All language subtitles for a.s.s02e03.1080p.hdtv.h264-cbfm_track3_[eng]

NARRATOR: On the border
of Europe and Asia, in Turkey

lies one of the most
majestic monuments on the planet,

the Hagia Sophia of Istanbul.

This monumental structure
was built more than 1,500 years ago

as a basilica,

transformed into a mosque

and then a museum,

and recently
converted back into a mosque.

Its 6th-century dome
soars more than 55m,

the equivalent of
a 17-storey building.

It had been
a challenge for architects

for the coming millennia.

How does it stand?

Today, researchers studying
the 7,540-square-metre monument

rely on the latest technology.

Wow. It looks so impressive.

Actually, from our level,
it looks much different.

By scanning from the aerial scale,

to the satellite scale,

and the molecular scale,

specialists can now unravel

the greatest mysteries of
this UNESCO World Heritage Site.

How did builders piece together
and transform this structure

over the centuries?

What is the secret to
keeping its gigantic dome standing?

This is a higher vault
than any of the Gothic cathedrals

that were built centuries later.

There is no precedent for it.

Positioned on one of the most active
seismic faults in the world,

the Hagia Sophia has resisted
earthquakes since the 6th century.

What architectural mysteries
can science reveal

to explain this
impressive resistance?

WOMAN: So what we learn is how
the structure behaves differently

at different locations.

And what incredible discoveries
have recently been unearthed

from the depths
of this iconic monument?

WOMAN 2: There is Hagia Sophia,
subterranean.

This is another world.

This is another marvel
of Hagia Sophia.

History, science, engineering,

exposed as never before.

Istanbul's Hagia Sophia

finally reveals itself
on every scale.

On the shores of the Bosporus,

the Hagia Sophia's iconic silhouette

stands out on the Istanbul skyline,

a massive building,

with a gigantic dome
and four towering minarets.

Builders carried out a series
of modifications

over its 1,500 years of existence...

..leaving a complex structure,

combining centuries
of engineering ingenuity

that still
fascinates historians today.

The building has been there
since 6th century,

and it had a very, very long
afterlife.

So it's like a patchwork.

Today, archaeologists
are using new technology

to solve the greatest mystery
of the Hagia Sophia.

They want to unravel
the sequence of building works

that form the building.

Bilge Ar is a professor
at the Department of Architecture

at Istanbul Technical University.

She thinks that
the key to understanding

the structure of the current building

is to investigate
its original construction.

Behind me is the 6th-century
Hagia Sophia,

but the 6th-century Hagia Sophia

is the third building with
the same name that is standing here.

The Hagia Sophia today is, in fact,

the third building
of this name to stand here.

The first was
completely destroyed by fire.

The second was built
in the 5th century,

and traces of it still remain today.

Behind me, we see
the original ground level

of the 5th-century Hagia Sophia.

You see some friezes back there
with some decoration on it.

And we have some remains
of its entrance portico.

The whole building occupied

about the same amount of
ground space as of here today.

3D modelling techniques

make it possible to visualise
the 5th-century basilica

that preceded the Hagia Sophia
we see today.

Builders made an immense
60m-wide structure

in a rectangular form,

fitted a wooden roof

and an entrance portico supported by
a series of monumental columns.

The structure suffered the same fate
as the first that stood here.

Hagia Sophia
of the 5th century was burnt.

So Justinianus immediately started
the project of a new cathedral.

After the second Hagia Sophia
burned to the ground,

6th-century emperor, Justinian,

ruler of the Byzantine Empire,

decided to build
a third version of the monument.

The Hagia Sophia that he started
would be transformed and upgraded

by generations of builders
over the next millennium,

leaving a gigantic puzzle

that can only be deciphered
using the latest technology.

Bilge usually studies the building
from the ground.

But today she's working with
drone operators

to investigate the building
from the air for the very first time.

MAN: It will take some shots,
wide angles and some close ups.

We are very ready to flight.

By flying over the Hagia Sophia,

almost 100m above the ground,

the aerial views change the scale
of her observation.

Wow. It looks so impressive.

Actually, from our level,
it looks much different.

It's very important

to grasp the buildings
in the three-dimensional scale.

The drone,
it's actually like a 3D-model.

It's a complete different thing.

And the three-dimensional figure

would give us much more opportunity
to understand and express

what we have to tell
about the building.

Today, it's difficult to recognise

the design chosen by
the monument's original builders,

but the drone views allow Bilge

to spot the original plan
of the third Hagia Sophia.

Well, basically, I mean,
look at the plan.

It's a basilica.

I mean, it's closer to a square,

which is typical for
a Constantinople basilica.

Well, the main challenge
is certainly to put

the round plan of a dome

over the rectilinear plan
of a basilica.

And we should look at
this building as an experiment,

and a building in a colossal scale

trying something
that has not been tried before.

3D modelling exposes

the unprecedented architectural
innovation of the Hagia Sophia.

This is the first time that

engineers building a basilica
of more than 5,000 square metres

would install a monumental dome

measuring 32m in diameter.

And they extended it by building
two semi-domes on each side.

Finally, they added
four massive buttresses

to each side
of the 6th-century building.

But today, the Hagia Sophia's layout

appears much more complex.

That's because
generations of builders

would go on to add new structures
to the original building.

These modifications are
easily spotted from the air by drone.

Could we go to the eastern parts?

We will see the later buttresses.

Attached to the narthex,

we see a series of buttresses.

They could be added
as a repair activity

during the Latin occupation

or maybe in
the restoration period afterwards.

These are flying buttresses
used in sequence,

as they would be
in French Gothic construction.

So it's the Latins
who are in control of the city

are importing a structural system
that is developed in France

and using this
as a way of stabilising

the older cathedral of Hagia Sophia.

Bilge uses the drone to investigate
a final addition to the structure,

a feature ordered built
in the 15th century

by Sultan Mehmed II
of the Ottoman Empire,

some 1,000 years after
the monument's initial construction.

When Constantinople
fell to the Turks,

Mehmet II transformed
Hagia Sophia into a mosque.

So it needed a tower
for the call for prayer.

So the first minaret
was added during his time.

And then the minaret
was expanded to four minarets.

In fact,
you don't need four minarets.

These are functionally superfluous.

But, visually, the idea of
the dome framed by the minarets

becomes a very prominent symbol.

WOMAN: Architecturally,

it was not surpassed
for many centuries to come.

The influence this building had

on world architecture
is immeasurable.

After a millennium of transformation,

this innovative building
became an architectural model

for the mosques
of the new Ottoman Empire

from the 15th century.

You know,
when I first arrived in Istanbul,

I could see the silhouette
of Hagia Sophia,

and my heart stood still,

and I thought, "I have arrived."

And then the bus
went around another corner

and there was another Hagia Sophia.

And then it ran around
another corner

and there was another Hagia Sophia.

And finally I realised

it's that all of the mosques
in the city look like it.

While the monument's
exterior appearance

became an iconic mosque design,

inside the building,
visitors are impressed by

the extraordinary volumes
of the structure,

and especially its gigantic dome.

A true architectural feat,

it is known as one of the greatest
masterpieces of antiquity,

with a diameter of over 30m
and a height of over 55m.

It has 40 windows that bathe
the whole interior in light.

When we look at 6th-century
attestors of the building,

their descriptions
also say the same thing.

Procopius, for example, says

the dome is like it is suspended
from heaven with a golden chain.

The effect of these windows,

making a break between the dome
and it's all structural elements,

so it feels like the dome
is just flying in mid-air.

How did builders manage to construct
this colossal structure

which seems to float in the air,

without modern techniques and tools?

The answers lie in understanding
the origin of Hagia Sophia.

When the Emperor Justinian
demanded it built in the 6th century,

he ordered his architects to surpass

a building he considered
the most impressive in the world,

A monument located
almost 2,000km away

in the heart
of the ancient city of Rome.

The first idea was to compete
the Pantheon in Rome

with its great dome...

..a building that has set itself up
to be admired,

but also to be imitated.

And with Hagia Sophia,
it's not so much imitation,

but doing something that
goes beyond the Pantheon.

PROF. AR: The base of the dome

is continuously carried
by masonry walls.

Here we have a skeletal system.

The dome is sitting
carried by four very large arches,

which are supported by
four very large piers.

So the architecture of the place
is actually a colossal canopy.

3D modelling exposes
the challenge that builders faced.

They would build a dome on top of

a square structure
formed by four pillars,

linked together by four Roman arches,

more than 30m high.

But the innovation didn't stop there.

To surpass the Pantheon,

Emperor Justinian
bets everything on the gigantic dome.

In contrast to the Roman dome,

which is built
in the form of a sphere,

the emperor wanted to build
a lower dome,

to symbolise a celestial vault

and to do something
never attempted before -

fit it with 40 windows.

What this means,
in terms of the design,

is that although
it appears transcendental,

airy, weightless and so on,

it is in fact a very heavy building.

The low dome collapsed
only a few years later,

after its construction

and was immediately rebuilt
in its present form.

But how did engineers allow this dome
weighing thousands of tons

to rest on only four pillars?

3D technology exposes the hidden
secret of its construction.

The dome is built with 40 ribs
that form a stone framework.

That is not a flat surface,

but the interior of the dome
is ribbed.

There are ribs,
like the ribs and an umbrella,

that will strengthen the dome.

3D modelling reveals

the material hidden
under the decoration of the dome,

a very special type
of lightweight brick.

Examining the brick
on a microscopic scale

reveals that builders made them
from a mixture of earth and tufa,

a very light material.

At the time,
these bricks were so light

that engineers claim that 12 of them

weighed no more
than an ordinary brick.

This is how the builders
were able to allow

a ring of 40 windows
around the base of the dome.

This would be impossible
at the Pantheon.

The 40 windows also made
the structure considerably lighter.

But there was another problem
that would challenge builders.

They needed to safely distribute
the weight of the circular dome

onto four pillars forming a square.

When we cover a square space
with a dome, which has a round base,

we need some transition elements

to make the change
from the square to the circle.

So the pendentives
are basically actually doing that.

So as we see behind me,

they are these spherical triangles.

They are part
of the structural system.

Between the arches
and the base of the dome,

they are covering the empty area.

The solution was to fit
triangular devices called pendentives

that work like a buttress arch.

They redirect the weight of the dome

and concentrate it at
the four corners, over the pillars.

But to complete
this innovative building,

builders needed to solve
one last structural problem.

With the weight of the dome,

there is a tendency that the pillars
and the arches tend to open up.

To withstand outward forces,

builders added two
huge half-dome structures

to the east and west.

Then they build four
massive exterior pillars

to the north and south.

In just a few years,

the builders of the Hagia Sophia
succeeded in their mission

of surpassing Rome's Pantheon
and building an iconic new monument.

It had been a challenge for
architects for the coming millennia,

and it hasn't been achieved
for a very long time afterwards.

This is a higher vault

than any of the Gothic cathedrals

that were built centuries later.

So it's a dome that is not surpassed
until the 16th century.

Architects designing and building
the massive dome of the Hagia Sophia

had to learn from their mistakes.

Over centuries,
they added reinforcements

to consolidate the building,

features preserved at this UNESCO
World Heritage site to this day.

Incredibly, this huge monument

has withstood
numerous and severe earthquakes.

One of the most dangerous
struck Turkey in August 1999.

The earthquake had a magnitude
of 7.4 on the Richter scale

and devastated
the region of Izmit near Istanbul.

Thousands of people lost their lives

and hundreds of buildings
were destroyed.

I was there in 1999.

The house next-door to me collapsed.

It was a...

I was out in the street in my
underwear at 3 in the morning,

not knowing what to do.

And the next morning,

my colleagues who were working on
the restoration of Hagia Sophia

were back on the scaffolding
at work.

There was no damage
to the building at all.

Why is the area
so prone to earthquakes?

Examining the region on
a satellite scale exposes the answer.

Istanbul is situated close
to the North Anatolian Fault,

one of the most active faults
in the world.

It's the border between the Anatolian
and Eurasian geological plates.

As these plates
slide against each other,

moving about 2cm every year,

pressure builds
and is released during earthquakes.

But despite this threat,

the Hagia Sophia
has remained standing until today.

Clues to explain how this is possible

can be found by examining something
invisible to the naked eye -

the stratigraphy of the soil.

The Hagia Sophia is built
on top of an anticline...

..a fold in the Earth's crust

created by the movement
of tectonic plates,

formed more than
400 million years ago.

Examining the rock
at a microscopic scale

exposes that it's a stone
known as compressed schist,

a particularly strong material.

But this alone doesn't explain
how the monument stays standing.

Eser Cakti is the director
of the Seismic Laboratory

of Bogazici University in Istanbul.

She has been commissioned
by the authorities

to assess the ancient and future
capacity of the Hagia Sophia

to resist earthquakes.

Her team has placed the monument
under high surveillance

using state-of-the-art technology.

She wants to understand
the building's movement

and its reaction to tremors.

The technology that we use
in the structure

is to monitor earthquakes
or earthquake vibrations

on Hagia Sophia, on the structure.

We put, all together, nine sensors
at various locations.

Of the nine seismic sensors
placed in the Hagia Sophia,

four are placed
at the top of the dome,

four at the arch levels,

and one is located on the ground.

The sensors operate 24 hours a day,

and continuously record and transmit
the structure's slightest vibrations

to the Seismic Study Centre.

Last week,
we had a 4.2 magnitude earthquake,

close to the city of Istanbul,

and this was actually recorded
by our stations

and mainly by the recording system
in Hagia Sophia.

Therefore, this earthquake
was actually felt by Hagia Sophia.

The researchers use daily information
collected by the sensors

to carry out 3D simulations

that amplify
the building's deformations

and expose
the Hagia Sophia's weak spots.

So what we learn is how
the structure behaves differently

at different locations.

Some points they vibrate more
as compared to the other points,

particularly
the eastern and western arches.

Also some of the piers, main piers.

So for those locations,

we pay more attention
in our analysis

and look at them in more detail.

In recent years,
the seismic research team

has been using a new technology

to monitor the arches and pillars
more closely,

and assess whether significant
deformation is taking place -

laser scanning.

The technology allows them
to monitor large surfaces

and discover details
invisible to the naked eye.

This is invaluable information

because, you know, the size
of the structure, it's huge.

It's colossal.

However, deformations
in the order of centimetres

may mean a lot.

But from the ground level,

you might not be able to sense

those deformations
in the order of centimetres,

while the sensors can.

By taking millions of
measurement points every second,

the researchers can create
a detailed 3D model of the building.

Every year they create a new model,

allowing them to compare,
point by point,

the slightest micro movements
in the superstructure.

We have compared the images
or the information,

and according to our analysis
and comparison of our results,

structure has not moved recently.

Today, the Hagia Sophia
seems unshakable.

But this wasn't always the case.

Engineers building the first dome
of the 6th-century Hagia Sophia

used a different design
to the one we see today.

With its peculiar geometric shape,

the forces exerted
by the original dome

were directed to the sides,

making the structure
particularly unstable

in the event of an earthquake.

With the collapse of the first dome,

the architect of the second dome

went safe
rather than daring in his design.

So that the geometry of the dome
as it was rebuilt

is different from
the original geometry of the dome.

Five years after its collapse,

architects built a new dome,

6.25m taller, reaching
its current height of 55 metres.

The new dome might be
higher and heavier,

but it's also much stronger.

It was built higher.

No doubt it was heavier,
but the form was more stable.

That is,
rather than pushing outward,

the thrusts of the vault
are pushing more downward,

because of the hemispherical form
of the dome.

The reconstruction of the dome
in this new form

was crucial to the final
earthquake-resistant shape

of the Hagia Sophia today.

Now, new technology
like 3D laser scanning

lets researchers peer back in time

and identify the challenges
that the builders faced.

What you see is the damages

or the reconstructed parts
of the main.

On base level, you can easily see

where the two parts
of the dome come together.

One part is the part that was
destroyed during the earthquake,

collapsed and reconstructed.

And then the other one,
the adjacent part

is the one that was original
that never collapsed, in fact,

according to our knowledge today.

After its first reconstruction,

the dome partially collapsed
twice more.

The damage allowed builders
to analyse and compensate

for the final weaknesses
of the structure.

3D-modelling techniques
expose innovative reinforcement

that the original engineers
dreamed up.

In addition to the half-domes

that held the main dome in place,

they added huge pillars
to the north and south.

In the 14th century,

a new type of reinforcement
was added to the east -

a flying buttress,

followed by a series of further work
in the 16th century.

The constant adaptation
and additions

that builders carried out
over the centuries

are the secret to
the Hagia Sophia's strength today.

There is no doubt that Hagia Sophia,
at the time it was constructed,

the best architects, constructors,
masons of the period was used.

There is no doubt about this.

And then it has been
very well kept over the centuries.

That's why it is still standing.

But the engineering of the building

is not the only explanation

for the incredible
earthquake resistance

of this monument.

Examining Hagia Sophia
on a macroscopic scale...

..reveals that builders chose
brick and mortar

to construct its walls.

This is a key feature
of the building.

Engineers chose to set bricks
into layers of mortar

of similar thickness
to the bricks they held.

The ancient technique
has been studied by Efdal Kirhas,

deputy director of
the Istanbul Conservation Laboratory.

His team studied the building's
mortar on a microscopic scale

in order to recreate
the original builders' techniques

and replicate it
during restoration work.

His team were able to figure out

the exact composition
of the material.

Investigating further,
on the molecular scale,

the team discovered that
the brick powder

was made from a rock
of volcanic origin called pozzolana.

This ingredient gives the mortar
special properties.

The elastic property of this mortar

allows the building's walls to deform
instead of crack,

absorbing shock waves and helping
the monument resist earthquakes.

But according to experts,

the mortar of the Hagia Sophia

has another important property.

Mortar will develop
micro-cracks in it

and will reset,
after, the cracks will re-adhere.

There's something
in the chemical property

that means this will be
a particularly stable form

once it's set
to its ultimate hardness.

But it can adjust.

Thanks to their
careful choice of materials

and upgrades to the Hagia Sophia
structure over centuries,

generations of engineers

succeeded in building
an iconic monument,

capable of resisting
frequent earthquakes.

Whether it will stand
the next big one,

only time will tell.

What we know from history, though,
is it never collapsed totally.

It has, definitely,
a remarkable earthquake resistance,

that's for sure.

Seismic engineers
can't predict with certainty

whether the monument
will withstand the next big one.

So the Hagia Sophia remains
under close surveillance.

The original 6th-century monument

was built in the space
of just six years,

an astonishing feat that
impresses specialists even today.

This is under six years

to build a building
that is larger than

any of the great Gothic cathedrals
in France.

You could put Notre Dame
inside Hagia Sophia.

I think only the towers
would stick out.

This is absolutely remarkable.

Today, the Hagia Sophia is
a magnet for visitors to Istanbul,

who marvel at its
incredible engineering and scale.

But the decoration of the interior
is equally as impressive,

and that's partly because of the
material choices that builders made.

Olcay Aydemir is responsible for
the restoration of the Hagia Sophia.

According to her,
the unique beauty of this monument

can be explained by the use of
precious decorative materials.

Examining the Hagia Sophia
on a macroscopic scale

exposes a palette of marble,

each one more precious than the next.

White, green and even red marble,

alongside an incredible display

of glass mosaics.

But where did the Hagia Sophia's
engineers source these materials?

Sometimes it takes centuries,
and new scientific approaches

to reveal
the secrets of the builders.

Nadine Schibille is studying

the origin of mosaics
at the Hagia Sophia.

By carrying out chemical analysis
of the glass,

She has revealed the geographical
origin of these materials.

90% or over 90% of the glass
does come from Egypt.

And the chemical composition
shows me that without any doubt.

This, in itself,
is not so surprising,

because glass was produced in Egypt.

We know that.

And Justinian gathered material
from all over the empire.

Examining this part of the world
on a satellite scale

helps visualise the origin of
material used by builders.

Blackstone came from the Bosporus.

Green marble from Greece.

Precious stones from Syria.

And glass from Egypt.

Builders chose materials
from all over the Byzantine Empire

to decorate its greatest monument.

But the choice of materials
offers more than just decoration

at the Hagia Sophia.

Inside, the light
seems to diffuse through its walls.

6th-century descriptions
emphasise exactly this aspect.

Of this vast space
that is suffused with light

and this is very impressive.

The Hagia Sophia's
four monumental arches

allow the structure to be opened up
with windows on all floors.

But it's the choice of
special marble,

visible on a macroscopic scale,

that contributes to
its unique appearance.

The marble is translucent

and has a high refractive index
due to calcite crystals,

the main component of the stone,

so light penetrates
the surface of the stone

before being reflected,

giving the material a unique glow.

The expertise of the architects -

that in itself, to me,
is just very impressive.

So the lower levels of the building
are covered in marble decorations.

The upper levels of the building
are covered in mosaics.

A vast amount of the mosaics
are actually gold-leaf,

and gold-leaf mosaic tesserae
are highly reflective.

So the tesserae are actually angled

to catch the light that comes in.

So they try to make the most
out of the natural

and artificial light
in interaction with the surfaces.

Builders fitted an astonishing

16,000 square metres of gold
decorations in the 6th century,

making the interior
of the monument shine.

Originally, these lavish
Byzantine embellishments

almost completely covered
the Hagia Sophia.

Then, in the 15th century,

the Ottomans transformed the building
into a mosque

and chose not to destroy the mosaics,

but to cover them over with mortar.

So what Byzantine decorations
remain hidden today?

What treasures of the Hagia Sophia
are invisible to the naked eye?

The Great Dome,
a masterpiece of Ottoman art,

has remained unchanged
since the 15th century,

but the majestic calligraphy

conceals another treasure
created six centuries earlier.

MAN: We know that by the 9th century
there was an image of Christ

where that calligraphy was located.

According to some researchers,
a huge artwork is still hidden

under the calligraphy of the dome,

a gigantic mosaic
representing Jesus Christ.

Other structures of
the Hagia Sophia,

invisible to the naked eye,

have spawned many legends
over the centuries.

There is a lot of interest

in what is
under Hagia Sophia today.

There are curious accounts from
people who visited historically,

who said, "Oh, there's a great
cistern under Hagia Sophia."

And that there is this network
of tunnels underground.

Your first thought is,
"A-ha! Mystery.

"This is exciting.

"Where does it go to?"

From 14th century,

the pilgrims ambassadors,
travellers,

they talk about this cistern.

They say that almost 3,000 boats

could sail in it until the sea.

Could there be truth in this legend

that has been passed down
for centuries by travellers?

What mysteries lie
deep beneath the Hagia Sophia.

Clues to this enigma...

..lie just a few hundred meters away
at the Theodosius Cistern...

..an astonishing
underground reservoir

built in the 5th century,

at the same time
as the second Hagia Sophia.

So this is a Byzantine cistern,
basically.

It is 40,000 cubic metres
of water capacity.

Cigden Ozkan Aygun
is an archaeologist and professor

from the Department
of Human and Social Sciences

at Istanbul Technical University.

She believes that the legend of

the Hagia Sophia's
subterranean secrets

stems from a feature of the city
during the Byzantine period.

We know there are 209 cisterns
in Istanbul.

Some of them are not existing,
of course,

but we know the existence of them.

But there should be
much more than this.

Examining satellite imagery
and 3D reconstructions

exposes what is
hidden underneath Istanbul.

Hundreds of underground tanks
were built

during the time that the city
was known as Constantinople.

These reservoirs were connected
to a vast hydraulic network

more than 500km long

that kept the city
supplied with water.

It's an amazing water work,

which shows us all the tradition

coming from the Roman technology.

This ancient water network
could have inspired the legend

of the giant cistern
hidden under the Hagia Sophia.

To solve the mystery
once and for all,

Professor Aygun assembled a team

to investigate the subterranean world
of the Hagia Sophia.

She wanted to explore
the depths of the superstructure.

She obtained special permission

to carry out one of the most
incredible excavation campaigns

that the monument has ever seen -

the exploration of passages
deep underneath the Hagia Sophia.

Of course, we were extremely
excited and curious,

because it was a chance
for us to discover

mostly about those structures
which has not been studied before.

For nine days, the research team
explored the depths

by every possible means.

Caving specialists and divers
joined forces

to discover eight
previously unknown wells.

Professor Aygun's
multidisciplinary team

recorded their exploration
into these unmapped areas.

We are in the well of Hagia Sophia,
in the nave.

It's full of water.

This well, most probably,
is the source of the legends.

Beginning from 14th century,

the pilgrims, travellers,
ambassadors

all mentioned about it,

because it's plenty of water.

It's always full.

10m of water, all the time.

So they thought, "OK, there must be
a big cistern there,

"because there is
always water in it.

"The water
is coming from somewhere."

So they thought
the source is much more bigger.

The water source seemed limitless
in the eyes of medieval visitors.

But where does it come from?

The team carried out
chemical analysis of water and rock

to make another discovery.

The water in this well does not come
from a cistern or from rain,

but from underground water sources,

a natural resource
located under the Hagia Sophia.

Revelations about what lies
underneath this monumental building

didn't stop there.

I began from the bath and I found
the world underneath.

We have found almost one kilometres
of water channels there.

Some of them are drainage channels.

They are mostly underneath,

because they are draining
the excess water from Hagia Sophia.

And under the western courtyard
there are the main supply lines,

and that makes Hagia Sophia

a part of the greater, longer
supply line of the city.

According to initial findings,

the Hagia Sophia does have
a hidden hydraulic network,

but not a large cistern
connected to the sea by canals.

To investigate inaccessible areas
of the underground network,

the team turned to new technology,

an ROV, or remotely operated vehicle.

They used the ROV to access areas
underground, including underwater,

that are too narrow or too dangerous
for humans.

Thanks to this
specially adapted machine,

Professor Aygun's team
was able to explore

galleries leading to
an incredible discovery...

..remains of
the ancient Hagia Sophia.

Under the exonartex,

there is a vaulted structure
supported by 23 piers.

It is most probably
prior than Justinian's Hagia Sophia.

Examining aerial images

reveals the location
of these hidden remains

under the Hagia Sophia,

a vaulted structure built
long before the current monument.

Evidence that when the original
and second Hagia Sophia basilicas

were built and burned down
in the 5th and 6th centuries,

engineers saved time

by building directly on top
of the old structures.

We can create more ideas,
more theories

about the prior Hagia Sophia.

So this is also very interesting,

for the unknown history
of Hagia Sophia.

Now the team has discovered
an even more mysterious structure

to the south of the building.

By taking precise measurements
in the field,

they've been able to model the
enigmatic room in three dimensions.

They believe it was
originally a tomb, called a hypogeum.

We have found that
it was a funeral hall,

not only one hall, three halls
together, connected to each other.

And most probably it's prior
to the first Hagia Sophia.

Hypogeum is
the most enigmatic, maybe,

because its walls are broken
on the east and the west,

and two water channels
are added into it.

So it is converted into
a water structure.

And why and when did they do this?

It's in the Byzantine era,
again, they did it.

But why they have neglected
this tomb, and converted it,

this is the question
that we are answering now.

Professor Aygun's team

have discovered treasures
from the Hagia Sophia's past -

thousand-year-old structures,
lost deep underground.

There is a Hagia Sophia
beneath the Hagia Sophia.

So there is Hagia Sophia
subterranean.

This is an underworld.

This is another marvel of
Hagia Sophia.

Now the team of archaeologists
are waiting for new permission

to continue their excavations.

They want to further explore
other discoveries they made,

such as a submerged hall located
nearby under the Topkapi Palace.

It's a tantalising glimpse
of the exceptional Byzantine remains

that have yet to be discovered
around and inside the Hagia Sophia.

Decade after decade,

archaeologists,
aided by the latest technology,

have unearthed many of the secrets
of the Hagia Sophia.

A monumental puzzle
modified over the centuries.

A robust structure
designed to resist earthquakes,

built using incredible
engineering innovations.

These are all characteristics

that make this
thousand-year-old monument

a world heritage of humanity
whose legacy must be preserved.

It is undoubtedly

one of the most important
architectural pieces,

certainly of the first millennium,

but probably
in history more generally.

It's a cradle in
the history of architecture

and history of human beings.

You have to see it
and you have to experience it.

Captions by Red Bee Media
(c) SBS Australia 2022