♪♪ -Submerged beneath the waters of Lake Iznik in Turkey, the ruins of an early Christian basilica sat undisturbed for nearly 1,000 years... until journalists on a routine helicopter flight over the lake spotted the remains in 2014.

♪♪ News of the reappearance of this incredible sunken building quickly spread around the world.

An international team of scientists is conducting an extraordinary investigation into the history of this long-hidden structure.

-[ Speaking in French ] -This drowned basilica raises quite a few questions.

We want to know when it was built, when it was destroyed, and how it was destroyed.

-From Turkey to the Vatican, in both France and England, scientists search for clues about the church and how it ended up underwater.

With exclusive access to the excavation site, this is also an examination of a major turning point in the history of the Roman Empire and Christianity.

-[ Speaking in Turkish ] -We work like detectives -- archaeological detectives.

-This will be a first for Turkey.

-Turkey is fresh, it's new, it's engaging!

-Working in the city of Iznik, once known as Nicaea, scientists want to answer one specific question -- Did the basilica host the First Council of Nicaea, one of the most important moments in early Christianity?

But as the world was reminded in February 2023, Turkey is an area prone to earthquakes.

The extreme seismic activity means the basilica could sink further at any moment.

Scientists are racing to unravel the church's many mysteries and to reconstruct the submerged basilica and experience its original grandeur.

"The Sunken Basilica."

♪♪ O0 C1 -It all began on a calm, windless day in 2014 on Lake Iznik, located in northwest Turkey.

Three Turkish journalists were taking a helicopter trip over the city.

Looking down from the air, what they saw shocked them.

-[ Speaking in Turkish ] -We were flying over the lake, and the water was particularly clear that day... ...so we could actually see the basilica.

-[ Speaking in Turkish ] -If it wasn't for climate change, we'd never have discovered it.

The water level's dropping every year.

-A drought, the result of climate change, has caused the lake to shrink, revealing the basilica.

-[ Speaking in Turkish ] -We're delighted.

Not everyone gets to make a discovery like that.

It's a real gift to humanity.

-Turkey is a land of fascinating historical riches.

But this is a discovery unlike any other -- a building submerged for centuries.

The scientific investigation is led by the director of the Department of Archaeology of Bursa Uludag University, Professor Mustafa Sahin.

-[ Speaking in Turkish ] -How is it that, 'til now, no one has noticed the presence of such a large monument?

I was really surprised when I first saw it.

I could see it was a church because of its typical basilical layout -- with three naves.

I immediately wondered who built this church, at what time, and why?

-To begin his search for answers, Mustafa goes scuba diving in the lake and finds a world frozen in time.

-[ Speaking in Turkish ] -It's impossible to describe how I felt that day.

-This investigation will be a real challenge -- a time machine taking scientists back to the beginnings of Christianity.

-[ Speaking in Turkish ] -Anatolia, later known as Turkey, is a real mixture of cultures.

Christianity may have emerged in Jerusalem or the Near East, but its name, creation, and transformation into a true religion took place here in Anatolia.

-Mustafa wants to know whether or not the basilica is where the First Council of Nicaea was held.

Ordered by Emperor Constantine, it is one of the foundational events of Christianity.

In the 4th century AD, Nicaea, as it was called then, was a part of the Roman Empire's eastern territory.

Christianity was still a relatively new religion at that point, defining its core tenets and beliefs, establishing its central rituals.

The conference would determine the rules that continue to govern the lives of billions of people nearly two millennia later.

-[ Speaking in Turkish ] -We still don't know exactly where the First Council was held.

It's still up for debate.

But it's believed the First Council may well have met here.

-But there are other aspects of the basilica that fascinate scientists around the world... scientists like geologist Julia de Sigoyer in Grenoble, who sees the structure as a chance to study how earthquakes disrupt history and civilizations.

She will team up with archaeologists, who can provide her with insight into the area's history -[ Speaking in French ] -What interested me was to see if this basilica had been buried because of a tectonic process -- in other words, because of an earthquake.

-Turkey's past is punctuated by earthquakes.

The country is at the convergence of three highly active tectonic plates.

It's also home to many artifacts that date back to the Roman Empire, which covered this vast seismic region.

Julia and Mustafa will conduct a double investigation.

Their team is composed of 15 researchers from around the world, working together, virtually.

Mustafa heads for Rome, where much of the recorded history of Christianity is kept -- the Vatican.

He's there to learn more about the First Council of Nicaea.

One magnificent fresco in particular catches his eye.

-[ Speaking in Turkish ] -Our starting point was the fresco in the Vatican that depicts the First Council of Nicaea.

♪♪ -It's a fresco painted in the 16th century by Cesare Nebbia.

♪♪ Emperor Constantine is included in the group of people shown.

Mustafa notices one small detail.

-[ Speaking in Turkish ] -It was something at the top left of the fresco that caught our attention.

You can see the edge of the lake and the city walls that go down to it.

That's the first clue.

So, what did that tell us?

That the Council was held on the shore of the lake, outside the city walls, right here where the basilica is located.

-For Mustafa, the fresco suggests the basilica may have played host to the historic Council of Nicaea.

The North Anatolian Fault stretches across the width of Turkey, dividing into three branches.

Iznik sits along the middle branch.

The devastating earthquake in February 2023 was caused by the East Anatolian Fault.

For the last 150 years, the North Anatolian Fault has fractured from east to west, with each quake creating a new segment of the fault line.

In 1999, the last segment to break caused an earthquake elsewhere in Turkey.

It measured 7.4 on the Richter scale and completely ravaged the city of Izmit, killing 17,000 people.

The shoreline bordering the Sea of Marmara sank nearly two feet, and a tsunami submerged part of the city.

♪♪ Despite being a small provincial town, Iznik contains many secrets from the past -- some of which are submerged in the waters of its lake.

Two archaeologists, Serkan Gunduz and Suha Cura, supervise the team of archaeology students.

-[ Speaking in Turkish ] -First, we measure the water level.

Sena, can you handle that?

-These underwater excavations of a submerged basilica are a first for Turkey.

The basilica is between 5 and 15 feet below the water's surface and is 135 feet long by 60 feet wide.

Before each dive, the young researchers carefully pinpoint the areas to be excavated.

-[ Speaking in Turkish ] -Let's go!

-As each object is found, its precise location is recorded, in order to create a map of the dig.

After 1,000 years, the remains of the basilica are covered in sediment that must be carefully vacuumed and then collected.

-Ready?

-Yes.

You can turn it on.

-The vacuum pumps are on, and the excavation can begin.

♪♪ The sediment is removed, layer by layer, taking the scientists further back in time as they search for clues to the basilica's history.

Merve Kurtbayam is responsible for sifting through the collected sediment in search of any possible artifacts.

-[ Speaking in Turkish ] -The church we're excavating here is one of the very first Christian churches.

And, above all, it's one of the few examples that has survived to this day without any changes to its architectural plan.

-A thick layer of algae and sediment covers the basilica, protecting the structure for 1,000 years.

Its remains can shed light on Emperor Constantine's transformation of the Roman Empire in the 4th century AD.

In Paris, Christophe Goddard, a specialist in Late Antiquity history, explains Constantine's significance in the history of the Roman Empire.

-[ Speaking in French ] -Constantine is the inventor of the Christian basilica.

He makes the decision to allow Christian worship outside of simple houses of prayer, and to bring this worship into the buildings known as basilicas.

What is a basilica?

It's an architectural structure that looks like a kind of market hall, with several large rooms that are separated by colonnades.

-Originally, basilicas could serve a variety of public and commercial functions.

But their purpose evolved over time.

-[ Speaking in French ] -Basilicas will become something else as well -- courthouses!

And pretty soon, any imperial palace has its own basilica -- not just a reception room, but a courtroom too.

-Already large open spaces, basilicas were then adapted to be places of worship, where congregations could gather together.

♪♪ The archaeological mission at the lake is well underway.

The French and Swiss geophysicists and geologists are getting to know their Turkish archaeologist colleagues.

They will work together to find out exactly how the basilica ended up underwater.

To get a better sense of the ruins, Julia and Mustafa suit up for a dive to explore the basilica together, up close.

♪♪ ♪♪ Julia notices some large stone blocks that might have moved during major earthquakes... ♪♪ ...while Mustafa is intrigued by the many tombs dotted around the interior of the basilica.

♪♪ ♪♪ -[ Speaking in Turkish ] -When we started our excavations here, the first thing we were surprised by was all the tombs inside the building and all around it.

What were they doing there?

Why were they there?

And some of them were even underneath the foundations of the building!

♪♪ -Elsewhere, another exploration of Roman ruins in Iznik has begun -- this time at a theater located in the center of the city.

And it's Yacine Benjelloun's job to catalogue the different traces left by past earthquakes.

♪♪ -[ Speaking in French ] -On this arch here, we clearly have, I believe, one of the most obvious signs of an earthquake.

Because this keystone would normally have been perfectly aligned with the rest, but in fact it's moved and been pushed out a bit.

-Yacine finds other, similar clues all around this theater.

For him, there's no doubt that they all point to the occurrence of one or more major earthquakes.

But just how big?

And when did they happen?

And where is the tectonic fault that generated them?

Back at the basilica excavation site, the investigation moves in a new direction... with the discovery of a small clay token.

To learn more about the significance of this object, Mustafa calls on the expertise of an American professor of biblical studies, Mark Fairchild.

For Fairchild, Turkey is an especially rich location for the study of early Christianity.

-I come to Turkey every year -- I've been here for more than 20 years, travelling through the back country, travelling to places that most people never go.

I began to do research in Turkey because it's been neglected.

Turkey is fresh, it's new, it's engaging!

There's so much to be seen that nobody's seen before, that has not been thoroughly researched, and that's the part of the story that needs to be revealed, part of the story that we need to tell.

-The newly found object shows the triumphant Christ Pantocrator seated on his throne.

-It's not a coin, but a token, which indicates that people came here because of its significance.

Literally, what "Pantocrator" means is "ruler of all" -- "Panto."

The term "Pantocrator" is in opposition to the imperial claims of the emperors.

The emperors claimed to be "Autocrator," which, translates, simply means ruler, monarch, dictator, and that is the imperial claim of the Roman emperors, that "we are in absolute control of the Mediterranean realm."

The Christians used that term, "Pantocrator," to describe Christ, and it's a way of trumping the imperial claims.

"Pantocrator" is... a political statement.

-But a political statement about what?

The fact that the basilica is built over a necropolis may shed some light on that... -Welcome to Turkey.

-Thank you, thank you.

Have you found tombs outside of the church or just inside of the church?

-Together.

Inside and... -And nearby.

-And nearby.

-Nearby.

-In all, 250 tombs will be found.

But there's one detail that puzzles Mark.

-Basilicas are usually built in the city.

This basilica is outside of the city walls.

That's unusual.

A necropolis is also outside of the city because of the stench of the decaying bodies.

When we find tombs inside of the basilica, that's very unusual, and the only conclusion that we can draw from that is that this basilica was a martyrion.

-As a martyrion, the basilica holds special significance.

-A martyrion is the place where a saint, an early Christian, was killed on behalf of his faith.

The early Christians commemorated that saint by marking that spot.

Soon thereafter, it became a place of worship, and after the Byzantine period, churches were built at these sites.

-As members of a minor religion in the pagan Roman Empire, Christians were often persecuted, which could explain the martyr's death and the construction of the basilica.

In the 3rd century AD, the Empire was under great pressure.

It was being attacked on both its western and eastern borders.

Military spending was consuming the country's finances, leading to political instability.

Diocletian was crowned emperor in 284 AD, and he tried to strengthen the Empire with far-reaching reforms, known as the Tetrarchy.

He had the Empire divided into four territories, each with its own leader.

But that only created more chaos.

Diocletian's next move was to ban Christianity in 303 AD.

-[ Speaking in French ] -Being a Christian actually became a criminal act.

-But persecution did little to stop the growth and spread of the religion.

And just three years later, in 306 AD, Constantine became emperor.

-[ Speaking in French ] -The Emperor Constantine didn't actually establish the Christian religion, but he was the first Christian emperor.

He saw the persecution with his own eyes.

♪♪ -Mark Fairchild wants to find out more about the events he believes the basilica was built to honor.

He travels to Rome to locate any records pertaining to the basilica that can be found in Vatican archives.

Fairchild studies an illuminated manuscript from the end of the 10th century, the Menologion of Basil II.

The manuscript contains descriptions of saints' lives.

Mark is interested in one particular young martyr named Neophytos.

-Neophytos is an interesting character.

Unfortunately, we don't have early traditions about Neophytos.

-According to the text, Neophytos was born in Nicaea to Christian parents and, while still a boy, he performed miracles, including bringing his mother back to life.

In 303 AD, the Roman governor of Nicaea, Decius, forced the city's Christians to pay tribute and make offerings to the pagan gods.

Neophytos refused.

The governor tortured him, to no avail, and ultimately had him put to death.

-The place of execution?

Some traditions indicate that Decius actually came here to Nicaea, and that's where the whole confrontation had taken place and that he was executed here on the shore.

-The basilica was built on the lakeshore where it's believed Neophytos was killed.

It became a place of pilgrimage, in homage to the young martyr.

♪♪ Research into Iznik's other Roman ruins continues.

Yacine has recorded more than 280 findings that indicate both destruction and reconstruction of the Roman theater of Nicaea.

Most of these damaged features are oriented north to south, which would make sense if the seismic fault that caused the earthquakes was located south of Iznik.

♪♪ -[ Speaking in French ] -I just felt from all my reading and my understanding of tectonics that there must be active faults within this lake.

Of course, intuition's all very well, but you have to be able to back it up.

-And after Yacine's field studies confirm her intuition, Julia focuses her research on the possibility there are fault lines beneath the lake itself.

♪♪ A new mission that requires heavy equipment begins -- locating the seismic faults.

♪♪ Two Swiss geologists, Flavio Anselmetti and Stefano Fabbri, specialists in lake tectonics, will produce a map of the bottom of the lake.

Within weeks, one-third of the lake is mapped, and two subaquatic fault lines have been identified.

The researchers will focus their work on the more southern of the two faults.

Lying just over half a mile south of the city, the Iznik Fault is a segment of the larger Anatolian Fault system, which separates the Eurasian plate from the Anatolian plate.

The line has a dangerous straightness to it, though.

-[ Speaking in French ] -A few years ago, it was discovered that on straight segments there can be a very particular kind of earthquake, one that involves sudden slipping -- which means that when the fault breaks, it slides apart at great speed.

-Seismologists call this phenomenon "super-shear."

On a straight fault, the wave of rupture that travels along it can advance much faster than the waves generated by the earthquake.

The "super-shear" phenomenon was first noticed following the Izmit earthquake of 1999.

With a magnitude of 7.4, the rupture wave surged along the straight portion of the fault, worsening the destruction.

-[ Speaking in French ] -If a super-shear earthquake were to occur on the fault segment that's just been discovered in Lake Iznik, it could lead to massive dissolving of the soil and the whole shoreline being engulfed.

That might explain how the Basilica of Nicaea was once completely drowned.

-To test this hypothesis, researchers must learn more about the characteristics of the newly discovered fault segment -- rupture length, past activity, and the magnitude of the earthquakes it has caused.

Details about these past events could explain what happened to the basilica, as well as what might happen in the future.

♪♪ ♪♪ Yacine Benjelloun is looking for clues to measure the magnitude of the segment's previous earthquakes.

Located three miles from Iznik, this funerary obelisk, which dates back to the 2nd century AD, could be helpful.

Miraculously, it has survived several earthquakes.

A few chips and slight rotations are the only visible damage caused by two millennia of earthquakes.

At the Grenoble Alpes University, researchers create a computer animation of the obelisk and simulate the effects of various earthquake scenarios.

The modelling shows what happens when seismic waves of varying magnitudes pass through the marble structure.

The model proves that no earthquake with a magnitude higher than 7.3 has occurred in the area in the last 2,000 years.

Otherwise, the obelisk would have been damaged or destroyed.

With this information, Yacine can begin to create a timeline of earthquakes along the fault.

For additional insight, he consults written testimony kept at the Ecole Normale Supérieure.

-[ Speaking in French ] -In Iznik, throughout Antiquity and the Middle Ages, there were members of the aristocracy, ecclesiastics, and monks who acted as chroniclers.

Over the years, they reported political events and wars, but also natural disasters, including earthquakes.

-According to these records, 14 earthquakes have struck the Iznik region over the last 2,000 years.

Of those 14, Yacine identifies four quakes that had a magnitude around 7... ...the tremors of 30 AD, 368 AD, 740 AD, and 1065 AD.

The one in 30 AD is ruled out, because it happened before the basilica was built -- which means one of the three remaining earthquakes is responsible for sinking the basilica.

The answer may well lie in Chambéry, home to a collection of precious sediment core samples.

-Do you know where they are?

-They're right here.

-The one taken from the bottom of Lake Iznik will give sediment specialist Pierre Sabatier and PhD student Renaldo Gastineau a picture of the last 2,000 years of the lake's seismic history.

-Ooh!

-Yes.

Very nice.

This one here just might tell us how the basilica was destroyed.

There are indeed historical earthquakes recorded in the archives of the city.

But we've no real idea of their intensity.

We know that if an event is recorded in the sedimentary core, it's probably significant enough to have done damage and perhaps destroyed the basilica.

♪♪ -The sediment is caused, in part, by erosion along the lake's edges that settles, layer by layer, over the centuries, providing a great deal of scientific data.

During an earthquake, the sediment slides like an avalanche and leaves a chaotic deposit on the surface of the lake bed.

Coring sediment layers allows researchers to identify and date anomalous events such as earthquakes.

-[ Speaking in French ] -You can see both events really clearly.

There wasn't much time between them.

-They're quite well marked.

There are two distinct levels.

-Geochemical analysis and carbon-14 dating of the sediment enable Pierre and Renaldo to identify all 14 earthquakes mentioned in the historical texts.

-So there's the top of the core sample -- and the date it was sampled.

-Two core samples, one from each side of the fault, are placed side by side.

-Pass me the C-14 dates from the sequence, will you?

-It's easy to see the anomalies that could have been caused by seismic activity and compare these disturbances in the sediment.

-[ Speaking in French ] -This deposit here is what really matters.

It's much lower down in the sample, at the foot of the fault.

And we can date that to the historic earthquake of 1065 AD that really rocked the city of Iznik.

-One thing's clear to the researchers -- of the two samples, the thickest and most offset is from the 1065 earthquake, making it the most likely cause of the basilica's destruction.

-[ Speaking in French ] -This would indicate that the basilica was destroyed by a possible "super-shear" earthquake on this rectilinear fault.

And that's a major discovery, both in our understanding of the seismic cycle and the history of the basilica.

[ Rumbling ] -The 1065 earthquake was a particularly devastating disaster -- and it's also the last large earthquake that's visible in the sediment core samples over the last thousand years.

-[ Speaking in French ] -It can mean two things -- either this segment has ceased to be active, or, on the contrary, we're at a critical moment, where enough stress has built up to bring us close to a breaking point, and we can expect to have a very big earthquake here on this middle segment.

That would have a big impact on the city of Iznik.

And Iznik is developing its tourist industry, so there are more and more hotels being built on the shores of the lake.

In fact, it's very close to the situation in Roman times.

♪♪ -Local authorities see the basilica as a major tourist attraction.

But as the lake continues to shrink because of climate change, the buildings become more exposed and fragile.

-Our underwater excavations will eventually turn into surface excavations.

-Kagan Mehmet, the mayor of Iznik, is clear about his hopes for the basilica.

-[ Speaking in Turkish ] -We still don't know, really, where the First Council was held.

But if, as the experts claim, it was indeed here, the sites of the 1st and 7th Ecumenical Councils will be in the spotlight.

For a small town like Iznik to have a monument like that is very important for us in terms of international tourism.

-The submerged basilica could soon become a popular site for the world's Christians.

It was here, in the city of Nicaea, in 325 AD, that Roman Emperor Constantine gathered the religion's bishops to lay out the foundations of their faith -- turning the pagan city into a center of Christianity.

Constantine organized the Council in hopes of resolving an issue that threatened to divide the church -- the nature of Christ.

An Egyptian priest and theologian named Arius claimed that Christ was not the same being as God, but rather was "begotten" by God, causing controversy throughout the Empire.

Constantine wanted the debate settled before it caused unrest.

To resolve the argument, he brought bishops from across the Empire to Nicaea.

-Constantine was not a theologian.

He was a politician, and he was very much concerned that Christianity was unified rather than fractured, because that would, of course, complicate his problems as a ruler.

-[ Speaking in French ] -The presence there of the Emperor himself... ...offers a glimpse of the solemnity of the occasion and the very political demands that it put on the bishops.

Those who did not agree with any decision would be exiled.

And in those days, exile was pretty much a sentence of death.

♪♪ ♪♪ -As he directs the new excavations in search of clues about the First Council, Mustafa is intrigued by other, unexpected finds.

-[ Speaking in Turkish ] -In my opinion, there was a temple here.

We've discovered architectural elements in marble that couldn't belong to a simple church.

-These marble items suggest a pagan temple was located here before the basilica was built.

And there are other clues to support this theory.

-[ Speaking in Turkish ] -What have we found recently?

Well, we've unearthed coins dating back to the time of Antoninus Pius, in the 2nd century AD, as well as oil lamps from the 1st century AD.

-All these objects predate the basilica.

-Hi!

It's wonderful to see you.

-One of the oil lamps in particular has caught the attention of the archaeologists.

-It looks like clay.

-Found inside the remains of the basilica, this terracotta oil lamp with an erotic motif is, for Mustafa, a significant clue.

-[ Speaking in Turkish ] -This type of oil lamp suggests it might be a pagan temple, because erotic lamps don't belong in a church.

-Although such lamps were commonplace at the time, it's more likely one would be found in a pagan temple than a Christian basilica.

-[ Speaking in Turkish ] -The Roman Emperor Commodus had ordered the construction of a Temple of Apollo at Iznik.

It was built outside the city of Iznik, near the lake.

But the Temple of Apollo has since disappeared.

-Mustafa thinks this 2nd-century-AD Temple of Apollo may lie under the foundations of the basilica.

Mark Fairchild isn't so sure.

-There's also a distinction in the understanding of sacred space.

In the pagan temples, the persons who come to the pagan temple to worship worshipped outside of the temple, they didn't enter into the temple.

In the basilicas, they were invited to come into the inner precinct and worship inside.

And I think it's also a different perception of God.

Whereas the Pagans thought of God as being transcendent -- "he's out there, he's distant from us" -- the Christians promoted a God who is intimate, who invited to have communion.

And the fact that you've got tombs that are inside of the church is a telling fact that indicates that it was originally a church, not a temple.

♪♪ -But Mustafa also consults Dominik Maschek, a specialist in the architecture of Late Roman Antiquity at Oxford.

Dominik, unlike Mark Fairchild, thinks it's possible the basilica was built over a temple.

-[ Speaking in foreign language ] -The basilica has a central part, here, in blue.

And it could very well be the "cella" -- that is, the central part of the temple.

This would have the advantage that the outer walls could support the columns of the temple.

-Dominik cites a similar case -- a basilica built on top of a temple to Aphrodite, also in Turkey.

-This is the Basilica of Aphrodisias, which dates from the beginning of the Byzantine period.

And if you superimpose the two, the temple fits into the basilica.

The temple is smaller than the basilica, which would also be the case in Iznik.

-Dominik has created a 3-D reconstruction of what the Temple of Apollo might have looked like.

♪♪ Continuing excavations should confirm whether or not a temple lay beneath the basilica.

♪♪ In order to predict how the Iznik fault will behave in the future, the scientists are studying how it has behaved in the past.

-[ Speaking in French ] -We want to look closely at how this fault line functions.

You can't do that with just one method and one specialist -- you need many different approaches.

-The first team to get to work is the pair of Swiss geologists, who will study the fault's movements.

They install a seismic reflection transceiver on a small fishing boat.

It will allow them to probe the layers of sediment on the lake bed and learn more about the fault segment.

-We do seismic surveys, which means that we produce acoustic waves.

And for this purpose, we have it here next to the boat, so it emits -- Every 0.3 seconds, there is a soundwave that's created, which travel through the water, which go into the sediments below the lake.

And we image these.

And we want to actually find the surfaces where the earthquakes slipped.

So, we see that the lake floor actually has a shift, and this was most likely caused by an earthquake that actually was the movement of this fault that might have caused the damage at the basilica.

-Their first results show that the fault has shifted vertically by an average of three feet.

♪♪ The second team of scientists, from the Grenoble Alpes University, is tracking the course of the fault on dry land.

♪♪ Along with geophysicist Stéphane Garambois, Julia de Sigoyer's team is taking ultrasound readings of the lake's shoreline, using electromagnetic waves.

♪♪ These electromagnetic echographies of the subsoil will provide important information about how the shoreline has moved.

Each of the images shows the same sedimentary structure -- but shifted by about 15 feet.

These two structures were one and the same -- until one or several earthquakes moved them apart.

The team concludes that if a single earthquake caused this horizontal slippage, it must have been of a magnitude of at least 7 -- which corresponds to the modelling done with the obelisk.

But their hunt for the fault line doesn't stop there.

The team heads further east, into the hills, to the nearby village of Cerkesli, which sits astride the fault.

♪♪ Another expert, seismologist Yann Klinger, has joined this latest search.

By studying the topography of the landscape, the team can track the fault.

They'll now dig a trench and examine the soil to date the different earthquakes.

Roughly 130 feet long, the trench stretches across the fault.

So how will they identify those past earthquakes?

-An earthquake is like a big fracture that spreads to the surface of the Earth.

Deep down, the fault itself is like the trunk of the tree.

By the time it gets to the surface, there are all these little branches.

And these are all the little fissures.

-The fault segment they're looking for is hidden at the heart of this mass of sedimentary layers.

On the exposed walls, you can see a lot of cracks.

But they're very hard to analyze, so the team is gathering as much data as possible -- such as this 3-D reconstruction of the heart of the trench.

♪♪ Raphael Paris, a specialist in tsunamis, is investigating the effects of the fault.

Could it cause a super-shear earthquake -- and potentially a tsunami?

Raphael and researcher Nur Deniz Unsal travel the shores of the lake that have remained undeveloped, in search of traces left by a tsunami.

-[ Speaking in French ] -I'm looking for an anomaly, a sedimentary anomaly.

This is an environment that has very slow sedimentation.

Then, all of a sudden, a tsunami comes along and deposits sediment that's completely different -- much coarser and with shells, shells that come from the lake, things like that.

-He's drilled into the soil to a depth of 10 feet.

At seven and a half feet, there's a deposit that's very interesting.

-[ Speaking in French ] -At precisely this depth, we've found a sandy level that's abnormally rich in shells that come from the lake.

And we reckon that's probably evidence of a tsunami on Lake Iznik.

♪♪ -But it will take months of research to find out if this tsunami occurred in 1065 -- and if it contributed to the sinking of the basilica.

♪♪ The scientists now shift their attention from the fault's past to its future, hoping to determine when it might shift again and protect people living in the area.

[ Ship horn blows ] ♪♪ As mist covers the Sea of Marmara, in the port of Yalova, the team of sedimentologists arrives to fetch an important research tool -- a National Research Centre barge that's left France for the first time.

It is capable of drilling much deeper core samples.

The barge must be reassembled after its journey from France.

The scientists hope it will help them better understand the fault.

Once assembled, the drilling barge is anchored in the lake, to the north of the fault.

-This fault hasn't moved for about a thousand years.

The last big earthquake was in 1065 -- and there hasn't been a major earthquake here since then.

♪♪ -The smaller 10-foot core samples the scientists studied previously have already confirmed the seismic activity of the last 2,000 years.

But the new drill should produce sediment cores that go much further back in time -- between 10,000 and 15,000 years.

-We want to see if these earthquakes are repeated every one or two thousand years, or even longer.

If they're repeated every thousand years and the last one was a thousand years ago, we risk having an earthquake quite soon.

-Since the last earthquake in 1065, the Iznik fault hasn't shifted.

If the pattern is every thousand years, is another quake imminent?

With that concern, the scientists race to collect as much information from the underwater basilica as they can.

And their efforts pay off with an unexpected discovery... -[ Speaking in Turkish ] -Two of these tombs are particularly important.

They're placed right under the bema wall, which means that they were there before the church.

-The bema wall, or "Seat of Judgement," is a foundation wall located between the central nave and the apse.

The archaeologists have unearthed tombs located directly under it.

If the tombs are beneath the bema, then they were there before the basilica was built.

New evidence raises questions about whether the basilica really hosted the First Council of Nicaea.

The tombs contain coins dating from the time of Emperors Valens and Valentinian, who reigned between 364 and 378 AD -- more than 50 years after the First Council of Nicaea took place.

-[ Speaking in Turkish ] -It means that the basilica was built, at the earliest, after 390 AD.

-It could not have hosted the First Council of Nicaea.

However, it does provide insight into the history of Christianity.

The basilica reflects the profound transformation the Roman Empire underwent as it shifted from pagan belief systems to Christianity -- led by Emperor Constantine.

♪♪ -[ Speaking in French ] -It's rare to have this mix of someone with enough vision to lead the whole of the immense Roman Empire toward something completely different.

-Constantine was determined to reunite the Empire after the chaos of the Tetrarchy.

He led the military into battles to unite the Empire as soon as he came to power in 306 AD.

Accounts vary as to what happened, but Constantine credited divine intervention for his defeat of rival Maxentius.

The night before the battle, Constantine is said to have ordered the Greek letters Chi and Rho to be painted on the helmets and the shields of his troops.

Chi-Rho is an early Christian symbol depicting the first two letters of "Christos," or Christ.

Crowned with that symbol, on October 28, 312 AD, Constantine's forces defeated Maxentius and entered Rome cheered as liberators.

The following year, after reuniting the entire Empire, he signed the Edict of Milan, which guaranteed freedom to all religions.

-The Edict of Milan did not establish Christianity as the official religion of the Roman Empire.

But it was a beginning, a new beginning for the Church.

And buildings were constructed, like this basilica, where Christians could assemble together and worship freely without fear of persecution.

♪♪ -In Oxford, Dominik Maschek, with help from Mustafa, has completed one of the most exciting elements of the project -- a 3-D reconstruction of the basilica.

What was only a dream for Mustafa, has now become a virtual reality.

The Basilica of Iznik, one of the earliest Christian basilicas built in the Roman Empire's eastern provinces, has come back to life.

-[ Speaking in Turkish ] -It was a very big dream for me -- to rebuild the basilica we'd excavated and to show it in all its stages.

♪♪ -While the Basilica of Iznik will never be physically rebuilt, the work of this international team has made it possible to see what it might have looked like before the earthquake of 1065.

♪♪ ♪♪ ♪♪ -[ Speaking in Turkish ] -We've shown the world how important it is to share research and work together on the international stage.

-For Julia, and all the other scientists who carried out this investigation with her, the mission is over.

A submerged basilica has been their starting point for a journey of historical and scientific discovery.

On February 6, 2023, on the East Anatolian Fault, two violent earthquakes, with magnitudes of 7.8 and 7.6, hit southern Turkey and Syria.

Thousands lost their lives.

Scientists now fear that these earthquakes have increased the risk of a new rupture of the Iznik fault.

With the destruction another earthquake could cause, research to prevent future disasters continues.

The next movement of the Iznik fault may happen sooner than anyone thinks.

We now know that the 1065 earthquake was very powerful.

The Basilica of Iznik illustrates the damage earthquakes can cause.

But it's also a real treasure, a precious testimony to a pivotal period in history, and a poignant vestige of the past -- as spectacular as it is fragile.