NARRATOR: Hagia Sophia... For nearly a thousand years, the largest enclosed building on earth.

Its heavenly dome soars 180 feet high, supported by arches that inspire awe to this day for their strength and resilience.

When it opened, gold mosaics covered over four acres of its walls and ceilings.

How did ancient builders construct such a magnificent monument?

ROBERT OUSTERHOUT: There's nothing practical about Hagia Sophia.

It's all innovation.

NARRATOR: Built nearly 1,500 years ago in Constantinople-- modern day Istanbul-- Hagia Sophia has survived clashing empires by transforming from church to mosque to museum.

JOAN BRANHAM: Hagia Sophia carries both the history of Christianity and Islam within its walls.

NARRATOR: Most remarkably, Hagia Sophia has survived centuries of city-busting earthquakes.

Did ancient architects actually design an earthquake-proof structure?

Or will the next big quake bring Hagia Sophia down?

To find out, a team of engineers is monitoring the building and constructing a giant model, placing it on a hydraulic platform, and hitting it with powerful simulated earthquakes.

Can they unlock Hagia Sophia's seismic secrets before Istanbul's next big quake?

(klaxon blaring) ESER CAKTI: There is always this fear factor, this fear of seeing unexpected collapses.

Right now on NOVA-- "Hagia Sophia: Istanbul's Ancient Mystery."

NARRATOR: Hagia Sophia completed in the year 537: is one of the most magnificent buildings ever constructed.

Its size alone is awe-inspiring.

Only the pyramids surpassed it in height for almost a thousand years.

Its ceiling is a glittering gold dome that spans over 100 feet across and soars 180 feet above its marble floor.

The Statue of Liberty can fit beneath its dome with room to spare.

How did ancient builders nearly 1,500 years ago construct such a gigantic dome?

Since its completion, Hagia Sophia has withstood the rise and fall of empires.

It has transformed from Christian church to Muslim mosque to secular museum.

BRANHAM: Hagia Sophia influences a number of mosques and it became a model for Christian churches as well.

Its innovative, ambitious design and its monumental scale speak to people across cultures, faiths and religions.

NARRATOR: How can one building be a symbol for two different religions and continue to inspire people to this day?

KORAY DURAK: Hagia Sophia is a unique building.

There are only a few structures in the world that present different layers of history in the last 2,000 years.

NARRATOR: But perhaps the greatest mystery is why it still stands at all.

For Hagia Sophia is in Istanbul, known as Constantinople in ancient times.

The city straddles two continents-- Europe and Asia-- and a major earthquake fault.

Over the last century, the North Anatolian fault has unleashed a series of quakes.

The most recent, in 1999, was just 60 miles from Istanbul.

And it was devastating, leveling hundreds of buildings across the city and killing thousands of people.

MUSTAFA ERDIK: The damage caused by the '99 earthquake was extensive.

Plus, there is a huge human loss, about... we lost about 17,000 people.

NARRATOR: But somehow, Hagia Sophia is still standing.

In fact, Hagia Sophia has withstood every major earthquake for nearly 1,500 years.

What is the secret to its survival?

As Istanbul braces for the next big one, a team of engineers searches for answers by building an enormous scale model... and hitting it with a series of simulated earthquakes.

In the process they will uncover the building's strengths and weaknesses... weaknesses that could ultimately threaten Hagia Sophia's survival.

ESER CAKTI: NARRATOR: Eser Cakti is director of the earthquake engineering lab at Bogazici University.

She is tasked with monitoring the structural integrity of Hagia Sophia.

Slanted floors and leaning columns may appear alarming.

But Cakti is most concerned about Hagia Sophia's core structure.

That core structure comes down to a few key elements-- the enormous dome resting on four huge arches, which in turn are buttressed by four giant piers and two semi-domes.

Of particular concern are the four arches.

If any fail, the dome could collapse.

To monitor the arches, her team has placed sensors at strategic points.

The sensors can detect the faintest of movements.

CAKTI: The data that we obtain from here is very important in terms of understanding the general structural behavior of this huge building.

NARRATOR: This information is transmitted to screens at Istanbul's Earthquake Center.

Each of the multicolored lines represents vibrations detected by a motion sensor.

Normally the lines are nearly flat.

But when an earthquake strikes, there's a dramatic spike.

From results of years of monitoring, Cakti sees two places of potential danger.

CAKTI: These are the vertical vibrations on both the arches on the east and west side.

NARRATOR: Two of the great arches are moving more than they have in the past, which could have serious implications for the future.

If an earthquake comes strong enough, I think there is a real chance it can receive damage.

NARRATOR: Will the next big quake finally topple Hagia Sophia?

NARRATOR: To investigate what danger Hagia Sophia might be in, Cakti is turning to a tried and true technique-- a seismic shake table test.

It's worked before.

In 2012, Cakti teamed up with engineering team Eren Kalafat and Korhan Oral to analyze the structural integrity of the Mustafa Pasha mosque in Macedonia.

They built this large-scale model, placed it on a motorized steel platform, then shook it violently to simulate an earthquake.

The idea is that wherever damage appears on the model is where damage would appear on the actual building, giving engineers important insights to protect the real structure.

It worked for the Mustafa Pasha mosque, but will it work with Hagia Sophia, a building larger, heavier and more complex?

The model team has doubts.

The main issue is scale.

The core structure must be precisely scaled down for the shake table experiment to be accurate.

If Cakti chooses a scale of 10:1, the dome, at just over 100 feet wide, would be 10 feet wide on the model.

But that's still too big for the shake table.

CAKTI: Each shake table has a capacity in terms of its dimensions and in terms of the power that it can create.

NARRATOR: The capacity of this shake table is ten tons.

The scale Cakti wants to use will make the model too big, so she must scale it down.

After intense recalculations it looks like a 26:1 scale could work, at least on paper.

CAKTI: We are always nervous at the shake table, whether it will work.

NARRATOR: The scale model is an ambitious project with no guarantee of success.

But it pales in comparison to the challenge of building the real Hagia Sophia.

Who built Hagia Sophia, and why?

Hagia Sophia is built at a major crossroad in history-- the decline of the Roman Empire and the rise of the Byzantine Empire.

In 324 A.D., after Rome is ravaged by civil war, Emperor Constantine establishes a new capital in the city of Byzantium.

It's renamed after him.

He embraces a new religion, Christianity, and Constantinople becomes the center of the Byzantine Empire as Rome fades in importance.

The empire thrives, but in the early 6th century, a power struggle erupts after a new emperor ascends the throne-- Justinian.

Riots break out, challenging his authority.

Theodora, his much younger wife and rumored ex-courtesan, persuades him to fight rather than flee.

OUSTERHOUT: Justinian rallied to the challenge.

He called the rebels, looking as if he was going to meet their demands, met them in the hippodrome, had the doors closed and had the army slaughter them all.

NARRATOR: Tens of thousands are killed and Justinian emerges victorious.

But during the riots, the rebels burn down much of the city, including an older imperial church also called Hagia Sophia.

This is all that remains.

OUSTERHOUT: Much of the city of Constantinople had been destroyed in the great riots, and this allowed Justinian the opportunity to, in effect, rebuild Constantinople and the church of Hagia Sophia in his own image.

Justinian needed a building to convey both his power as emperor and piety as a Christian.

So what to build?

Joan Branham is a professor of art history at Providence College and an expert on how builders design sacred space.

She is at San Giovanni Evangelista, a church in Ravenna, Italy.

Although rebuilt many times, its floor plan dates to when Christianity becomes a state religion.

BRANHAM: For the first few centuries, Christians worshiped in private, in homes and small buildings.

But this completely changes in the fourth century.

NARRATOR: Christianity had been an underground cult and Christians persecuted.

But when Christianity becomes the official religion of the Roman Empire, Christians face a different problem.

What should a church look like?

BRANHAM: Early church builders looked at Biblical prototypes like the Temple of Solomon described in the Hebrew Bible.

But it's actually a secular Roman building that is adapted for early Christian use.

NARRATOR: That building is the basilica, used for courts of law and other public gatherings.

Its floor plan-- a large central nave flanked by two aisles, and culminating in an apse-- becomes the model for churches, an ideal space for worshippers to gather.

Justinian embraces the church's rectangular shape to demonstrate his Christian piety.

But he still needs something to symbolize his imperial power.

He looks to the dome of the Pantheon in Rome, the ultimate symbol of the might and glory of the Roman Empire.

But the Pantheon's dome sits on a thick circular base.

Justinian wants his dome to be centered over a rectangular Christian basilica.

BRANHAM: Justinian sets out to do something that has never been done before.

He wanted to merge two architectural structures into a mammoth hybrid space.

NARRATOR: So where do you find builders to create something on a scale that's never been done before?

Justinian turns to Greek mathematicians.

AHMET CAKMAK: Justinian hired Anthemius of Tralles and Isidorus of Miletus.

Both had experience as mathematicians and physicists and scientists of their day.

They were asked to create the most impressive, biggest building ever built.

NARRATOR: Justinian puts these university professors in charge of 100 contractors and 10,000 workers, and gives them the entire treasury of the Byzantine Empire.

The emperor is taking a big gamble.

OUSTERHOUT: There's nothing practical about Hagia Sophia.

It's all innovation.

It's geometric flights of fancy beyond what a practical architect would ever attempt to build.

NARRATOR: Their first challenge is how to support the dome and still keep a huge space for worshippers below.

CAKMAK: If they built walls or columns, the space would not be open like this.

It would be much smaller and narrower.

In order to make it as large and as heavenly as possible, they need a big space.

And that can only be accomplished by building large arches.

NARRATOR: A giant dome will need giant arches to support it.

CAKTI: The original architects should have been very concerned about how to support this huge dome over there.

NARRATOR: The team back at the earthquake center face the same challenge building their model.

Will their arches be strong enough to support the dome?

To find out, they add sacks of cement to simulate the weight of the dome.

Each bag weighs about 50 pounds, and they expect the arch to support about ten of them.

But as the fifth bag is placed... Luckily nobody is hurt as the arch collapses with only about 200 pounds on it.

CAKTI: It collapsed before we were expecting the collapse to take place.

I think that happened because we didn't wait for the mortar to set fully.

NARRATOR: While this might seem to be a setback for the team, Cakti insists this kind of unanticipated collapse illustrates one of the main advantages of building a physical model.

CAKTI: It is always interesting to see the failure mechanism in real life.

When you do it on computers, you develop an idea of how the failure is going to happen.

But it is only during a test of this kind where we see the collapse pattern.

NARRATOR: The slow-motion replay of the collapse shows that the downward force of the sacks pushes the arch out sideways.

The weight of the dome exerts the same force on the arches in the real Hagia Sophia.

CAKMAK: The arch wants to push out and fall down.

So you have to hold the arch together like bookends.

NARRATOR: To create those bookends, Anthemius and Isidorus, the Greek mathematicians, build four buttress piers-- massive weights of brick and mortar-- and two semi-domes.

These push back against the arches, canceling out the sideways force caused by the dome.

But Anthemius and Isidorus still have one more problem to solve: how to rest the dome on the tips of the arches.

CAKMAK: The architects had to transition from a circle to a square.

This they accomplished by building what is called pendentives.

It is this triangular shape that fills in the corners of the square.

NARRATOR: The pendentives together with the arches transform the circular base of the dome into a square.

And the semi-domes stretch that square into a rectangle.

Justinian has it all-- the classic rectangular shape of the basilica capped by the enormous circular dome.

Anthemius and Isidorus complete Hagia Sophia in only six years and do indeed spend nearly the entire treasury of the Byzantine Empire.

In 537, Emperor Justinian and his wife Theodora unveil their church to the world.

All who enter are awed by its size and the richness of its decorations.

Columns crowned by capitals so finely carved they look like lace.

Floors and walls of marble dazzle worshippers with patterns of swirling colors.

OUSTERHOUT: Justinian brought marbles from all parts of the empire.

The great purple columns that we see in the corners, for example, come from the imperial quarries of Egypt.

Elsewhere in the building we see stones brought from as far away as the Pyrenees in Spain.

NARRATOR: An eyewitness account reports that the dome looks "as though it were suspended from heaven by a golden chain."

Like the church before it, Justinian christens this monument "Hagia Sophia," which in Greek means "holy wisdom."

But the dome that Justinian first sees is not the same dome that sits atop Hagia Sophia today.

Just 20 years after Hagia Sophia's unveiling, its dome collapses in a catastrophic earthquake.

OUSTERHOUT: We really don't know what Justinian did when the first dome collapsed.

We can imagine he wasn't very happy.

Fortunately for Isidorus and Anthemius, they were dead by that point.

CAKMAK: When the dome collapsed in 558, the business of rebuilding it was given to the architect Isidorus the Younger, a nephew of the original architect.

NARRATOR: Cakmak believes Isidorus the Younger redesigns the dome.

To reduce its weight, he installs 40 windows at its base.

CAKMAK: The windows serve two purposes.

One is to get rid of the bricks that you needed, which add additional weight, and to let light in.

NARRATOR: Hagia Sophia is put to the test in at least another dozen major earthquakes.

The dome suffers two partial collapses, which were repaired, so visitors today cast their eyes up to the same dome built by Isidorus the Younger nearly 1,500 years ago.

But Hagia Sophia has withstood more than just seismic activity.

It's also been resilient to cultural upheavals.

600 years after Justinian, Constantinople continues to flourish, but its riches inspire envy.

In 1204, European Christian Crusaders on their way to the Holy Land sack the city and loot treasures from the Byzantine Christian Hagia Sophia.

Then a new religion challenges the old order-- Islam.

Its forces lay siege to Constantinople seven times over eight centuries.

Finally, in 1453, Sultan Mehmet conquers the weakened city and makes it the capital of his Ottoman Empire.

Mehmet enters the church of Hagia Sophia on a Tuesday, and by that Friday he is praying in the mosque of Hagia Sophia.

OUSTERHOUT: For Mehmet the Conqueror, Hagia Sophia was really the ultimate conquest.

That was the symbol he was after for his new empire.

NARRATOR: But how can a church become a mosque?

From an architectural perspective, it isn't difficult.

BRANHAM: There was the addition of the minbar from which the imam would give the sermon.

The mihrab gave the sacred direction orientation to Mecca.

NARRATOR: Later, the Ottomans add large discs calligraphied with sacred words from the Koran, plaster over Christian mosaics, and outside construct minarets for the call to prayer.

But Hagia Sophia's vast dome most easily makes the conversion.

BRANHAM: The dome itself had religious meaning for both Christian worshippers and now Muslim worshippers.

For both, it was a symbol of the heavens.

The structure at the heart of Hagia Sophia-- the round dome on the square base-- works as powerfully for Islam as it did for Christianity.

Hagia Sophia is so admired in the Islamic world, it becomes the classic model for mosques throughout the Ottoman Empire.

Today, Hagia Sophia is a museum, a showcase of its religious and cultural history.

DURAK: When you enter the building, you look to your left and you see a beautiful mosaic panel from the Byzantine Empire.

And you look at your right and you see a wonderful calligraphic quotation from the Koran.

You see the history of the whole city, in a sense the whole region, in a nutshell.

NARRATOR: But deciding which layers of its history to display is a battle that continues on its walls.

Stepping onto the battlefield is researcher Hitoshi Takanezawa.

He's on a hunt for Christian mosaics that were plastered over when Hagia Sophia was converted into a mosque.

His challenge is how to find the Byzantine mosaics without damaging the Ottoman decorations.

Takanezawa's secret weapon is this electromagnetic scanner.

Normally, it's used to find structural faults in things like bridges.

Nobody has ever used it to find Jesus.

(speaking Japanese) HITOSHI TAKANEZAWA (translated): We're developing new equipment for investigation.

It is crucial we find a technology that can deduce whether a mosaic exists without destroying anything.

NARRATOR: Takanezawa and engineer Satoshi Baba carefully run the scanner against the wall.

What might they find?

A tantalizing taste of Hagia Sophia in its full mosaic splendor is here, the church of San Vitale in Ravenna, Italy, also built during Justinian's reign nearly 1,500 years ago.

BRANHAM: Byzantine visitors would be transported into an entirely different world.

And it was through the mosaics that this happened.

They were a vehicle to bring the visitor into contact with the divine.

NARRATOR: That divine glow of Byzantine mosaics is what makes them so awe inspiring.

And the mystery material that gives them that glow is what will help in the search for Hagia Sophia's hidden mosaics.

Luciana Notturni and Gabrielle Warr are using the same materials to make mosaics today.

They begin with glass discs, carefully breaking them into smaller pieces, until they become tiny cubes called tesserae.

Notturni places each tessera piece by piece into a design she's drawn on the mortar and carefully angles them to reflect the light.

(speaking Italian) LUCIANA NOTTURNI (translated): It is believed that especially in the Byzantine mosaics, the positioning of the tesserae was directly connected to where the light was coming from, so where the windows were, where the main light sources were.

NARRATOR: And to make that light shimmer, they add something else to the mix.

It has a thin layer of gold leaf.

And the fact that it does have gold in it makes it very reflective and very luminescent.

NARRATOR: The gold tesserae give the Byzantine mosaics a heavenly glow.

And because gold is metal, it may be the key to rediscovering the lost mosaics in Hagia Sophia.

TAKANEZAWA: NARRATOR: Takanezawa's scanner sends electromagnetic signals below the surface of the plaster.

If the waves strike a buried metal tessera, they are reflected back, creating an image of the inside of the wall.

TAKANEZAWA: NARRATOR: The scanner is working.

It has detected a mosaic circle beneath the plaster.

But Takanezawa isn't searching just for circles.

TAKANEZAWA: NARRATOR: The walls of Hagia Sophia are hiding more than mosaics.

They also hold secrets to its seismic strength.

High above the streets of Istanbul, a team is repairing a wall as part of Hagia Sophia's ongoing restoration.

Sonay Sakar is the lead architect.

(speaking Turkish) SONAY SAKAR (translated): What we're doing is removing all the cement from the surface you see here.

Then we'll repair the layer of bricks we've uncovered.

NARRATOR: Her team must replace crumbling cement from a restoration in the 1950s.

She's using a more resilient mortar, one formulated from the original recipe-- limestone, sand, water and a secret ingredient: ground-up bricks.

It turns out the best way to preserve Hagia Sophia for the future is to use materials from the past.

SAKAR (translated): The mortar in Hagia Sophia is certainly more flexible than modern mortar.

So it adapts to the structural deformations caused by earthquakes.

NARRATOR: The flexibility of the mortar is crucial, but so is how it's applied.

SAKAR (translated): Hagia Sophia differs from other structures because the layer of mortar is thicker than the bricks.

NARRATOR: Modern brick buildings have thin layers of mortar, but Hagia Sophia's layers are so thick they act like cushioning.

Hagia Sophia's bricks also play a role in earthquake protection.

CAKMAK: Here is an original brick from Hagia Sophia, and here is a modern brick.

As you can see, the original brick is significantly lighter than the modern brick.

NARRATOR: Which turns out to be very important.

CAKMAK: If you make the weight light, then the building can sway with the earthquakes, like a tree in the wind-- flexible, but strong.

NARRATOR: 1,500 years ago, other architects built heavy and massive to protect against earthquakes.

Anthemius and Isidorus, the Greek architects, did the opposite.

They built light and flexible-- the principle of modern seismic engineering.

But will this world treasure survive into the future?

Eser Cakti and her team are building a model of Hagia Sophia's core structure to investigate.

Their arch problem solved, they move on to their next challenge: the semi-domes.

They create a mortar that mimics the materials of the real semi-domes, and spread it over a wooden mold.

We have worked on paper for a long time on how to get it right, how to make it, and then what would be the thickness, what would be the material.

NARRATOR: After the mortar dries, they remove the wooden mold.

But as they take off the mold, suddenly... a crack appears at the top.

Cracks at this stage mean the semi-dome is clearly too weak for the shake table test.

They break apart the semi-dome to get a closer look at the mortar.

We have four centimeters coming from that side and four centimeters coming from that side.

But the failure part was too thin.

It is almost a half-centimeter.

NARRATOR: A problem with the way the mortar was applied caused the top of the semi-dome to be much thinner than planned.

CAKTI: Some shrinkage occurs after drying of the mortar.

We may consider to introduce some elements to the mortar so that its strength properties will improve.

NARRATOR: The team must rebuild the semi-dome.

And they'll need to come up with a better method for building the final piece of their model-- the large central dome-- and that will take some time.

In his hunt for hidden Byzantine mosaics, Hitoshi Takanezawa is heading to the uppermost level of Hagia Sophia, a thin ledge that runs beneath the main arches.

The building is so huge, he must narrow down his search.

TAKANEZAWA: NARRATOR: Because these niches on the northern wall are filled with mosaic figures, Takanezawa believes the southern wall may have been too.

But have the mosaics survived?

To find out, they run the electromagnetic scanner along the wall.

TAKANEZAWA: NARRATOR: The team takes a closer look at the scan.

They find metal behind the plaster, but not the gold Takanezawa is hoping for.

TAKANEZAWA (translated): There are only horizontal lines.

It would seem it is not a mosaic, but rather a metal structural support.

NARRATOR: Takanezawa's guess is wrong; no mosaics have survived in this niche.

The challenge is that his scanner measures about two feet at a time, and Hagia Sophia's surface area is over 200,000 square feet.

To narrow down his search, Takanezawa has come to Bellinzona, Switzerland, to explore the state archives.

Inside, archivist Carlo Agliati shows him an astonishing record of Hagia Sophia's Byzantine mosaics.

(speaking Italian) CARLO AGLIATI (translated): In 1847, the Sultan entrusted the architect Gaspare Fossati with the task of restoring the mosque of Hagia Sophia.

NARRATOR: These drawings were made by the Fossati brothers, Swiss architects who were hired to renovate the aging building, which was then a mosque, in the 1840s.

The Fossatis began stripping plaster from the walls and were astonished by what they found.

(speaking Italian) (translated): Fossati's big discovery during the restoration, hidden under the plaster, was definitely these extraordinary Byzantine mosaics.

NARRATOR: They quickly documented every image before covering them with plaster once again.

While some of the mosaics recorded in the drawings have been uncovered, others have never been found.

One in particular catches Takanezawa's eye.

(speaking Japanese) (translated): There is a circular sketch from the Fossati, but the exact location is still the subject of debate.

NARRATOR: The sketch depicts Christ, framed by a cross in a circle.

Near Hagia Sophia, the church of Chora contains a strikingly similar image found in the crown of a dome.

Takanezawa believes the Fossati sketch depicts a similar mosaic in a dome in Hagia Sophia.

And he has a hunch where to find it.

(speaking Japanese) (translated): A very plausible hypothesis is that there is a large depiction of the face of Jesus Christ at the top of Hagia Sophia's immense dome.

NARRATOR: But there's a problem.

TAKANEZAWA (translated): Currently it's covered by plaster and by Koranic verses, but one day, with our scanner, we would like to discover this image.

This is my dream.

NARRATOR: It's a dream Takanezawa could realize because as part of Hagia Sophia's ongoing restoration, this enormous scaffold is about to reach the dome.

But if Takanezawa does find Christ beneath the Koranic verse, what should be shown?

It's a question at the heart of Hagia Sophia's identity... a question with a long history.

CAKMAK: Religiously, it was a Greek Orthodox church.

And during the Fourth Crusade, it was taken over, became a Catholic church.

When the Muslims came, they made it into a mosque.

Finally it became a museum, which we thought was a solution to the problem.

But, unfortunately, the Greeks would like to make it back into a church.

And the Muslims would like to make it back into a mosque.

And the conflict, controversy continues.

NARRATOR: But whether Hagia Sophia remains a museum or is converted back to a church or mosque could prove irrelevant if there is an earthquake.

The more pressing question is will it be converted into a pile of rubble?

Eser Cakti hopes the shake table test will provide some answers.

The semi-domes are carefully rebuilt from mortar.

But mortar will be too fragile for the main dome, which, like the real thing, will be built from brick.

CAKTI: We came to the conclusion that having a brick dome is much easier to construct and it's more realistic.

So dome-wise, I'm confident with what will happen.

But with respect to the semi-domes, there I have doubts, because it's much more fragile.

NARRATOR: Before the test, the seven-ton model must first survive the move to the shake table.

CAKTI: This is the largest model ever to be made in our lab.

We need to be very careful that during lift-up everything should be perfectly horizontal.

Otherwise, we may damage the model.

NARRATOR: The model is so heavy it bends the steel plate that supports it, which puts pressure on the structure.

It settles onto the shake table, but has it suffered any damage?

As the wooden molds come away, Cakti looks for cracks.

If the model breaks at this stage, they will be unable to perform the earthquake test and months of work will have been for nothing.

CAKTI: Okay, there is some, yeah.

We have observed some cracks on the semi-domes.

But we don't see them from outside.

These are just interior cracks.

NARRATOR: Cakti believes the cracks do not compromise the structural integrity of the model.

So the team moves on, installing motion sensors in similar locations as the sensors in the real Hagia Sophia.

CAKTI: We will be able to compare the vibrations that we record during the shake table test with those obtained from the real structure.

NARRATOR: The model is a scaled-down version of Hagia Sophia's core structure-- the main dome, four great arches, four buttress piers and the two semi-domes.

But will the model move on the shake table in a similar way as the real building moves in an earthquake?

Astonishingly, overnight, the sensors get an unexpected trial run: a real earthquake.

CAKTI: At about 4:00 a.m. we had an earthquake near Istanbul.

Its magnitude was 3.6.

So, by pure chance, we have now recordings of that earthquake recorded on the model.

And we have the same earthquake recorded by our instruments in Hagia Sophia.

NARRATOR: The parallel recordings verify that the sensors on the model and in the real building are reacting in a similar way.

Now it's time to see how the model will react to a more powerful quake.

(speaking Turkish) They calibrate the shake table to simulate the impact of the devastating '99 earthquake, magnitude 7.4.

The duration of the test is scaled down to match the size of the model, about three seconds.

The sensors capture every twist and turn.

The model seems to have taken the impact without damage.

But what everyone really wants to know is how will it stand up to an even stronger earthquake?

To find out, the team must push the power of the shake table beyond anything they've tried before.

The simulated quake is measured in g's-- its gravitational force.

We are increasing the amplitude of our earthquake one more step so that now we aim 2.2 g. You said two was the maximum.

Now we are going more than two?

If we can do it, we'll go for 2.4.

Perfect.

NARRATOR: They hit the model with a simulated earthquake stronger than any in Istanbul's recorded history.

Cakti checks out the damage.

I see one new crack in this arch.

But surprisingly, there is nothing new with the semi-domes.

We were afraid about them.

But they are as they have been before.

NARRATOR: The Hagia Sophia model has survived two enormous earthquakes in quick succession, with minimal damage.

But the team isn't done yet.

We have passed the known capacity of our shake table.

And then it appears the mechanics have allowed us to go further.

NARRATOR: Can the shake table push the model to the point of collapse?

They hit it with everything they've got.

At this stage, the model has been hit by the equivalent of a major earthquake every day for a week.

And although it teeters on the edge of collapse, it still stands.

There are two vulnerable parts-- the semi-domes and then the arches.

It is just a matter of time to see which one will go first.

NARRATOR: With everyone's eyes on the semi-domes and arches, nobody anticipates what happens next.

The great dome comes crashing down.

I am a little bit surprised now because I would have expected the main arch to go, and then instead of the main arches, the main dome went.

NARRATOR: The slow-motion replay reveals that the semi-domes separated from the structure... and with the main arches damaged, support for the dome was severely compromised.

KORHAN ORAL: My masterpiece is collapsed now.

But for scientific observation, I can accept it.

CAKTI: Now we have come to its end.

But, at the same time, we know that we have lots of things to do in terms of data analysis and interpretation.

This is a little bit frightening, but it needs to be done.

NARRATOR: It is too early to draw any firm conclusions.

But the model going 15 rounds against the most powerful simulated earthquakes the shake table could produce explains Hagia Sophia's supposedly miraculous survival.

CAKTI: If there is a miracle, it is in its design.

It was constructed to survive.

The balances between its structural elements appear to create a dance.

The domes, arches, semi-domes, buttress piers, they behave in harmony.

NARRATOR: Though the model lies in ruins, Cakti believes the data captured in this experiment will provide new insights into Hagia Sophia's structural strength and how it can be preserved for the future.

OUSTERHOUT: Scientists have spent decades trying to analyze the structural system of Hagia Sophia.

But when you go inside Hagia Sophia today, you don't see structure.

We're not meant to understand how the great dome is supported.

We see only the weightless quality of the building.

That was what was most important.

We understand the interior of the building as an experience that's completely different from anything else on earth.

NARRATOR: After nearly 1,500 years, Hagia Sophia continues to astonish modern builders with its ancient secrets of seismic engineering, and for its resilience not only as a structure, but also as a symbol of the great civilizations that have adopted it.

SAKAR (translated): We don't think of Hagia Sophia based on the meanings other people assign to it.

Hagia Sophia has an identity of its own.

It is a monumental building; it is a special building.

Our goal is to pass it down to the next generations.

NARRATOR: Hagia Sophia will have to endure many more shifts in the ground that lies beneath it, and the cultures to which it is entrusted.

Hopefully its majestic beauty and innovative design will inspire people of all religions and cultures to protect it for generations to come.

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