Original subtitles

(dramatic music)

Lyon is the third largest city in France.

During antiquity, it was the largest

and the capital of the Gauls.

It was then called Lugdunum,

a city that would establish itself as a showcase

for Rome abroad.

With its theater, which could seat 10,000 people,

five times as many as the Paris Opera House

thanks to vaults constructed in Roman concrete.

It was this technology that allowed the Romans

to achieve such things.

With its status as capital of the Gauls,

Lyon and its Sanctuary of the Three Gauls

symbolize the massive Romanization

of the defeated Gallic tribes.

When you went to the sanctuary,

you weren't worshiping Jupiter,

you were worshiping Rome and the emperor.

A sanctuary that backed

onto a huge amphitheater,

which put on plays and gladiator shows.

(crowd cheering)

It had a seating capacity of 20,000.

Mammoth scale work was needed

to construct such a huge edifice.

Lugdunum with its network of aqueducts

of more than 200 kilometers long,

one of the greatest technological feats of the Roman Empire.

It was the second largest water supply system

after that of Rome itself.

The aqueducts were among the longest in the empire.

The Romans also constructed underground.

A unique structure as long as the Champs Elysees

the so-called Fishbones.

The Fishbones are inside an 80 meter slope

and measure two kilometers.

This engineering work is the only one of its kind

in the world.

Lyon owes its existence

to Julius Caesar's conquest of the Gauls

between 58 and 50 BC, during one of the longest

and bloodiest military campaigns in history.

So how were the formidable edifices and monuments

of Lugdunum built, and for what reasons?

(dramatic music)

Lyon.

At the confluence of the rivers Rhone and Saone.

Today it's one of France's largest urban areas.

A crossroads chosen by Rome to establish

its first colony in Gaul here on the Fourvière Hill.

Lugdunum was the showcase for Rome in Gaul.

That's why the city was endowed

with all the buildings and facilities

worthy of any Roman metropolis.

Its forum, circus, amphitheater, Odeon

and most importantly, its theater,

one of the first buildings to be constructed.

Today, it's the most impressive of all the remains

of ancient Lugdunum.

Remains that were long forgotten,

but which are now a reminder of the former glory

of a city that was none other than the capital of the Gauls.

It was a large city and given preferential treatment

by the Roman authorities as soon as they founded it,

so it became the very example of a Romanized town

with the full panoply of typical Roman monuments.

Its development was important,

because it was supposed to represent the Romanization

of all of the recently annexed Gauls.

Why did the Romans decide

to make Lyon so monumental?

And how were its monuments constructed?

To answer these questions,

we must go back to the founding of Lugdunum.

The history of Lugdunum began in Vienne in Gaul in 44 BC.

A few years after Caesar's conquest,

it was there that the Roman veterans of his legions

were temporarily installed.

(shouting)

Romans that abandoned rebel Gauls

were determined to kick out.

(shouting)

The Republic of Rome appointed one man

to find these veterans and their families a refuge.

The governor of Gaul, Lucius Munatius Plancus.

His task, to found a colony,

the first in these newly conquered lands.

Plancus decided that the colony would lie

30 kilometers to the north of the Roman town of Vienna,

today's Vienne at the confluence of two rivers,

the Rhone and the Soane,

a strategic crossroads at the extremity

of transalpine Gaul, already under Roman control.

He had to control this crossroads,

which gave fast and easy access

to the northern provinces by river.

The Romans called northern Gaul Gallia Comata,

which means heavy Gaul,

because that was where the people were considered

to be a bit more barbarous than in the south.

And so Lyon was the transition point

between the civilized south and the north.

(ominous music)

43 BC.

Lucius Munatius Plancus founded his new city.

The colony was given defensive walls

and a right-angled layout typical of Roman cities.

With its cardo, the north-south road,

and decumanus, the east-west road.

This allowed them to organize circulation

inside the city, and it dictated the sites

of the public spaces like the forum

and the surrounding dwellings.

But how did they layout the right angles

that would shape the new city?

Roman surveyors used an ingenious topographical tool

called a gruma.

The principle was simple.

At the top of a pole planted in the ground,

two perpendicular arms with plumb lines at the ends.

You line up the strings until one hides the other,

which means you have a straight line.

Then I move my string and line it up with the other

perpendicular plumb line.

Then I can trace the second perpendicular line.

It was a very good system and pretty precise.

It was on this basis of right angles

that Lyon was laid out and began to grow.

But the city was yet to be given its first known monument.

The theater.

With its 108 meter diameter and height of 30 meters,

this gigantic structure stood out from the entire cityscape.

It was a political symbol of Rome,

the Empire of Rome and the civilization of Rome.

In Roman cities the theater stood out architecturally

as a showcase for Rome,

but also for its dimensions and the huge number of people

it could seat.

This architectural giant

could seat 10,000 people,

five times more than the Paris Opera House.

It comprised a stage at the foot of an ornate sculpted

and decorated backdrop wall.

Facing it were three seating sections.

Below them, the orchestra,

reserved for the most prestigious spectators,

such as the emperor and the notables of the city.

To what do we owe its existence?

To what principles was it constructed,

and with which materials?

To find out, we need to go back

to one of the most important moments

in the history of Rome.

(dramatic music, swords clashing)

27 BC.

After almost 20 years of political instability

and civil war, the Republic of Rome became an empire

under Julius Caesar's nephew, Augustus.

A man would weigh like no other

on the fate of the Roman colony.

In a way, Augustus was the main figure

in the history of the Roman Empire,

because he was the one who first imposed the system

of dynastic governance.

What he put in place would define the next 400 years

of Roman history.

Augustus reorganized the empire

and divided northern Gaul into three Gauls.

Gallia Belgica, with its capital Durocortorum,

today's Reims,

Gallia Aquitania, with its capital Mediolanum Santonum,

today's Saintes.

And the largest, Gallia Lugdunensis,

with its capital Lugdunum, Lyon.

Augustus would give Lugdunum a special place

by placing it at the heart of the new Gallic road system.

This was a very important political move,

making Lyon the heart of the road system,

but also of the military and economic system

of the three Gauls.

Within the empire,

Lugdunum now enjoyed a unique status.

But here as in Rome, Augustus needed to impose adhesion

to his new imperial ideology.

To do so, he would rely on the only mass media

available in antiquity, the theater.

The theater was a genuine propaganda tool

regarding the relationship of Lyon to Rome

and in a more general sense, of Gaul to Rome.

It was during the reign of Augustus

that the first theater was constructed between 20 and 10 BC.

It then culminated at the height

of the first section of seating and numbered 4,000 places.

A century later, its capacity would be increased to 10,000.

And yet, like in the rest of Lyon,

this monument had totally vanished from the cityscape.

In the 19th century, all that was visible

was the semicircular section of the Odeon wall.

All the remains you see now had been buried.

You have to imagine the site covered with

vineyards and pasture with a theater buried beneath

several meters of earth and rubble.

It wasn't until the early 1930s

that the great theater of Lyon was discovered

after a series of important excavations.

Excavations the like of which Lyon would never see again.

They unearthed the original structure,

that of a type of edifice never constructed before in Gaul.

And the shape of the theater was due to its purpose,

because here everything was based on one need, acoustics.

It's like dropping a stone into a pond.

The waves radiate out in circles,

the same with sound.

So it was imperative that the theater was semicircular.

So how does it work?

From the stage, the sound made by the actors

is transmitted directly to the spectators.

The constant slope of the seating

allows for this transmission without any obstacles.

The most surprising thing is the role

of the smooth paving in the orchestra.

If the actors spoke at the floor of the orchestra,

the flat part in front of the stage,

the sound bounced off it.

And as it was quite close,

the direct sound and reflected sound combined,

thus increasing the volume.

That also meant that the theater had to be of limited size,

because if you were too far away,

the sound and image would become out of sync.

There are no records to show

who ordered the construction of this monument.

And as is often the case with Roman edifices,

we don't know the name of the architects.

The reason we don't know the names of architects

is that they were not really considered to be

very important people.

They might even be slaves.

And yet, they were particularly ingenious.

Beginning with their choice of location for the theater.

There's a meaning behind the topographical location

pointing directly at Italy towards the rising sun.

That's why this site was chosen.

Etymologically, Lugdunum means hill of light,

and the theater plays on this name.

The theater was undoubtedly the biggest building

up on the hill,

the biggest in surface area, and the biggest in height,

so you couldn't help but see it standing out

from the landscape.

It was colossal.

While the architects chose this site

to emphasize the monumental nature of the theater,

they also made considerable gains

by avoiding unnecessary work.

By building the theater into the hillside,

they made a lot of savings on a large part of the structure.

The theater simply backed onto the hill

with no need to construct all the rear sections

like on flat land.

They did have to excavate the hill, however,

to make room for the theater,

which allowed them to recuperate building materials.

The excavated parts of the site served as a quarry

to supply the construction of the theater itself.

The Roman builders set the lower part

of the seating directly on the hillside.

But when the theater was enlarged,

it was impossible to proceed in the same way

for the upper seating.

So what did they use for support?

Their answer was an invention of engineering genius,

the so-called hollow structure.

This technique was invented in Italy

and employed since the second century BC.

It's known as hollow structure.

The walls fan out and are joined together by vaults,

and these vaults supported the seating.

As astonishing as it may seem,

the vast majority of this limestone seating

therefore rests on empty space,

space created by self-supporting semicircular vaults.

The Roman invention of stone arches and vaults

mark the advent of what we could call

modern architecture.

With large halls and spacious interiors,

which weren't possible before that,

when wall spans were limited by the size

of the stone blocks.

There are still traces

of these revolutionary vaults

among the remains of the theater.

I'm standing in one of the theater's vaults.

Their purpose was to support the masonry

on which the large limestone blocks

that formed the seating were placed.

This masonry was composed

of one of the strongest and most malleable

construction materials ever invented,

Roman concrete.

A mixture of lime mortar and rubble stone found on site.

This concrete would form the skeleton,

the main structure of the grand theater.

The advantage was, once the mortar had set,

the wall was the equivalent of a single block, a monolith,

a mass of set cement.

It was incredibly solid compared to a brick wall,

which could crack.

These walls were formidable.

It allowed them to build high, increase solidity,

spread the forces exerted by the edifice,

save on materials and thus free themselves

from the natural topography of the construction site.

The widespread use of this

extraordinary invention was a direct consequence

of the Roman conquests in the last two centuries

before our era.

Notably the capture of Carthage from Hannibal.

Then that of Corinth in Greece.

With these conquests, the Roman Republic

would inherit thousands and thousands of slaves.

Rome inherited a huge workforce, and what would it do?

Cut lots of small plain blocks.

Nothing beautiful or ornamental, just small blocks.

Go on, go cut me some blocks.

It was non-specialized labor.

Small blocks, more small blocks,

even more small blocks.

The genius behind the theater Lyon

was the combination of the hollow structure

with the incredible material that was Roman concrete.

To build the edifice, the workers first erected

the cladding of the straight piles for the vaults.

Inside, they poured the Roman concrete

made of lime mortar and rubble stone obtained on the site.

These piles then served as supports for the centering

on which the arches of the vaults

would also be set in concrete.

Once set, they removed the wooden centering

through small openings so it could be used again.

The aim was to recycle the construction materials.

Above this concrete structure,

the theater was given a bright white covering

of limestone blocks.

The construction of the theater must've been a massive job

made even harder by the gradient of the hill.

Most of the blocks arrived by water,

but then they had to be hoisted up the hill,

and this limestone has a density of seven,

which means that one cubic meter

weighs almost three tons.

The reason Roman theaters

like the one in Lyon were so numerous and imposing

was because they served the imperial power.

In this showcase of Rome, where plays, fables

and lines were performed,

the spectators were seated according to social ranking.

Senators and city notables in the orchestra

had seats nearest to the stage,

while citizens, plebes, free men and slaves

filled the less prestigious places higher up.

Augustus wanted inequality.

He didn't want people having the same status.

That way, it was he who held the keys to social promotion.

And for his system to work correctly,

there needed to be rungs of a ladder to climb.

Inside the theater,

one feature embodied the power of the emperor.

The stage wall, worthy of a palace.

More than 30 meters high, three superimpositions

of the different architectural orders

and numerous niches containing statues of the gods,

but also those of the emperor and his family.

All that remains of this sumptuous wall today

are a few bases and parts of the marble columns.

One of the theater's roles was to arouse wonderment,

and the decoration was part of this magic,

because you were entering a different world,

filled with pomp, the gods, the emperor,

the imperial family, and important figures of the city.

This theater,

one of the most beautiful monuments of Gaul,

still bears witness to its former pomp.

The walls carry the traces of the marble cladding

that covered them 2,000 years ago.

But where is the rest of the monument?

How could such an edifice fall into such ruin?

And why was it dismantled?

At the top of the theater is an unusual block of limestone.

This block has two characteristics,

an unusual decoration on the front and sides,

and on the top, a hole with the diameter

about 30 centimeters.

This hole was made to hold a wooden pole.

This block tells us that the Lyon theater

once had a velarium,

a system of ropes and canvas awning

that sheltered the spectators from the sun.

Unfurling the velarium was part of the show.

The spectators were wowed by the technological prowess.

What's more, it involved them in the show,

because the awning of the velarium was colored,

and these colors reflected down onto the characters

in the play.

Another of these characteristic blocks

was discovered in a nearby Christian burial site

where it had been reused as a sarcophagus.

This block is extremely interesting,

because it has the same decoration as the block

that was part of the theater's velarium.

This block was certainly reused as a sarcophagus

after the semicircular theater wall had been dismantled

to use the materials for other purposes.

Sadly, we don't have a precise date,

but the sarcophagus was probably made

in the late fourth, fifth or sixth centuries,

which is the only clue we have

to the dismantling of the theater.

Although we do not know the exact date

of the dismantling of the theater Lyon,

it is certain that it ended up as a quarry

for the heirs of its original builders.

In the heart of Lyon's Presqu'ile district,

the Basilica of Saint Martin d'Ainay.

To construct its 11th century facade,

limestone blocks removed from Roman edifices were used,

perhaps even from the theater.

Inside, beneath the dome, stand four monumental

granite columns of about one meter in diameter.

These were originally nine meters tall

and weighed 10 tons each.

The first remarkable thing is that the stone

used for these columns wasn't local.

They're made of an extremely dense granite

typical of the quarries in the Aswan region of Egypt.

How did these two monolithic columns

reach Lyon and finally end up in this church?

All we know is that the Roman builders imported them.

The use of monolithic blocks is very typical

of Roman architecture in that every task

was a genuine challenge.

Extremely heavy and compact monumental materials

were preferred so as to underline the remarkable nature

of the architecture.

According to tradition,

these columns came from one of the most

important Roman edifices in Lyon.

Situated at the foot of the Croix-Rousse Hill

was a gigantic amphitheater capable of seating

20,000 spectators.

This huge ancient stadium backed onto

the Sanctuary of the Three Gauls.

A monument implanted by Augustus to symbolize

Gallic submission to the empire.

It was a sanctuary dedicated to the imperial cult

for all the people of the three Gauls.

It ensured a permanent link to Rome,

and it was one of Augustus's most brilliant ideas.

We don't know the exact location

of the sanctuary, but we do know it contained

a huge altar dedicated to Rome and its emperor,

an altar most certainly flanked by two monumental columns,

and all around, 60 statues representing

the 60 Gallic tribes.

Delegates would come every year in August,

the month named after the emperor

to swear an oath of allegiance, recognition and affiliation,

so total adhesion to the imperial ideology.

Every year, the delegates

of the 60 Gallic tribes elected from among them

and for the coming year a high priest of Rome and Augustus.

One of them was a certain Gaius Julius Rufus.

Early in the first century,

he oversaw the construction of the city's

most colossal Roman structure.

Gaius Julius Rufus

was one of the highest ranking delegates

of the Gallic aristocrats, who alongside the Romans

took part in taking Gaul further and further

towards a form of Romanity.

In the late 1950s, early 1960s,

archeologist Amable Audin

discovered a trace of this monumental structure.

They were the ruins of the Amphitheater of the Three Gauls,

a Lyon Colosseum with its arena

financed by the Gallic high priest.

Oval-shaped, it was constructed in 19 AD.

At first, 81 meters long by 60 meters wide,

it was enlarged during the second century,

increasing its capacity to 20,000,

almost as many as those in Arles and Nimes.

It rested partially on the hill

and partially on a hollow structure

with Roman concrete vaults,

a showcase that served as a gathering place

for the delegates from the 60 tribes.

They would make decisions of general interest.

They would present petitions to the governor

who would pass them on to the emperor.

It was also a time for executions

and for gladiatorial fights.

(cheering)

Gladiator fights,

wild animals

and executions.

Yet another expression of Rome's power,

but also a way of encouraging the total adhesion

of the vanquished Gauls to the culture of their victors.

I'm in the chapel dedicated to Nemesis,

a goddess who is highly respected by the gladiators.

What's more, it was the only room with direct access

to the arena.

So every gladiator had to come through here,

stop to reflect a while before going out into the arena

to face combat.

To watch the fights and take part

in the discussions of the council of the three Gauls,

the delegates from the 60 Gallic tribes had reserved seats.

These limestone blocks, weighing up to one ton,

were moved from place to place

with state-of-the-art hoisting equipment,

such as the treadwheel crane,

a machine which owed everything to a Roman invention

that's still used today, the pulley.

The most important Roman invention for construction

was the block and tackle,

a system of pulleys which enabled them

to lift heavier and heavier weights

with less and less labor,

thus gaining in efficiency and profitability.

Found during the archeological digs,

the blocks were returned to their rightful places

in the amphitheater during restoration work.

On the first part of this block are the three letters,

A, R, V, standing for the Arverni,

present day Auvergne.

Then there's B, I, T, C,

standing for the Bituriges Cubi,

present day Bourges.

And there are vertical lines that show the separation

between seats.

Three delegates of the Bituriges here,

and one for the Arverni here,

almost certainly with his colleagues to our left.

It was due to these discussions

which the walls of this amphitheater undoubtedly overheard

that in 48 AD, the emperor Claudius,

who was born in Lyon, spoke out for the Gallic elite

to be incorporated into the Roman Senate.

A historic speech given in Rome

and reproduced on these bronze tablets

discovered at the foot of the Croix-Rousse Hill.

After Claudius became emperor,

certain Gallic nobles submitted a petition

demanding that they have the full rights of Roman citizens.

What's remarkable about this,

is that an emperor in the first century AD

should stand up for the integration of foreigners.

Like the Roman colony of Lugdunum,

the Lyon amphitheater would forever change

the face of the Gauls.

Gauls for whom Lugdunum had been presented as a showcase

for a new Romanity during the first

and second centuries AD.

Other than its theater, it had all the attributes

of a modern Roman city.

It's Odeon dedicated to the lyrical arts

built in the late first century.

It's circus for chariot races,

the remains of which have never been discovered,

but whose existence is confirmed in records.

And its numerous mosaics,

which testified to a wealthy city through trade.

Lyon was undoubtedly one of the biggest cities

in the empire.

We know this from ancient texts,

but also from the remains still present in Lyon today.

It was probably one of the 10 largest cities

in the entire Roman empire.

You must imagine lots of merchandise,

especially foodstuffs being shipped up the Rhone Valley

to Lyon, where it was then sold.

This made Lyon a commercial hub.

It was a very active cosmopolitan city,

which drew a lot of foreigners.

A number of monuments

of this cosmopolitan city remain on the surface,

but also magnificent structures underground.

Beneath the slopes of Croix-Rousse Hill,

an underground maze of shafts and galleries

as long as the Champs Elysees

the Fishbones,

a name which this feat of engineering owes to its shape

with one main gallery and 32 secondary galleries

placed like bones growing out of a backbone.

The fish bones are inside an 80 meter slope

and measure a total of two kilometers long.

There are 400 meters of shafts.

This engineering work is the only one of its kind

known in the world.

An underground superstructure

unique in the Roman world.

It was discovered in its entirety in the 1960s,

during the digging of the Croix-Rousse tunnel.

It has since fueled all kinds of fantasies,

the hiding place for the treasure of the Templars,

a secret military defense system.

Today it's used to drain rainwater from the hill.

In 2008, using carbon-14 dating,

archeologists became certain that it was constructed

by the Romans sometime in the early first century AD.

We're now pretty much sure that it's a Roman structure

built in several phases,

but with the first phase quite early,

probably during the reign of Augustus.

It must've been extremely expensive

and difficult to construct,

and it remains a complete enigma to us today.

It was thanks to this stonework shaft

that the network was first discovered in 1959.

The shaft measures two by two meters

and is 25 meters long,

half the length of an Olympic swimming pool.

There are a total of 16 shafts

like this one in the Fishbones.

They were used to remove the rubble

produced by the digging of the two kilometers of galleries.

The network was linked to the surface.

Were the shafts solely for the evacuation of rubble

from the digging?

That seems unlikely.

There was clearly a need for communication

between the surface and the underground network, but why?

Then there's the question of its connection

to the Sanctuary of the Three Gauls.

Was this just a coincidence?

And if they were connected, why?

To construct the network,

laborers started by digging the galleries,

which they then reinforced with semicircular vaults.

The galleries were then laid with mortar

mixed with rubble stone.

It seems as if they were intended

as some kind of storage space,

like a gigantic underground Fort Knox of antiquity.

They used the typical Roman vaulting system

for these underground galleries,

which is somewhat puzzling,

because the vaults are absolutely perfect

and employ the same system as for the theater

and other edifices in the city.

Inside these vaulted galleries,

another detail catches the eye,

the particularly thick and solid floors.

It's what's known as a hedgehog sub-base,

essentially made up of rubble, bits of granite and nice,

which were used a lot in Roman construction in Lyon.

And the sub-base is covered in a layer of concrete.

The total thickness of the sub-base and concrete apron

is about 80 centimeters.

That may have been to make the structure more solid,

but also perhaps for the storage

of particularly heavy materials or objects.

Heavy materials like those used

in the Lyon mint,

the only place to strike coins in the empire

outside of Rome.

A mint striking gold and silver coins like in Rome

to pay the Roman soldiers stationed in Gaul and Germania.

To protect this mint, Lyon was sent

an elite force of 500 men,

an urban cohort.

Lyon's importance is very much emphasized

by the presence of an urban cohort.

In other words, a garrison to protect the city.

And it's also the only provincial city apart from Carthage

which has any urban cohorts.

Were the Fishbones used to protect

the stock of coins struck in the Lyon mint?

The only certainty is that they were connected

to the Rhone River via which the heaviest materials

were brought in and out of Lugdunum.

The fact that the Fishbones,

clearly some kind of storerooms are so deep down

remains something of a mystery,

but because it's difficult to reach such a depth,

there was obviously some reason behind it.

While struggling to find the reason

behind the Fishbones, underground and protected,

they provide one of the most moving relics of Roman Lyon.

These fingerprints left by a child

possibly visiting the works,

but also this graffiti left in the fresh mortar.

Here we have one of the clearest pieces of graffiti

in the network,

(speaking foreign language)

It was scratched into the mortar just after it was laid.

Bituias was certainly the name of one of the laborers

who constructed this gallery.

How many men did it take

to build this gigantic underground structure,

unique in the Roman world?

History cannot tell us,

but it does testify to the flourishing life

of the former capital of the Gauls.

During the first century AD,

the population of Lugdunum continued to rise.

Historians estimate that it was around 30,000 inhabitants.

Aristocrats, citizens, plebes, freed men, slaves.

Their water needs were colossal.

Why did they need so much water?

Notably for the public baths and fountains,

but especially the baths, which used vast amounts of water.

To meet Lyon's needs,

Roman engineers constructed a network of four aqueducts

with a combined length of 220 kilometers

between the reign of Augustus and that of Hadrian

in the early second century.

Their total flow rate was 40,000 cubic meters a day,

20 times more than the aqueduct of Lutetia, today's Paris.

The water supply was the second largest after Rome

and its aqueducts were among the longest in the Roman world.

So it's incredible that the Romans

had better water supply in Lyon

than Lyon itself had until the 19th century.

How were the Roman engineers

able to achieve such a technological feat?

And with what means did they get around

the numerous obstacles in their way?

40 kilometers southwest of Lyon

is a tunnel dug by the Romans to allow the aqueduct

to cross a rocky outcrop.

These are marks left by Roman pickaxes.

There are also places where they were able

to hang their oil lamps.

This place is filled with life,

and is a very moving testimony to the laborers

who dug the tunnel during the Roman period.

Inside the tunnel, the water channel

is pretty much intact.

The tunnel through which the aqueduct runs

was solidified with a vault of Roman concrete.

They proceeded section by section.

They used wooden centering over a length

of two to three meters, then moved it further on

once the piles and the vaulting of that section

were completed.

A Roman invention, opus signinum,

a coating of mortar mixed with broken tiles and pottery

made the channel completely watertight.

One of the main characteristics of opus signinum

is the presence of broken pottery,

ground down to dust or slightly larger fragments.

It was this pottery which made the mortar watertight,

allowing water to flow through the channel with no loss.

The great Was this material

was readily available everywhere,

and it could be made of broken tiles, bricks, or pottery.

Water flowed along this channel

thanks to gravity by following the most constant

downward gradient possible of about one meter per kilometer.

Well, the idea is that the aqueduct starts

somewhere that's higher than where it finishes.

So the trick was to actually create a channel

that had a constant gradient of that sort

all the way from the source of the aqueduct

to the finishing point.

To follow the most constant gradient possible,

Roman engineers carried out extensive work on the ground.

Firstly, in locating water sources,

then tracing the best route for the aqueduct.

Their goal to limit the amount of heavy labor required.

The surveyors looked for two things.

One, the shape of the terrain and how to obtain

the longest possible straight lines,

and two, the parts of the route

where they might need to build a bridge.

But how were they able to maintain

this constant downward gradient?

The Roman surveyors, who were often called soldiers,

used an ingenious tool called the chorobates.

It acted like a modern day spirit level

with a beam, two rods and two plumb lines.

With the chorobates, they could calculate

the height difference between two distant points

and thus establish the most suitable gradient

for the aqueduct.

It was a simple tool, but handling it

took a lot of skill on the part of the surveyors.

And after that, they could dig the foundations

and start erecting the edifices.

Skill on the part of the surveyors,

but also of the builders

to which these bridges to cross rivers and valleys

and maintain the sacrosanct constant gradient

bear witness.

And always with the same principle, the arch,

to save on materials.

Works constructed with Roman concrete,

but with meticulous cross-link cladding,

just like in Rome.

The construction of an aqueduct in a Roman city

was a symbol of civilization.

Don't forget that Lyon was the capital of a Roman province,

and therefore considered a very important city,

and that justified the presence of such a vast

sophisticated water supply system.

On the piles of these arches,

one detail catches the eye.

These layers of bricks that run through them.

What was their purpose?

They marked a first full level during the construction.

Then the builders laid the casing on top

and placed the cladding on the outside

before pouring in the Roman cement

made of mortar and rubble stone.

But for the water to reach Lyon,

the biggest challenge lay elsewhere,

in the region's rugged terrain with numerous deep valleys,

including those immediately surrounding the city

higher even than Fourvière Hill.

You would have to pile up 15 bondegard

to actually fill the gap represented by the valley.

So clearly a bridge was out of the question.

To cross these deep valleys,

The Roman engineers used an ingenious system

based on that of communicating vessels.

The siphon.

Key to this system was a flush cistern,

which sent water under pressure into lead pipes.

This is the aqueduct channel

where the free flowing water arrived.

It's accumulated here in this tank,

and then it was released under the built up pressure

into the lead pipes.

Now why lead pipes?

Because nothing else could hold the pressure.

And also not one lead pipe, but a number of lead pipes

to divide the pressure so that they didn't all burst

when they got to the bottom.

Once under pressure in the lead pipes,

the water crossed the valley and its momentum enabled it

to climb up the other side to another slightly lower tank.

This second tank regulated the flow

before the water was released into the channel

and onto its final destination.

I have to say that out of pride for Leon,

we actually have a total of nine siphons at Lyon,

and in the whole Roman world there are only about 30.

So we have one third of all the siphons in the Roman world

are at Lyon.

So a lot of people know Lyon as the capital of the Gauls,

but we call it the capital of the siphons.

Capital of the siphons.

One figure testifies to that.

10,000 tons, the quantity of lead used for their pipes.

That's almost the weight of the Eiffel Tower.

This figure alone embodies the height of Lugdunum

in the second century,

before a terrible event marked the beginning of its decline.

(ominous music)

197 Ad.

Battle raged at the gates of Lyon.

A battle that opposed two claimants

to the title of emperor of Rome.

The battle of 197 was the climax of a civil war

in which Lyon sided with the eventual loser.

Lugdunum, which had sided

with the vanquished was ransacked.

Archeologists have found traces of this ferocious battle

hidden in the soil of Lyon.

These are very well-preserved arrowheads and a pilum

now kept at the Center of Restoration

and Archeological Studies in Vienne.

This is a fine example of a pilum.

Why is the metal shank so long?

Because on impact with the target, it would break,

making it unusable for the enemy.

So it's very rare to find one intact.

We usually just find small parts of them,

so this is a very rare find.

Among the traces of the battle

in the Lyon soil were these hastily made arrowheads.

This is the type of weapon made during a siege

when you needed to make a lot of them very fast

to keep the invading army at bay.

It's consistent with the siege and capture

of the city in 197.

It's an interesting find,

because it confirms the violent nature of the events

that took place at the gates of Lyon

and then inside the city.

(swords clashing)

This decisive battle marked the beginning

of a transformation for Lyon,

with the gradual abandoning of Fourvière Hill

for the banks of the Saone and the Presqu'ile district.

The abandonment was gradual,

but complete by the end of the fourth century.

And the hill then saw the development

of religious communities.

The urban population decreased,

the city shrink further and further

into the reduced city walls,

and these reduced walls ring what is now

the Presqu'ile district.

During the Middle Ages,

the former capital of the Gauls was methodically dismantled,

then forgotten.

But the remains of its edifices

constructed between the late first century BC

and the second century AD

still testify to its ancient glory,

like its theater, which now hosts the festival,

Les Nuits de Fourvière,

its Odeon,

its 20,000 capacity amphitheater,

its aqueducts and their ingenious siphons,

and the Fishbones,

that mysterious underground superstructure

and reminder that Lugdunum has yet to reveal

all its secrets.

(dramatic music)