-I'm Darius Arya, and I'm off to explore three of the most amazing cities on the planet -- Cairo, the gateway to Egypt's ancient wonders; Athens, the cradle of democracy; and, in this program, Istanbul, a boisterous treasure trove of architectural genius.

For over 2,000 years, East has met West here in Istanbul, the strategic middle ground fought over and contested by some of the world's mightiest empires.

But many of its secret treasures are hidden... Oh!

...out of sight or underground.

How deep is it?

[ Chuckles ] -Be careful.

-Using the latest 3-D imaging technology, our scanning team will help us see the city as no human eye ever could... diving through layers of history to show how the city has had to reinvent itself over and over.

As a professor of Roman history, I'm floored by the Hagia Sophia Cathedral... and the drama of its Roman racetrack.

♪♪ I'll be using virtual reality...

I'm going through the dome.

...to get to the heart of the city's hidden wonders.

This is Istanbul as even the locals may never have seen it.

Welcome to "Invisible Istanbul."

♪♪ -Istanbul, Turkey -- the meeting place of East and West.

The bridge between Europe and Asia.

And I'm arriving on the mighty waterway that put this city on the map.

♪♪ This is the Bosphorus.

It runs north to the Black Sea, down south to the Mediterranean.

Asia is on this side.

Europe is on this side.

Everything comes into focus right here.

Now, over 2,500 years ago, this was a Greek town, Byzantium.

Then it was renamed Constantinople, capital of the Roman Empire.

Many considered it the center of the world.

Perhaps Napoleon said it best -- "If the world were a single state, then Constantinople would be its capital."

I love how the city's tempestuous past has created one of the most vibrant cities on Earth.

This place has been built by Romans, rocked by earthquakes, sacked by Crusaders, and besieged by Ottomans, and you can see evidence of all of that all over the city, but there's also an invisible world hidden under the surface of modern Istanbul, and that's where the key to understanding this ancient city can be found.

Even the most famous places have their surprises.

The Hagia Sophia is Istanbul's most iconic building.

It embodies the history of the entire city.

♪♪ It began life as a Christian cathedral built by the Romans.

It was then converted into a mosque in the 15th century.

And now it's a secular museum.

Called "Ayasofya" in Turkish and "Hagia Sophia" in Greek, it means the Church of the Holy Wisdom of God.

In its day, it was the largest religious building in the Christian world.

♪♪ ♪♪ Your eyes are drawn up to the beauty and the size of the great dome.

This is absolutely massive.

It's 110 feet across.

It's 180 feet above the marble pavement.

It was the highest dome of the world.

Can you imagine to be a person living in 6th century Constantinople, how overwhelming that experience would've been, how its massive size dominated the entire city?

I'm blown away by this place.

This 10th century gold mosaic shows the two Roman emperors responsible for the existence of the Hagia Sophia.

This is Constantine the Great.

He moved the imperial capital here from Rome in 324 when the empire was in turmoil.

Thanks to his conversion to Christianity, it soon became the official religion of the entire empire.

But it was this emperor, Justinian I, who built this cathedral in the 6th century.

♪♪ Its size and open spaces make it an astonishing piece of engineering for the time -- and even today.

Our scan team is using laser technology to reveal the genius of Justinian's engineers.

By recording each location as data, we can analyze how it was built and changed over time, and this place has survived 1,500 years of earthquakes, riots, and wars.

Hey, Matt.

-Darius.

-Hey, man.

-Good to see you.

You okay?

-Nice to see you.

Yeah, good, man.

-This is Tom.

-Hi.

Nice to meet you.

-So, how's work?

How's scanning?

-Yup, scanning away.

The laser is firing out over a million times a second now.

-God.

Now, I've got a question for you.

I've always wondered why this structure's actually still standing despite all the different things, like earthquakes.

Do you think the scan can tell us some of those answers?

-That's the plan.

So, we'll scan from multiple positions around the whole building.

By the time we finish that, we'll have over a billion points all tied together in this millimeter-perfect 3-D model.

-Unbelievable.

Now, I'm going to be going to, like, a lot of other sites throughout Istanbul.

You guys are going to be doing the same thing?

-Yep.

We will scan every location with exactly the same technology, so we'll get all of these individual sites down to that level of millimeter-precision.

-Unbelievable.

Well, I can't wait to see the big thing, but if you don't mind, if I steal you away for just a minute, I'll show you something really on a micro scale -- real gem of a detail.

-Okay.

Fantastic.

-Right over here.

You know, Justinian spared no expense when he built this church, and, I mean, he had 10,000 guys working to build it day-in and day-out.

For example, maybe he used up to 40,000 pounds of silver to decorate it, to say nothing of the gold and the marble.

-Yeah.

The most expensive building in the world, I hear.

-Absolutely, and this thing -- he said that this was more magnificent than Solomon's Temple in Jerusalem.

And we have a lot of stories, but here, let me show you a piece of history.

I can step right down into it.

-Okay.

-And there's this weird little groove that you can see here.

-Yeah?

-What made this?

Well, this is the emperor's gate.

So, one story that's told is that actually, from all the years and all the centuries of all the guards that stood here wore this groove.

That's the neat thing -- the mystery that we still have with this history.

It's up to us to unravel it.

-So, speaking of mysteries, I need to go and solve some.

Let's get the scanning done.

-All right, take it easy.

See you later.

-See you later.

Take care.

-All right, ciao.

♪♪ I've always thought of the Hagia Sophia as a symbol of the city's ability to adapt and to survive.

For me, it's a powerful pivot point, still today, between East and West -- religion and politics.

I mean, it's been a church, a mosque.

Today, it's a museum.

It's an important mixture that embodies all of the great history of this city.

To reveal the secrets of Istanbul's past, our scan team is creating a 3-D map of the Old City.

-We're going to employ a process called photogrammetry -- taking thousands and thousands of individual photographs, and we pump all of those images in, and these are very clever algorithms, and then we're able to compute a really, really accurate 3-D model of Istanbul.

-When they add in the scans, we'll have a complete picture of how Constantinople was built and how it survived.

♪♪ I can't wait to see what the Hagia Sophia scans will show us.

Hey.

-Darius, hi.

-How's it going, Matt?

-Very good.

How are you doing?

-Good to see you.

All right.

-Take a seat.

-Oh!

This is the show.

-Yeah, this is our photogrammetry.

-Phew!

-We have this 3-D map of the extent of Constantinople, I guess.

And, obviously, at the core of that is Hagia Sophia.

-Beautiful.

-So, here it is.

Just think of everything on the screen here as digital dust.

It's millions and millions of individual points, and that means the laws of physics have kind of stopped applying for us.

So, we can just zoom on in here straight through the ceiling... -Wow.

-...and we're inside.

-Incredible.

You can get any perspective you want, even from beneath the surface of the floor.

-Exactly.

Yeah.

-Huh.

-Our eyes are always drawn up to the central feature of this space, which is, of course, the dome.

-Yeah.

-So, there it is.

And it's here that we started to notice the first kind of slightly strange things that we weren't picking up on.

So, this dome -- this seemingly perfect spherical structure actually is not a perfect circle down at the bottom here.

So, you see, as you take this circle around the edge here, it's got quite a lot of deformations.

-Yeah.

-Quite a lot of kinks.

There's a flattening off.

There's this really quite substantial dink into the side of it here.

And there's one place where these anomalies are actually really, really quite clear.

-Mm-hmm.

-I'm just going to strip off the outside.

♪♪ So, what we're left with here is...

I mean, look at that alone.

-That is quite deformed.

My God.

That is incredible.

Because you can't do that in real life.

You can't -- We don't have X-ray vision.

-Yeah.

No.

-Incredible.

-I mean, we've literally skinned the building here.

We're just left with this inside surface.

And that is a major lump.

-It's like it's gotten bashed in a bad fight or something.

-Yeah.

Well, you say a fight.

I mean, it's this earthquake shaking this thing, and, you know, this dome first collapsed 26 years after it was completed.

-Right.

-Rebuilt, repaired, patched up over the centuries.

-Of course, there's just got to be so much weight, right, from the dome all the way down, being transferred?

-Yeah.

Those ribs are transferring the load from that dome down past these windows onto this circular area here.

-Mm-hmm.

-But that's not where it ends.

That then comes down to these triangular features.

There's four of these, and they are the pendentives.

These pendentives take the load from a circular position down into a square.

So, each pendentive leads to an enormous column that transfers the load down to the ground.

-And each one has an angel on it, so that's like the angels are supporting the dome of Heaven up above.

-Yeah, exactly.

So, I'm just going to put us a slice right the way through the middle... -Ah, nice.

-...of the building there.

There's still this enormous pressure pushing down and outwards, so we can start to see some of the solutions that stop the building from splaying out there.

And that's this whole series of buttresses that turn this almost into a mountain.

-It really has revealed why the Hagia Sophia is still standing, sometimes rebuilt and buttressed, changed and adapted over time to survive.

♪♪ Good morning.

-Oh, welcome.

-Hi.

When Emperor Constantine built this city, he wanted to outdo Rome itself.

Thank you.

He built public baths, aqueducts, a library, marketplaces, and palaces.

-Thank you.

-Okay.

Bye-bye.

The population soared from 20,000 to 200,000.

The emperor also had big plans for the city's racetrack, the Hippodrome.

Great public places were ways to attract rich and poor alike, and back in the day, this was the most popular attraction in the entire city.

Today, this space is a park and a school, but our graphic shows how the Hippodrome once dominated the heart of ancient Constantinople.

This immense structure held twice as many spectators as Rome's Colosseum or Yankee Stadium.

♪♪ Hello, Kerim.

How are you?

-Hi, Darius.

-Nice to see you.

-Nice to see you, too.

-Professor Kerim Altug is an expert on the Hippodrome.

-It will be fantastic.

-So, if the starting block's over there, where's the actual racetrack?

-The racing track was the road running around today's square.

It perfectly follows the line of chariot-racing track.

-Perfect.

That's amazing.

So if we go back in time, we're right in the line of fire.

That's not where you want to be.

And then screaming fans are up in the stands.

How many?

-Some say the hundred thousands.

-So, that's a big part of the population here watching the chariot races, watching someone race to victory or crash and burn and die.

So, there's a lot of drama that's here, people that are pushing in the stands.

It's just like any Roman city -- like Rome, like the Circus Maximus.

-Exactly.

-It's intense.

-Exactly, and now, I've got something even cooler to show you in the underground.

-Oh.

Lead the way.

That sounds great.

-Yeah.

Let's go.

♪♪ -Amazing.

This is just so enormous.

-Yes.

I'm going to show you the archways forming the entrance and exits of the Hippodrome, now walled-up.

-Oh, yeah.

-And now, we are going to the foundations.

♪♪ -Ah, that's a little door.

-Yeah.

We have a lock here.

-Doesn't look like it's too frequently visited.

No, no.

Wow.

Now, this is really interesting.

You've got all these massive foundations.

What was this place?

-This is the substructure of the seating.

-Ah.

Okay.

So the Hippodrome is above, and this is what's supporting all those stands.

Now, what about -- I see some other holes and passageways.

What about over there?

-Yeah, there are a lot of them, but I don't want you to get lost down there... -Okay.

-...so you'd better follow me.

-All right.

You're the boss.

What was this place right here?

-These are the passageways, but some say these also used as a prison... -Hmm.

-...or for the executions.

-What about down here?

Does it continue?

What is it?

-Let's take a look inside.

-Should I go?

-Go on.

Why not?

-All right.

♪♪ Ooo-kay.

I think we're going to need a little more equipment.

I think we're going to need some better lights.

Slipping into something a little more comfortable.

Shimmy, shimmy, shimmy.

Not exactly what I expected I was going to do today.

All right, Kerim, I'm ready to go.

-All right.

-Let's do this.

-All right.

Good.

♪♪ -Wow, this is...

Incredible staircase here.

♪♪ How deep is it?

[ Chuckles ] -Be careful.

-Whoa.

This place is huge.

-This vault chamber converted into cistern, probably in the 6th century.

-This improvised water tank is over 30 feet high and could hold over 2 1/2 million gallons of water.

-So, this is one of the largest covered cisterns of Constantinople there is.

-And I actually -- I'm getting stuck in mud and water... -Yeah, be careful.

-...which is -- Thank you.

It still holds the water, that's for sure.

-Yeah.

-What about over there?

Can we -- Ugh!

Can we check that out?

-Be careful.

-Yeah.

-So, you can see how the walls are plastered with waterproof hydraulic mortar.

-Yeah, amazing.

-Clearly shows us this a cistern.

-And obviously it still works, because we're walking in a place that's filled with water and mud and God knows what else.

-Yes.

You can also see how the hydraulic plaster ends.

This indicates the maximum storage level of the cistern.

-Pretty big capacity here.

Pretty impressive.

Constantinople had few natural sources of fresh water.

Concealed beneath the Hippodrome's seating, these abandoned passageways were converted into water-storage tanks.

Ingenious.

For over 1,000 years, this was the city's insurance against drought, fire, and siege.

So, this one really impressive cistern.

It surely couldn't be the only one, though.

-We were able to identify more than 200 cisterns in the historical peninsula.

-This place is simply amazing.

You know, thanks so much for bringing me here, because I can't believe that something that was created as a hippodrome in the 4th century -- you showed me that it's been converted into a cistern in the 6th century, and then we're standing right here today, in the 21st century.

-Yes.

This is one of the hidden treasures of the city.

-Thanks to our scan team, we can now see how the Hippodrome was adapted to the changing needs of the city.

This vast structure entertained the first citizens of Constantinople, and, later, as a cistern, it kept them alive by storing precious water for 1,000 years.

Now, this reservoir is all but forgotten, except to those who know invisible Istanbul.

The Bosphorus has always provided unrivaled transport and trade routes that allowed this city to thrive, but finding fresh water has always been a problem.

-Was in 2012... -Professor of Art History at Istanbul University Ferudun Ozgumus, has promised to show me how fresh water took on a sacred significance in the ancient city.

-Something important in this carpet shop.

-All right.

I'm intrigued to find that our journey begins in this carpet shop.

So it's, like, a recent discovery.

I mean, that's just like the other day.

Yeah, yeah.

We're going down pretty deep here in the basement.

-Yes.

♪♪ -Oh, and it just keeps on continuing down.

Oh, this is getting pretty deep.

Right.

Oh, thanks.

♪♪ All right.

So, what is this?

Is this it?

Ah, very impressive.

Yeah, a big portion of it, too.

Now, what is this part of?

What was it?

♪♪ Our photogrammetry shows a layer cake of civilizations, with today's carpet shop sitting right above the Great Palace of Constantinople.

And there's another hidden level below that.

Okay.

Okay.

Ah, and we're going to go down further for this?

-Yes.

Yes.

-Amazing.

[ Key jingles ] Wow.

Now, this -- this is impressive.

Actually lower than the level of the church.

-Yes.

♪♪ -All right, something else.

Ah!

Yeah.

But this is a spring?

That, I would not expect underneath a church.

Oh, yeah, right down there.

What is that there?

Okay.

But it looks like Greek.

Okay.

Mm-hmm.

So, for a very long time, then, people are coming to this church, coming down below here, and they were able to potentially purify themselves... -Yes.

-...before their... That's fascinating, because this was a precious commodity.

This was not a city of water.

So, this is a real rarity to have a spring that still flows even today.

Our scans show how Istanbul is built on layer upon layer of history, with only a tiny trickle of fresh water deep in the foundations of the city.

But if Constantinople was to grow, its people would need the miracle of Roman hydraulic engineering.

♪♪ One of the most magnificent places to observe some of that hydraulic engineering is not underground, but rather while just sitting here, sipping a tea, or chai, as they call it, if you're underneath an aqueduct.

♪♪ This is the Valens Aqueduct.

It's the city's major water supply in Roman times.

It's over 100 feet high and over half a mile long, and the water channel descends on top into the city.

It was a masterful engineering feat, and today it still dominates the city landscape.

Our scanning team is already at work on this mighty aqueduct bridge.

It was completed by Emperor Valens in the late 4th century.

The Western half of the Roman Empire was in decline, but Constantinople was still on a building spree.

This is just one of 30 such bridges, part of a vast and complex system of water channels and underground piping bringing water to the heart of the city.

I think it's one of the greatest engineering and surveying feats of the entire ancient world.

-Hi, Darius.

Come on up.

-All right.

The man to tell us about it is Professor James Crow from Edinburgh University.

It's a big ladder, man.

-Yep, just keep on holding.

-Oof.

Yeah.

He's doing a research project to map that water-supply system... All right, don't let go.

...the crucial structure in keeping the city alive.

Whew!

Nice!

Great view.

And sunny.

-Yep.

-So, Jim, how did you and your team figure out all of this complicated system?

-Well, we knew -- it was known there were sources outside of the city, but they'd never been properly mapped before, so we set about sort of creating maps, finding out more evidence for where the bridges and channels were.

It's a deeply forested area, so it's not laid out in front of you, so you really have to work hard.

You have to find from traces and then join the dots in many instances.

-So much of Constantinople is colossal -- I mean, the Hagia Sophia, the Hippodrome, and now this.

This must've carried a lot of water.

-Yes, this huge bridge, nearly a kilometer long, has two channels in it.

-Unbelievable.

-One we know about is probably as tall as you and this wide, and this brought water for over 150 kilometers in a direct line with the spring, but in total length, probably exceeding 300 kilometers.

-Amazing.

I'm a Roman guy, so don't tell me, "It's better than Rome."

-It's different to Rome, but in some ways, it exceeds Rome in terms of the length of the channels and the way that the channels tend to cut through really difficult countryside.

So, it's a real engineering achievement in the way -- much more than Rome is.

-Our scans reveal that the water flowed through two great channels inside the aqueduct bridge.

Jim's research confirms that the water was then directed into cisterns across the city -- 209 of them -- some small, some improvised, like the one in the Hippodrome, and some purpose-built and as big as a football field.

They were fed by two supply lines coming all the way from forest springs up to 125 miles away.

Altogether, there were over 360 miles of channels in the longest and most sophisticated water system of the ancient world.

♪♪ Aqueduct bridges like this were essential to Constantinople's growth into one of the greatest cities in history.

♪♪ ♪♪ The city's success inevitably attracted envy...and enemies.

Today, people drive past walls like this and think nothing of it, but it was once a beautiful palace.

This overgrown mess is the Palace Boukoleon.

It was once part of the great Palace of Constantinople.

Now, when it was built in the 5th century, the sea came right up and crashed against these walls.

We're told that there were 500 rooms covered in gold mosaics, but all that's left of its faded glory are these few marble window frames.

♪♪ I've been told that you can get into the palace through here.

What a garden.

When this palace was built, Constantinople was said to possess two-thirds of the world's wealth.

The remains of the Boukoleon Palace are a melancholy reminder of the city's lost glory.

This is major architecture.

The scale is huge, and, of course, this must have been some sort of estate room.

The walls would have been lined with marble.

You could imagine mosaic floors, painted vaults, and, of course, in antiquity, it's right on the water's edge.

What a spectacular view you would have had.

Still pretty spectacular today.

♪♪ ♪♪ Our graphic gives us just an idea of the grandeur and scale of this magnificent palace.

So far, our scan team has recorded ruins that tell the story of the rise of Constantinople.

Here, the ruins tell us about its decline.

And the people who caused the destruction here were Western Christians.

♪♪ Rome and Constantinople were at the center of a dispute over religious doctrine in 1054.

From that point, Crusaders from the West saw the people here as Eastern heretics.

This was once a chapel covered in gleaming, resplendent marble, but it's all charred black from fire.

Today, this is a refuge for the homeless, but the real destruction took place over 800 years ago, when, in 1204, this city was sacked by Western European Crusaders.

They pillaged.

They trashed this palace and this city.

Constantinople was on the ropes.

♪♪ The Western invaders occupied Constantinople for 57 years.

They desecrated Eastern Orthodox churches, burned down the library, and looted the palaces.

By the 15th century, the population of Constantinople had plummeted like a stone down to 70,000, tops.

The Roman Empire, the greatest power the world had ever seen, had now shrunk to this single city.

Those who remained soldiered on, growing their own food, surviving the best they could.

With the city's cisterns for collecting water and agricultural land for raising crops within the city walls, the city was virtually self-sufficient within a ring of stone.

The tradition continues today.

The people of Istanbul still raise vegetable crops in the moat and in the area between the city walls.

Constantinople stood alone... and the East was rising.

The Muslim Ottoman Empire had the Christian city completely surrounded.

By 1453, the only way supplies could reach the city was by ship, and the Ottoman leader, Sultan Mehmed, was about to increase the pressure.

He built this huge fortress to lay siege to the city.

♪♪ It's on the European side of the Bosphorus at its narrowest point, less than 800 yards from Mehmed's fort on the Asian side.

♪♪ The name of this stronghold, Rumelihisari, literally means "fortress of the Romans."

Now, let's think about that for a second, because the year was 1453.

I mean, 50 years later, Columbus discovered America, and still Constantinople was the capital of the Roman Empire.

The original name is even better -- Bogazkesen Castle, literally "throat cutter," because we're at the choke point here at the Bosphorus, and it was meant to cut the throat of the Bosphorus.

And it worked.

When a Venetian ship sailed right out front and ignored the warnings from the fort, a single cannon was fired from one of the towers, hitting the ship.

The crew was taken off and beheaded, and the captain, well, he was impaled, and his cadaver was hung out front like a scarecrow as a warning to all others.

Mehmed didn't want any assistance to get to the city.

But Constantinople had a final line of defense -- an iron chain laid across its harbor inlet, the Golden Horn.

Mehmed's ships couldn't get past it to attack.

But Mehmed had a brilliant solution.

He ordered the construction of a road made of logs, heavily greased, around the chain barrier.

Then, on April 22, 1453, he orders his troops to drag the ships off the water, on the road, around the chain barrier, and into the estuary.

Now the city's 5,000 defenders were entirely surrounded and trapped in a siege of bombardment and starvation.

Despite this -- despite being surrounded by 200,000 enemy troops -- Constantinople still resisted.

The main tower of the Rumelihisari fortress was a strategic command post for Mehmed's whole operation.

Its secrets are locked away, but we're getting to see them... Shall we take a look?

-Yes.

-...with local historian Ali Zirek.

♪♪ It's kind of dark and spooky.

-Yeah.

-Wasn't expecting to find one of the most important artifacts in the history of the city just lying on the floor.

Is this the chain that Constantinople used to stop Mehmed?

-Yes.

-That's incredible.

I mean, this is the real chain?

It's the real thing?

-Very strong.

-Very strong, and it's rusty, but, I mean, it's hundreds of years old and still intact.

[ Grunts ] It's very heavy.

I'm really curious to find out what else Ali wants to show me, and he's taking me up all the way to the top floor.

Wow.

This is an amazing space.

What happened here?

Oh, so it's, like, a council chamber.

It's unbelievable.

I'm thinking about the decisions that he made in this castle, maybe even in this very room.

I mean, this is where history happened.

I think it really -- kind of get a sense of that, can almost smell it.

♪♪ Mehmed's fortresses were among the most advanced of their time.

Our scans allow us to really get under the skin of this tower.

♪♪ They reveal a seven-story weapon of ruthless efficiency, with walls up to 23-feet thick that protect Mehmed's council chamber... crenellated lookout positions, arrow slits and firing positions, toilets, grain storage, living quarters for 400 soldiers, a water cistern, of course, a chute to drop boiling oil onto an invader at the front door, and an unrivaled view to guard the narrowest part of the Bosphorus.

On April 6, 1453, the sultan launched a 53-day siege of constant bombardment, and he was using the biggest cannons in the world.

Mehmed concentrated his efforts on a section of old wall in the northwestern part of the city, and he hammered with his cannons that section relentlessly until eventually a part of the wall collapsed.

Now, the defenders defended themselves valiantly, but they were no match for the sultan's elite troops, the Janissaries, and with the prospect of the horrors that would follow, many of the city's inhabitants decided to commit suicide, jumping from the city walls.

The pillaging and looting went on for three days.

The legend goes that instead of letting their enemies get the loot, the people of Constantinople threw so much of their wealth into the Golden Horn that the waters sparkled with gold.

♪♪ The invaders closed in on the Hagia Sophia.

They smashed down the door and burst in on the city's women, children, old, and injured.

The last citizens of Constantinople were raped, slaughtered, and enslaved.

Mehmed himself lamented the devastation he had caused, saying, "What a city we have given over to plunder and destruction."

This was the final fall of the Roman Empire.

The people we know as the Byzantines were finished.

Constantinople was no longer a Christian capital.

It became Muslim Istanbul, Mehmed's capital of the Ottoman Empire.

The East now held sway.

♪♪ The best place to see how the city changed is back at the Hagia Sophia.

From that day, the cathedral became a mosque.

Over time, the four minarets were added, and the mosaic crosses were decorated to disguise them.

Eight enormous panels were erected much later, spelling out the name of Allah, the Prophet Muhammad, his grandsons, and the four caliphs.

To this day, they are the largest in the Muslim world.

There are lots of Islamic additions.

Case in point is this recessed niche right here.

It's off-center from the apse, off-center from the whole church.

There's a reason for that.

It's the mihrab, and it indicates Mecca, and that is the direction that Muslims pray.

♪♪ Sultan Mehmed became known at Fatih, "the Conqueror."

But he was also a builder.

He repaired the water supply, rebuilt the city walls, and established one of the greatest economic centers of its time -- the Grand Bazaar.

♪♪ ♪♪ Hi.

-Hi, sir.

-How you doing?

-Fine.

You?

-Good.

-These look okay?

-Yeah, these are very nice.

Within two centuries, the population rebounded from 70,000 to 700,000.

Okay.

Thank you.

-You're welcome, sir.

♪♪ -As the Ottoman Empire grew, so did the bazaar.

Today, it has over 4,000 shops on 61 covered streets, visited by up to 400,000 shoppers in a single day.

The Grand Bazaar became the center of Istanbul's wealth, and Istanbul became the center of the Ottoman Empire.

It must have felt like the center of the world.

-Thank you.

-Thank you.

-[ Speaking native language ] -Oh, thank you.

Thank you.

♪♪ Islamic and Turkish cultures certainly have added to what the Romans and the Christians had established.

Turkish cuisine is famous for a rich variety of ingredients, but sometimes you just want something simple.

You want a balik ekmek, a fish sandwich, and the best place to have it in Istanbul is in Eminonu, right here at the Galata Bridge.

And certainly, eating this at the Golden Horn amidst all of this history adds to the flavor.

♪♪ ♪♪ But not everywhere in Istanbul is so hectic.

Islands of peace can still be found in the Old City, like this mosque, just north of the Grand Bazaar.

♪♪ ♪♪ This is so peaceful.

It's hard to believe that I'm in the midst of a teeming city of 15 million, but this place gives you a sense of calm, tranquility.

I see community.

It's about being with yourself.

It's about having opportunity to meditate on many things before going forward to prayer, and this is an amazing opportunity to start that reflection.

♪♪ A century after Sultan Mehmed, his successor, Suleyman the Magnificent, had a new mosque built in 1557 to outdo anything that had come before it -- the Suleymaniye Mosque.

Inspired by the grandeur of the Hagia Sophia, this mosque is the epitome of the Ottoman Empire's golden age.

The genius of the mosque is found in its grand design and its smallest details.

Much is invisible, but not to my guide -- architectural historian Olcay Aydemir.

♪♪ Wow.

Amazing.

This is spectacular.

-Yes.

-I mean, there's so much light.

There's so much space.

Just an incredible volume.

It's just so massive.

It's so tall.

Absolutely.

Though inspired by the Hagia Sophia, Suleyman'S mosque had innovations of its own.

So, I'm looking at the illumination here, and, of course, I mean, this is electric, but what was it like in the 16th century?

Okay.

Now, I see, you know, interspersed between these electric lights, I mean, there's some sort of suspended ball or egg-like thing.

What is that?

Ostrich shells, okay.

-Yes.

-And is that intentional as well, or is that something from -- Okay.

It's very beautiful and eco-friendly.

-Yes.

-So, technology, sustainability, incredible architecture, incredible design.

-Yes.

It was genius.

-It's so impressive.

-Yes.

-But there's more.

Our scans reveal an invisible ventilation system hidden in the walls.

It drew up the soot from the lamps through hidden funnels leading into a secret room.

The soot was scraped off the walls to make black ink for precious manuscripts.

Today, only graffiti remains.

♪♪ Sultan Suleyman wanted his mosque to be a social-welfare complex.

A hospital was built, six schools, Turkish baths, a travelers' inn, a bakery, a medical college, and a public kitchen for the poor.

In the 19th century, there was even a place called Addicts Alley where cafés sold hashish and opium, though I doubt if that was part of Suleyman's plan.

-So, as we go through...here, there are lots of monuments, and they're all over the empire.

-This is the mosque's public kitchen, or imaret.

It fed over 1,000 of the city's poor every day in a way that allowed people to keep their anonymity and their dignity.

Very nice.

So, I can take this anonymously, as well?

-Yes, of course, please.

-Thank you.

Buon appetito.

This was a sultan's mosque meant for everyone.

Suleyman's kitchen was designed to feed Muslims, Christians, and Jews alike.

Today, it's a commercial restaurant in this modern city.

♪♪ I've been invited by Matt to go back to the Old City, but this time, I'm going to experience it in a whole-new way.

The scan team are eager to show me how the city comes alive in virtual reality.

Cool.

I've never seen -- -It doesn't look like much now, but this little backdrop here, together with this headset, this is your portal into our digital model of Istanbul.

There you go.

-Wow.

This is amazing.

-So this is the whole peninsula.

-Beautiful.

And right down there, that's the curve of the Hippodrome.

That's right where we went into the cisterns.

Oh, that's right where we entered, ha, and down the stairs.

Oh, look at that corridor.

[ Chuckles ] You just can't smell it, but otherwise, it's like I'm there.

I can get underneath it too.

-Yep, worm's-eye view.

-Ah, man.

And now, I mean, I'm just walking right over top the entire Hippodrome towards the Hagia Sophia.

-So, you're free to drop your head inside.

-Whoa.

There's everything inside.

Oof.

Space is incredible here.

I see all these little outlines of people -- children, adults, just in motion.

-These are the ghosts of the tourists that were there on that day.

They're all moving around a little bit quicker than we can scan them.

-Okay.

-And we can actually start to go and inspect this building a little more thoroughly.

-Okay.

Oof, going up the elevator.

Oh, slow down.

Yeah, you get a little vertigo here.

Wow.

Really fast.

Wait.

Okay.

Going through the dome?

All right.

Okay.

Whoa.

-So, here we are.

-Wow.

I'm going to need to take your hand here.

-[ Chuckles ] -It's really disorienting, though.

You feel like you could just fall right through.

-And check out -- I mean, the lumps that we saw previously, they've become this kind of golf course of mounds around us.

-Oh, my God.

Yeah.

Oh, back down again?

Okay.

Whoa.

The VR makes me really appreciate what an extraordinary feat of engineering the Hagia Sophia is.

Oh, coming down.

It's like this whole city and its people -- They've been knocked around, but they've always found ways to survive.

♪♪ They've adapted buildings like the Hippodrome to stay alive through drought and war... ♪♪ ...and kept fresh water flowing to everyone through their elegant aqueducts.

♪♪ This is a city that has built upon its layers of history and culture.

As the battered dome of the Hagia Sophia reminds us, the people of Istanbul have fought both armies and earthquakes to preserve their city against all the odds.

♪♪ This ancient city has been the fulcrum of the world for two millennia.

And in the 21st century, the tug-of-war between East and West remains as intense as ever.

Turkey is rising in prominence in international politics and looking to strengthen its Islamic ties, looking to the past, the Ottoman Empire, for inspiration.

All eyes are focusing once again on the city at the center of the world.

♪♪ -"Ancient Invisible Cities" is available on DVD.

To order, visit Shop PBS or call 1-800-PLAY-PBS.

This program is also available on Amazon Prime Video.

♪♪ ♪♪