Original subtitles

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