to have managed to reduce the thickness

of the vaulting enough to beat the height record

and beat it by a significant margin.

(tense music)

For the new restoration of Notre-Dame,

today's architects want to replicate the original work

as closely as possible.

(metal clanking)

First, they need to construct a massive scaffold.

(metal clanking)

(soft ethereal music)

Within a few months,

this would reach the nave's damaged vaulting.

Now the architects must devise a way to construct

the medieval vaults exactly as they were before.

We're working according to a logic

of reconstruction that's,

I don't know if you can call

it archaeological reconstruction,

but let's say faithful reconstruction,

restoring it as far as we're able to,

to the state before the fire.

They begin with a great transverse arch

which forms the skeleton of the vault.

There's about 30 to 35%

of the arch remaining.

That obviously gives us a starting point for the geometry.

Especially since of the 79 arch stones

that fell, we've recovered 70.

One by one, the recovered arch stones

are washed to remove their covering of lead dust.

Damaged by fire and water,

and after falling from a great height,

experts wondered if they could be used again.

The big question the architects

need to answer is, how solid are these blocks?

How well can they sustain the weight of the cathedral?

Several of the stone blocks are analyzed

by geophysicist Thomas Dabat

at the Historical Monuments Research Laboratory

in Champs-sur-Marne.

So, with this selection of 30 blocks

of stone, taken from the three vaults which collapsed,

my work consists of taking cylindrical samples

out of each block.

(drill whirring)

We can study the effects of any damage,

both at the surface and deep inside the block.

(soft celestial music)

So, the aim here is to establish the blocks

that can be used again and those that can't.

(grinder whirring)

This analysis is quite complicated,

because the damage done to the stone blocks

is not consistent.

(celestial music)

Here, I have three samples

from the same block of stone,

and you can clearly see that they look different.

This first one is severely damaged,

it's a dark gray color and it's extremely fissured.

It comes from the surface of the block,

which underwent extreme heat.

So the question now is how its resistance compares

to a sample from an undamaged part of the block?

(celestial music continues)

Thomas Dabat measures how energy moves

through each stone.

The results are sent to Lise Leroux,

who is responsible for monitoring the state

of the cathedral's ancient stones?

(celestial music continues)

There was uncertainty about whether

these arch stones, these blocks,

would have the same mechanical properties

as when they're initially used.

And what we were able to show

was that many could be used again.

Not all of them, but some.

(celestial music continues)

Of the 70 blocks recovered,

only 15 of them seem stable enough to be reused.

But first, the usable stones must be reassembled

on the ground.

Like pieces of a gigantic jigsaw puzzle,

the experts need to work out the correct placement

for each one.

(group speaking French)

We started this trial reassembly hoping

that we'd be able to replace some of these arch stones

in place, because that was one of the major goals,

to preserve the same components

as they were originally used in the edifice.

(group speaking French)

There are critical decisions to be made.

Each of the missing blocks needs to be cut

and then arranged using the original techniques.

(group speaking French)

This experiment was requested

by the architects in charge of the restoration,

and you can understand why,

because architecture is an experimentation with space.

An architect isn't just someone who builds some walls,

it's someone who, using those walls,

creates a space.

And to get a real understanding

of what that arch represented,

they'd have to put it back together for real.

And this has never been attempted before

on a Gothic cathedral.

Never before have all these different skill sets

and specialties of all kinds

been brought together in the service of a national monument.

Well, at least not in France.

It's an incredibly difficult challenge

because there are no blueprints from the Middle Ages

and no model for a transverse arch.

(group speaking French)

The experts are forced to seek answers

from the ruined cathedral itself.

They scan every part of the building stonework

using drones equipped with cameras.

(celestial music)
(air whooshing)

Suspended above the vaulting,

rotating scanners digitize the details of what is left

of the roof and its framework.

By combining these two techniques,

they can create a precise virtual copy

of the cathedral after the fire.

Livio De Luca, the research director

at the National Center for Scientific Research in Marseille,

heads the digital team.

In the quest to restore Notre-Dame,

virtual reality becomes a tool

for archaeological investigation for the first time.

You can no longer think

of digital technology as just virtual.

There's a sort of complete symbiosis

between physical reality and digital reality.

What we're composing is a sort of digital memory.

And it's not only the digital memory

of a physical dimension,

it's the digital memory of a scientific adventure.

(celestial music)

The digital team relied on another asset,

a 3D model of the cathedral done in 2010.

It was created by a young American historian

We can superimpose the points cloud,

which was made before the fire by Andrew Tallon, okay?

We have it right there.

And we can fill in the holes with the point cloud from-

Can you turn it around a bit?

Yes, it's perfectly aligned as well.

Can you come back to the post-fire bit,

just to see there.

There. Right there.

That's it.

We gathered, integrated,

and combined data on the different states

of the cathedral before the fire,

during the phase when the vestiges were being recovered,

and of course, after the cleanup of the surfaces.

This 4-dimensional representation

of the cathedral we created

lets us look at everything

in a uniquely geometric framework,

while also traveling through time.

(celestial music)

The combination of data enables

the engineers to establish a plan

to rebuild a part of the arch that has collapsed.

This plan is transposed at full size

on an immense ground sheet.

(celestial music continues)

The team can now determine how to position

the 70 arch stones according to their dimensions.

For archaeologist Cedric Moulis,

seeing the plan laid out reveal

the unique arrangement of the stones.

This plan was a great help,

because we have an arch that isn't standardized.

By that I mean that the arch stones are all different sizes.

The smallest are 10 centimeters thick,

and the biggest are up to 28,

but the sizes don't increase progressively.

It's not necessarily the biggest ones at the bottom

and the smallest at the top.

So the whole idea is to understand the sequence

of how they were organized.

Here you have a stone of 19

in a place that's 18.5,

and one that's 18 in a place that's 19.

The archaeologists continue to be surprised.

Out of 66 recovered arch stones,

49 had thicknesses from 14 to 18 centimeters.

13 had thicknesses from 18.5 to 28 centimeters.

And only four measured from 10.5 to 13.5 centimeters.

The team needs to understand why the thickness varied.

The answer will lie deep underground

in an ancient quarry which had provided stone

for other Parisian monuments.

(soft tense music)

Archaeologist Marc Vire is an expert on quarries.

And he spots clues that Notre-Dame stonemasons

had been there.

(soft tense music continues)

So we have coarse limestone here,

sedimentary rock, which formed on the bed

of a warm and shallow sea,

and which formed into these banks,

these banks of rock.

Why is this?

Because, from time to time,

the sea doesn't deposit the sediment so well,

and so we get these horizontal joints,

which geologists call stratigraphic joints.

And between these joints,

the rock is in one solid piece,

a massive bank of rock.

And this is what determines the height of the blocks

dug out of the quarries.

Here, for example,

this is as high as a block can be,

but down here the bank is deeper,

so the quarry workers would've been able

to dig out much bigger blocks.

Even with the advantage

of stratigraphic joints,

removing the massive blocks

would've required extremely grueling labor.

They inserted iron wedges

and walloped them with a sledgehammer,

and that way cutting out a block from the rock face.

This is what we call the extraction.

Even at the quarry stage,

there was this basic work done on the shape of the block.

The block had to be square,

this was the basic shape.

And this was done right here inside the quarry.

Once this basic shape was extracted,

the block was placed on a cart

and taken from the quarry to the work site.

And there, the stonemason would do the secondary cutting

so it could be used in the cathedral.

As the scientists make progress

with reconstructing the transverse arch,

they discover an ancient

but efficient organizational system.

So a block would come from the quarry,

from which they'd cut two arch stones.

So from a standard block,

they'd cut out two arch stones lengthwise

but to the same thickness.

Is that what you mean?

Yes, the same thickness.

So from a big block this size,

they were able to cut out two arch stones.

We're really learning things here.

We don't know anything anymore.

It's like we're only just starting

to learn about vaulted ceilings.

(person speaking French)

As the puzzle slowly comes together,

a detail on some of the blocks

demands further investigation.

We're finding out more,

for example, from these small cavities

which were cut into this part of the arch stones

at their junction with the rest of the vault.

These notches, which sometimes

go across two stones together,

were slots to fit wooden trusses into,

to stabilize them during the construction phase

while the mortar was drying.

These trusses were a temporary construction

that would then be removed.

(soft brooding music)

Yves Gallet wants to fully understand

what role the notches played

in the cathedral's construction.

The answer might be found at the top of Notre-Dame.

(brooding music)

Notre-Dame has completed the first phase of its restoration.

Over just a few months,

the cathedral of stone has become a cathedral of iron.

(brooding music continues)

Nearly 1500 tons of metals were used to construct a skeleton

over which the restored legend would be rebuilt.

It's a colossal frame,

designed for the massive task ahead.

(brooding music continues)

When the scientists first ascended in the iron cage,

they came face to face with something no one

had seen up close for over 150 years.

The vaulted ceilings of Notre-Dame.

Going up like that in the lift,

when seeing it from that perspective,

the state that the cathedral is in,

you get a shock.

You get a shock.

You think you're prepared for it,

but you still get a shock.

(soft celestial music)

(moves to ethereal music)

Also, seeing the damaged state of the stone,

it's hard to imagine from down below.

This stone has already been doing its job for centuries,

and now water has gotten into the masonry,

the salts have been leaking out,

the skin of the stone is starting to blister,

starting to deform.

Up close, you can see all that.

(soft melancholy music)

Yves Gallet uses this opportunity

to further his investigation into the strange notches

on the fallen blocks.

He had to get close enough to see if they were visible

in the remaining arches.

Can we look over here?

(drone whirring)

That one's wonderfully clear.

You can see the notch perfectly across two arch stones.

(drone whirring)

(person speaking French)

Statistically, they're mostly concentrated

in the upper part of the vault.

Finally, the reason behind the placement

of the mysterious notches was becoming clear.

These notches became more necessary

in the construction of the vault

the more overhanging the arch got.

The closer you got to the summit,

the more pronounced this overhang was,

with, consequently, a greater danger

of collapse without that supporting wooden structure.

There's one at every fourth, fifth,

or sixth lock depending on the thickness of the blocks.

So it's quite a regular distribution.

A temporary wooden structure held

the vault in place during its original construction

and helps archaeologists determine how to assemble the arch.

That's a really good notch.

The idea was that the number of stones

with these notches is precise.

It's exact.

So we divided the number by two

to get the number of notches on the right side of the arch

the same as on the left.

And we saw that the space between the notches,

into which the wooden trusses were placed, was 3.5 feet.

(soft music)

This equals one meter and 10 centimeters.

With the reconstruction of the arch underway,

another detail requires clarification.

(soft music continues)

The experts need to know the significance

of the crosses carved into the blocks.

The crosses had been hidden under mortar for 800 years.

What role did they play in the assembly?

(soft music continues)

Clues are found in Noyon Cathedral's attic space,

where stone fragments that were damaged during World War II

had been stored.

(soft music continues)

Historian Arnaud Timbert finds similar crosses

on some of these pieces.

(soft music continues)

In the 13th century,

there was a sort of production process in the quarry

for turning out standard, prefabricated pieces.

There were men already working the stones at the quarry,

and they had to convey information

by means of certain signs to the men

on the work site who were going to assemble the pieces.

And these men wouldn't know each other.

There will be no other communication between them,

so they'd make this type of mark,

a cross, for example,

to show that this stone was intended to go next

to another stone with the same mark

in the corresponding place.

(gentle music)

The crosses served as guides

for the stonemasons by indicating

which side of the block was to be placed

onto the previous one

and which side would remain visible

until the next block was placed.

What matters here is the cross.

The crosses, the notches,

and the dimensions of the blocks

all have equal importance.

But it seems there may be multiple versions

of how the blocks could have been arranged.

The reassembly presented a monstrous challenge.

(group speaking French)
(gentle music continues)

As we tried to position a stone

in such and such a place,

other questions, other problems would pop up,

and we'd suddenly realize that a stone actually went here,

and that would knock all the others out of position.

As the difficulty of the task

became more apparent,

the team reaches out to the digital experts again.

This time they need to scan each piece

of the mammoth puzzle

so that they can attempt to solve it virtually.

(determined music)

Engineer Eloi Gattie invented

a special device for this process.

Under domed lights equipped with four cameras,

the arch stones are photographed from every angle.

These photos are then processed

using photogrammetry software

to create a virtual copy of the object.

Each stone block is captured

as an incredibly precise 3D model.

(determined music)

In Marseille, Livio De Luca and Antoine Gros

begin to reproduce the state of the cathedral

one day after the fire.

(determined music continues)

They organized thousands of photos taken

by the team following the catastrophe.

(camera whirring and snapping)

By repositioning the photographs within the virtual space,

they create 3D models of the fragments

in such detail that it's possible to identify

the digitally scanned arch stones.

So, you see this photo?

We have stone X-001,

and we can go looking for it.

You can clearly see its tours and moldings.

(determined music continues)

(camera snapping and whirring)

The investigation continues.

From the position of the arch stones on the floor,

the scientists can calculate the vertical drop,

locating where they fell from.

(determined music continues)

They pinpoint the stone's original position in the arch.

But it's difficult to reposition every block,

because some were deflected when they fell.

They need a way to confirm the original positions

of all 70 arch stones.

Once all the data and all the observations

have been gathered and processed,

we're able to work through all this information

and make sense of it.

As regards to the assembly of the transverse arch,

we can offer, in parallel to the traditional methods,

a mathematical method.

Livio De Luca's team creates an algorithm

that integrates the pathways

of the arch stones' trajectories

with the discoveries made by the archaeologists.

So we have this tracking criterion

whereby we can work out the position

of a stone from its fall.

And added to this,

the archaeologists have found notches and crosses

in the stones by which they can work out their relation

to each other and reassemble them.

(celestial music)

But there were countless ways the stones might

have been assembled,

and no human brain can accurately process all the data.

They turned to a computer for a review

of the algorithm's information

to produce the most accurate result.

(celestial music continues)
(computers whirring)

Here we can see part of the solution

on the screen, where we have the different arch stones

put back inside that envelope.

Let's take N-222 as an example.

Using the algorithm,

we've been able to put that in position R-3.

No, R-4, right there,

and see it next to other arch stones in our solution.

(gentle accomplished music)

The researchers now have a solution

that seems 80% accurate.

But there's still some margin for error.

It's impossible to be certain of the precise position

for each of the 70 arch stones.

The secret code to unlocking this medieval masterpiece

still resists modern technology.

Nonetheless, this incredible process

has provided a wealth of information

about the way in which Notre-Dame's builders

were able to construct an arch over 30 meters high,

a first in the history of Gothic architecture.

The fact of physically attempting

this reassembly helped us to understand things

that we wouldn't have understood any other way.

We've got a real understanding

of the weight of these blocks,

of the methods used for lifting these blocks

over 30 meters above the ground,

and of the difficulties involved in manipulating

such heavy blocks in order to orient them

in the right positions on the wooden centering.

What I'll gain some understanding of here

is how the work site,

the work site of that period, would've functioned.

Pascal Prunet needed that in order to understand

how the arch worked,

but that also helped us to understand

how it had originally been constructed,

which will, in turn, help us to reconstruct it.

(gentle accomplished music continues)

The archaeological investigation was a success

because of the unique and unprecedented collaboration

between the multidisciplinary team and the architects.

(Pascal speaking French)

Now a decision has to be made.

It's unlikely that the salvaged blocks

can be placed into the reconstructed arch

without knowing the precise placement of every stone.

The solution for the reconstruction

is something that came about gradually.

In the beginning, we felt a sort of moral obligation

to put these arch stones back in place.

We wanted to show that what had been recovered

could go back where it belonged.

But we don't know that.

And if we're not sure that it's going back

to its correct position, then in a way,

just placing it in some arbitrary position

isn't necessarily a good thing.

(celestial music)

The exquisite scan of the cathedral

done before the fire will allow it

to be reconstructed identical to the original.

But the arch stones must all be new,

cut to the same dimensions as the medieval blocks.

The team now needs to find the source

for the replacement stones.

(celestial music continues)
(hammer knocking)

And search for limestone with the same characteristics

and composition as the cathedral's original stones.

The answer came from examining the transept crossing,

because the architect who rebuilt the vaults

in the 18th century had faced the same problem.

Paris quarries had been stripped clean

during the Middle Ages.

So where did he find his replacement stones?

Geologist Lise Leroux analyzes a sample

from an 18th-century block

at the Historical Monuments Research Laboratory.

The sort of observations a geologist

can make regarding a sample of stone taken

from a building are in the order of determining

what are the principal minerals that can be seen,

that can be recognized,

and also whether there are fossils or not.

It so happens that in this piece

there's a whole series of different microfossils.

The sample contains billions

of fossil skeletons and remains of microorganisms

that have collected over eons

during the formation of the limestone.

On this image in particular,

we can see a tiny bit of debris of a fossil.

It's a fossil of Orbitolites complanatus.

(soft celestial music)

There are also minerals present,

which can give us some clues.

We know in what kind of environment this rock formed,

and it's quite probable that it comes

from the Vexin area of northern France.

(soft celestial music)
(lever whirring)

Lise Leroux has access to a rare asset

that allows her to pinpoint the limestone's origin.

Generations of geologists have accumulated

a unique stone collection with more than 6000 samples

from monuments and quarries all over the country.

It's a geological map of the monuments of France.

Amongst these stone samples

from a sculpture on the Pont d'Iena in Paris,

we have exactly the same assortment of fossils

and minerals as in the sample taken

from the arch stone from Notre-Dame.

And it just so happens that for the Pont d'Iena,

we have a written source telling us the actual quarry

that the stone came from,

which is at a place called Sagy in the Val-d'Oise.

So this confirms that for the stones of that arch,

we have a source which is a quarry in the Vexin region.

(dramatic music)

Based on the geological investigation,

the stone for the reconstruction can now be sourced.

Work begins in the Val-d'Oise quarries to extract limestone

from which Notre-Dame's new arch stones will be cut.

(dramatic music continues)

The plan links the past to the future,

ensuring that the majestic arch will once again soar

in the name of Notre-Dame.

(dramatic music continues)

As the work progresses,

today's builders continue to discover

more about the incredible accomplishments

of their medieval predecessors.

It's one of the very first cathedrals,

and it's like they pulled all the materials up

into position to the very limits of their capabilities,

and yet it didn't settle over time.

The flying buttresses remained,

the vaulted ceilings were still there,

with the roof's framework above.

Eight centuries, and it hadn't moved.

Notre-Dame keeps its secrets close,

and the methods builders used for a structure

that survived throughout centuries remain mysterious.

(ethereal music)

It's impossible for the architects

to rebuild the edifice without understanding

and confirming its stability.

When the cathedral's roof was destroyed,

the building's framework

and careful balance was compromised.

The fractured vaulting was dangerously weakened

by fire and water,

yet it continued to support over 500 tons

of damaged lead and wood.

Scientists, along with wood,

metal, and stone specialists unite

to determine how these materials worked together.

They will probe the stone vaults,

burrow into the masonry to examine the metal skeleton,

and investigate the destroyed roof's wooden frame.

Slowly, they will learn the laws of physics

that governed and gave life to this gigantic

but fragile house of cards.

(brooding music)

(air whooshing)
(brooding music continues)

(air whooshing)