Rome's Invisible City

Rome, for 1,000 years

the beating heart of the ancient world.

Capital city of the most powerful empire on the planet.

But this iconic cityscape tells only half the story.

Every modern city is served by its underground spaces.

2,000 years ago, the Romans got there before us.

Deep beneath Rome's glorious domes

and columns lies a secret underground powerhouse that

made life possible for a million citizens up above.

I love Rome. Of all the places in the world this is my favourite.

Every time I visit I find myself just newly bewitched by

this fantastic ancient city.

But this time I'll go beyond the surface world that

the tourists see and the archaeologists scrape at.

I'll be digging deeper to explore a whole new invisible world deep

underground that reveals how the first metropolis was built and run.

Wow. It leads down eight storeys.

Xander, are you all right down there?

Fine.

'I'll be working with a team of experts who'll use the latest

'technology to reveal this secret underworld.'

Look at this.

That's incredible. Look at the detail.

'We'll explore the underground engine rooms that built

'and powered the extraordinary world above...'

Everything that made Rome tick is coming along these passageways.

'..the hidden wonders below the Coliseum that made it the greatest

'and the goriest show on earth.

'And in long-lost labyrinths we'll uncover underground cults.

'This invisible treasure trove will reveal the secrets

'of the world's most remarkable ancient city both below...'

I've never been in a sewer before. I hear it's great(!)

'..and above ground.'

Welcome to invisible Rome.

Now, this is enormous fun.

I used to drive one of these for about seven years in London.

My wife made me get rid of it when we had children. Why?

Did she think it was dangerous or something(?)

Anyway, this is the best way to get around Rome.

If you've been around Hyde Park Corner

getting around Piazza del Popolo is going to hold no fear for me.

Here we go. Let the dog see the rabbit.

I've always loved this city,

ever since I was a child and my grandfather used to read to us

from the legends of ancient Rome. I even had my honeymoon here.

Who could resist the majesty of the Forum, the Coliseum

and St Peter's Basilica?

But now I'm going to dive into the underground spaces

we don't see on the surface to discover exactly how

the underworld powered the first metropolis.

My walking Wikipedia for this exploration of invisible Rome

is Dr Michael Scott.

Michael's been coming here for 15 years to study the city's

amazing monuments.

And where better to start our journey into Rome's

underworld than in its largest and most famous building,

the Coliseum?

It opened in 80 AD, just as Rome reached the height of its powers.

Look at that.

'For 500 years it hosted a gladiatorial carnival of combat

'and carnage.'

-So, Xander, welcome to the Coliseum.

-Wow!

This is where you hear the roar, isn't it?

Something like 60,000 people on the seats all around us...

60,000 in a city of a million is a significant percentage, isn't it?

..all baying and shouting loudly for what was going to take place

right here on the arena floor.

And this place opens with 100 days of games.

And can you imagine what it must have felt like to be standing on the arena itself?

And the noise, the wall of noise of the people all around.

'Standing on the arena floor gives me

'a spooky sense of the spectacle that unfolded here.

'But to understand how the Coliseum really worked,

'we have to go down into the bowels of the beast.'

This is called the Hypogeum.

It just means underground space.

There's a tunnel that goes all the way out there

-and that leads to the gladiator school.

-Right.

So we're walking in the footsteps of the gladiators

who would have been coming into the Coliseum.

A tiny proportion of who might get to walk back that way, as well.

Yeah, there was also an exit to the morgue.

'These tunnels aren't the only hidden secrets of the Coliseum.

'Beneath the stage was a labyrinth of holding pens and lift shafts.'

Well, if I show you.

This is the arena floor, this is the bit above us, right.

There would have been these holes that opened up, about 40 of them,

and these correspond to these tunnels that we're looking at directly.

Underneath each of these, we can start to see the mechanisms.

'These crumbling ruins were once at the cutting edge of technology.

'In the central corridor, sloping rails guided monumental

'scenery up onto the stage above.

'On either side, numerous lift shafts disgorged animals

'and humans to their deaths in the arena.'

-Can you see that hole in the ground, the central hole?

-Yeah.

That's probably the hole where a capstan pole went up two floors

and had big arms so that two teams of men could turn it

and that would be used as a winch to lift up cages.

How incredible!

These lifts would have been big enough to take anything up to a lion.

These animals would just magically appear.

This would have been a sea of machinery, toil, effort, noise.

The animals starved so that they were extra hungry

when they got out there onto the arena floor.

You know, I've been backstage in a lot of theatres

and the atmosphere backstage, particularly with a big show,

nothing as complex as this but, you know, you've got people

running around with clipboards getting terribly panicked.

I guess if you times that by 100...

If these guys down here, the guys operating the machinery,

the guys calling the timing, got it wrong, they could find

themselves not running the spectacle but being the spectacle!

-They were the next ones on(?)

-The next ones to be fed.

-Blimey!

-It's not just about not getting a bouquet of flowers at the curtain call.

-No.

It's just so macabre, isn't it? The spectacle ultimately is death.

'Over the lifetime of the Coliseum, it's believed that up to

'500,000 people and one million animals were slaughtered here.'

To have something like this right at the heart of Roman life,

it smacks of a certain hedonism spiralling out of control, slightly.

There was nothing out of control about this environment.

The Emperor paid to put on these games to demonstrate his power,

-his control.

-Mmm.

He did it to ensure that the people,

the mob of Rome in some ways, were on his side.

How do you keep the people happy?

-You feed them, you keep them entertained.

-Bread and circuses.

Stopped them giving him trouble.

A none too subtle way of saying,

"You stick on side or else it'll be you hanging out of the lion's mouth!"

I think if I'd just seen this above ground,

I'd have seen the spectacle, the scale.

I'd have thought of this as a piece of bravura architecture

but then you go below stairs...

You get that horrific macabre sense of this being a machine

that spewed people out to their certain deaths.

'After the underground world of the Coliseum,

'I can't wait to see what more this worm's eye view

'can tell me about the rest of invisible Rome.'

'But first, time for a more modern traditional Roman pick-me-up.'

You can come here and see so much of Ancient Rome

up on top of ground. The Coliseum is a perfect example.

-You knew the Coliseum.

-I did. I had seen it, visible Rome.

Visible, Ancient Rome is all around us

and the thing that really excites me about this is we have got to go back underground

if we want to really understand how Rome became the amazing city it did.

We are going to be working with a team of 3D laser scanners,

-cutting edge technology.

-This is very exciting, indeed.

That is going to, for the very first time in Rome,

be mapping some of the underground spaces we are going into,

in a level of detail, texture, colour that has never been done before.

OK, so I can get my head around that, of course, underground,

one accepts that a great deal going on under there.

How do we get underground?

We're going to be walking through the streets of everyday Rome and

just there's going to be something completely that you'd pass by without

even noticing, access points for us to the world of invisible Rome.

This is like the Time Bandits.

Sometimes it might be slightly more complicated.

-You're not claustrophobic, are you?

-No.

How's your abseiling?

Abseiling(?)

You brought the Flavia.

I don't drive a Vespa...

'So, now I'm buzzing with espresso,

'I'm ready for invisible Rome to reveal itself.

'I'm hitching a lift with the Prof.'

How do we know what's there? I mean...

Sometimes we need some kind of like natural disaster

to uncover a bit of underground Rome that we didn't even know existed.

'It seems the city is so peppered with undiscovered subterranean spaces,

'that people and buildings just keep falling into sinkholes.

'We're on our way to the Aventine Hill,

'one of the seven hills Rome was built on.

'According to legend, it was founded by brothers Romulus and Remus,

'who were suckled by a she-wolf in a cave close by.'

Now we're going to see a little sink hole.

'Marco gets the call when random bits of the city

'disappear into the ground.'

Look at this!

-What actually has happened?

-This collapsed in the night.

When one of these people go in the morning to work,

and found this situation.

-It's gone.

-I'm assured that it is man-made.

-Man-made?

-Yes.

So Marco, how many sinkholes like this appeared last year, let's say?

Last year, we have 80 sink holes.

-80?

-80.

-Eight zero?

-We have many more.

-Already?

'Michael isn't surprised that Ancient Rome is devouring

'so much of the modern city.'

There was a city of one million people in Ancient Rome.

The population density was ten times what London is today.

There is so much still to find

and it's moments like this that open up new windows.

So when something happens like this, it's a sort of God-given opportunity

for archaeologists to roll up their sleeves and have a sneak peek.

It's very exciting.

'Marco has got a lead on what's causing the trouble.

'It's just around the corner.

'This is where our 3D scanners will start revealing

'the secrets of Rome's underground spaces.

'Not so long ago,

'another collapse revealed an ancient underground quarry.

'Today there's precious little sign of it

'in these quiet, suburban streets.'

I'm not quite sure where this quarry's going to be.

-The quarry is there.

-It's in there? Is it a quarry?

No, it's under your feet.

No, that(!)

That's what's underneath all these manholes.

I had no idea, I thought it was utilities.

I think it's 20 metres.

That's about an eight-storey building underground.

I'm going to step back and think about that for a moment.

That's extraordinary.

Just in this very unassuming little side street,

there's this manhole cover, unlocked,

that leads down eight storeys.

'It's time for the scanning team to swing into action.

'They're making the first of our 3D scans to help us

'reveal this invisible world.'

As we stand as the blue and red men.

Hot and cold, Michael. Hot and cold.

How do you feel?

Hot and cold, actually.

It was very good of you to agree to go down first.

I think once I'm over the first...

Argh! I think it might be quite fun.

Good. That's spot on.

There we are.

Xander, are you all right down there?

It's fine!

It's absolutely fine.

OK, down I come.

Oh, blimey!

HE CHUCKLES

This is great.

You knew all about this? Oh, God, it's amazing.

I wasn't expecting anything as big as this.

Look at that. Amazing!

Look, it just goes on.

And on.

'This place would once have teemed with hundreds of slaves

'working under the lash.

'Today, we're setting our scanners to work.'

'Matt Shaw explains how the technology works.'

We're laser scanning the caves

so what that means is taking millions and millions

of measurements of the surface down to a level of detail of every millimetre.

It allows us to assemble a model of the complete 3D

geometry of the caverns.

These places are incredibly complex and very strange shapes.

The laser is amazing at understanding those strange forms

but we're also able to scan above ground

and relate those above ground spaces to the places down here.

'As the lasers map this jumble of rocks,

'Michael shows me why this underground space

'was so important to the Roman world above ground.'

What particularly were they quarrying here?

Everything that surrounds us is a particular volcanic rock.

It's tufo, and that's what most of Rome is built on.

That's what they wanted from down here.

'Tufo hardens when exposed to the air,

'an ideal building stone.

'But also, in layers between the tufo,

'is a less compacted volcanic ash.

'If tufo built Rome,

'then this stuff helped it conquer the world.'

It's called in Italian, pozzolana.

A secret Roman ingredient in making Roman concrete.

The Romans were making concrete(?)

The Romans were making concrete 2,000 years ago.

'Concrete, as we know it, wasn't rediscovered until the 19th century.'

No wonder that's why they send people down 20 metres to mine the stuff.

With concrete they can build structures that no-one had

ever dreamed possible.

'Here the miners' pick marks are still just visible.

'But to make sense of this space, we really need the scans.

'They can record everything from the most minute detail to

'the labyrinth of interconnected chambers where the slaves

'would have toiled.

'Above a giant spoil heap, there's another intriguing feature.'

What we come to, we think,

is an Ancient Roman exit.

Look, and there it is.

The timbers covering it.

The timbers of probably some guy's basement.

He doesn't realise that underneath his floorboards is the entrance to a Roman quarry.

When they walk across and hear that weird creak, they never realise...

"Here are a couple of presenters below. I wonder what's going on?

"Oi! Turn it down!"

Can you see these little handholds, footholds,

-as some poor guy had to clamber his way out...

-Yeah.

..taking the stuff to the surface to turn it into...

-Turn it into an empire.

-Yeah.

'Scrambling around an ancient quarry has given me

'an insight into Rome's geological good fortune.

'Now I'm keen to seek how the 3D scan shines a light

'on this part of invisible Rome.'

Matt, how are you doing?

'Matt has been processing the results.'

I'm so excited about this.

You should recognise, I think, this little place that we're looking at.

That's right, the house where the sinkhole had appeared.

If we pull out there.

We are looking at a large section of Aventine Hill here.

'The team have stitched together individual scans to make

'a 3D model of the hill.'

That is just a terrifyingly sophisticated tool, isn't it?

It is amazing.

From a millimetre detail, right the way out to a view

that's spanning half a kilometre of the city.

You may recognise...

Look at this.

This little manhole cover.

There we are. Yes, I do recognise...

And something lurking below the screen, as well.

I'm never going to stand on a manhole cover again.

There it is!

'This is Rome like I've never seen it before.

'The city and its invisible underground spaces

'connected to each other.

'And now we're in the quarry, it is like a light has been turned on.'

Blimey, Matt, look at the detail.

How extraordinary, and this maps it completely

accurately in terms of its relationship with the above ground?

Exactly, yeah.

I see exactly where you are now,

that's where we went up to what was possibly the original access.

It looked very much as though that might be someone's cellar,

or something like that. You were tempted to knock.

'We learnt later we were under a convent.

'Now that would have given the Mother Superior a shock(!)'

If I were a householder on Aventine Hill,

one of these rather smart residences, I think

this is a map that would probably keep me awake at night.

Yeah, I think it's amazing, this kind of network of strange,

-organic spaces underground.

-Yeah, yeah.

And then this very rigid street pattern.

-The grid of streets above and this ginger route beneath ground.

-Yeah.

-Exactly.

-It's extraordinary.

So we're building up this kind of strange underground map

and above-ground map simultaneously

and they're starting to fill in all these patches together.

That's never been done before. God, that's incredible!

'So an amazing amount of information for archaeologists,

'surveyors and...

'..burglars(?)'

Wow, I feel like I should be sitting back in my chair

and stroking a cat and saying, "Moo, ha-ha, ha..."

We've taken this quiet, residential area of Rome

and we've turned it into our plaything.

This is incredible, isn't it?

And to have that above and below ground perspective

and see how they interrelate.

I mean, what a resource.

I can think of so many uses for this.

But, most importantly,

it allows us to build up our map of invisible underground Rome.

'This quarry alone produced at least 6,000 tonnes of tufo and pozzolana.

'It was part of a network spreading underneath the city,

'like this one we scanned next to the Coliseum.

'So far we've discovered 94 of the underground quarries

'that helped build Rome.

'It's like the metropolis is constructed

'on an enormous Swiss cheese.

'I'm rather fascinated by this pozzolana.

'I really want to find out what the Romans did with it

'and where better to discover how humble pozzolana

'helped build the Roman Empire,

'than in the most enduring of all their monumental buildings?

'For 2,000 years, the Pantheon has been a pagan temple,

'a church and then a tomb for Italian kings.

'It was completed in 126 AD when Rome ruled an empire that

'stretched from Portugal to Persia,

'from Scotland to the Sahara.

'I'm exploring this remarkable building with architectural

'historian Professor Ettore Mazzola.'

This is quite magnificent.

-Even without knowing its antiquity.

-Yeah.

-It is staggering.

-It's fantastic.

It's one of the greatest ancient Roman buildings,

extremely well preserved.

-Such an acoustic.

-Fantastic.

'The interior is a vast cylinder

'but its crowning glory is the dome.

This is still today the largest,

unreinforced concrete dome

ever built on this planet.

It's something that it is possible only because of the material

and the technique used to build it.

'Without steel to reinforce the concrete from within,

'surely this dome should collapse under its own weight(?)'

The whole structure is standing on eight pillars

and you have load-spreading arches that are concentrating all

the forces vertically into the pillars.

'What ensures these walls can bear the load without buttresses,

'is the construction of the dome itself.

'Each of its layers is made with a slightly different mix of concrete.'

The trick of these is to use different materials with

a different weight.

In this model you can see how step-by-step the dome is growing,

so the first part is made up of Roman concrete

that has inside fragments of travertine stone

and tofu stone, which are very compact.

And step-by-step these materials are getting lighter

and then, at the very end, there is only the pumice stone

which is closing the structure.

-It gets lighter and lighter.

-Lighter and lighter.

Then, at the very top, there is this big hole,

which is nine metres in diameter,

which is necessary structurally also because all the forces, step-by-step,

are going down vertically into the pillars.

It's such a clever use of the raw materials

found beneath Rome's streets.

And it's still standing.

Our modern concretes can succumb after as little as 20 years.

What is it that makes Roman concrete last two millennia and beyond?

Michael has come to a modern quarry outside Rome

to find out how Roman concrete changed the world.

The same types of rocks dug out of the quarry we explored earlier

are still being quarried here today.

With experimental archaeologist Lara Comis,

he's going to make concrete

using a Roman recipe that's 2,000 years old.

-We have the recipe that says that we need quicklime.

-OK.

-This is quicklime.

-This is the one.

And it's basically made of rocks which have been fired in a kiln.

-And here we have pozzolana.

-So, this is pozzolana.

-This is the secret ingredient in Roman concrete.

-Yes, absolutely.

The aggregate, the material that bulks up

and strengthens the concrete,

is the good old Roman tufo

we also found in the ancient underground quarry.

Do that there.

When the quicklime is mixed with water and pozzolana,

a reaction occurs that binds them together.

Pozzolana contains naturally occurring oxides

that create an even more durable mesh than modern concrete.

Ah, now we're starting to create something.

One...

Two?

So, how long will this take to dry?

Well, actually, we think that the minimum

-should be for 24 hours.

-OK.

2,000 years ago, the Romans discovered

that their special concrete mix doesn't even need air to dry it.

Pozzolana has got his wonderful property to dry under the water.

-The miraculous pozzolana. It really can do everything, can't it?

-It can.

And if you want,

I can show you an experiment that has been set in the water.

-OK.

-So, we can actually try and see what happened.

-OK.

It's been setting for just 36 hours.

..That one. Stuff on there.

But will the concrete pass the test?

-BOTH:

-Wow!

OK, I don't know about this colour. I'm not convinced. Hang on.

-Well, try.

-Actually, wow!

-You see?

-My God, you can just...!

Bellissimo.

-Yeah.

-I'm beginning to be a believer.

-I think that you can actually try, you know...

-Three, two, one...

-Wow!

-Wow! That's ama... Look at that!

-It's incredible.

Standing on Roman concrete that has set underwater in 36 hours.

The Romans, what technology! 2,000 years on, this is sensational.

Using concrete that could set underwater,

the Romans built the harbours and bridges

that enabled them to dominate one quarter of the world's population.

Wow!

The Romans were the first to take this technology

and to use it on a scale that others had only dreamt of,

whether it be above ground or underwater.

And in doing so, in thinking big,

they were able to create an empire that controlled the Mediterranean

and to create monuments and structures

that have lasted for 2,000 years

and will probably last for a lot longer to come.

The Romans were so passionate

about their buildings, harbours and bridges

that the emperor took the title of Pontifex Maximus,

the greatest bridge-builder -

a title still used by popes to this day.

The resources found beneath Rome's feet

helped it build the world's first metropolis.

But the city also needed the underworld

to help it survive and prosper.

Ancient Rome required

up to a billion litres of clean water every day.

All this water got here courtesy of Rome's finest engineering triumph.

The aqueduct.

One of the most iconic structures in the Ancient Roman landscape.

Above the Spanish Steps

is one of the ancient city's most spectacular examples.

But it remains hidden

from the thousands of tourists who come here every day.

Now, when you said we were going to an aqueduct,

-I, obviously, was picturing something...

-Up on arches.

Exactly that.

-I'm now beginning to get a hunch that we're going...

-Underground.

-..underground!

-Underground.

-Is that right?

In fact, we're looking for the entrance

and it should be coming up...

Here we are.

-2B.

-2B.

-Should have brought a bottle, or something.

-Do you want to knock?

-Ciao, Adriano.

-Hello. How are you?

-This is Xander.

-How do you do? Alexander.

-Ciao.

'Our guide is underground archaeologist Adriano Morabito.'

-So, Adriano, where are you taking us? All the way...

-Whoa!

-24 metres down.

-OK.

-To the Virgin Aqueduct.

This wonderful spiral staircase is one of the few access points

to an aqueduct that's hidden beneath the city.

There it is. Look at that.

-Crystal clear, Adriano. Virginal, you might say.

-Exactly.

And if you think about it, if you go that way, back to the source,

it's something like 20km of this aqueduct tunnel.

But that's just phenomenal engineering, isn't it? How...?

And think that all the other aqueducts are much longer.

-Some of these are going up to 90km.

-Seriously?

RUMBLING That's the train. Can you feel it?

-This is the metro line.

-It's the metro line.

-Wow.

-Wow.

The Aqua Virgo was built before the birth of Christ.

It's the only Ancient Roman aqueduct still in use today.

Oh, Michael, it's freezing.

HE SPLUTTERS

-No, it's lovely.

-Is it nice?

-What about that?!

-Is it cold?

-Clean, fresh and pure and virginal.

Amazing.

So, Adriano, this is one of how many aqueducts coming in?

-There were 11 imperial aqueducts...

-Yeah.

..built during 500 years,

from 315 BC to 226 AD.

-And they were built with the town growing.

-Yeah.

So, obviously, they needed more water for more people.

-And the more water was helping the town to grow at the same time.

-OK.

-So, one fed the other and it was a virtuous circle, then?

-Exactly.

How many of those aqueducts were underground?

-All of them run, for a certain part, underground.

-Right.

And this particular one, the Vergine, is 99% underground.

This is 99% underground.

It was coming out of the ground

only basically when it was arriving in town,

-just a few hundred metres after here.

-Yeah.

'And all the water is held in place by an old friend.

'The pozzolana in the waterproof Roman cement.'

This is just such a fluke chance that you have in your volcanic soil

the presence of these oxides that gives it this fantastic quality.

I mean, that's just a gift from the gods.

It's a jaw-dropping engineering achievement

hidden away underneath the city.

And all powered by gravity.

Along its 20km length, the aqueduct drops just four metres.

So, I mean, think about these aqueducts like arteries,

that are bringing, if you like, the lifeblood into the city of Rome.

They're bringing not just the drinking water

but the water that was used for their baths

and for all the public fountains,

and everything that made Rome tick

is coming along these passageways, these pathways.

The Virgo Aqueduct still feeds the spectacular Trevi Fountain.

Its route to its source...

..takes it north past our spiral staircase...

..then east for another 19km into the hills.

It's all part of the underground network of 11 aqueducts

that quenched the thirst of one million toiling Romans.

But it also supplied water for another essential of Roman life.

The baths.

And they, too, had a secret underground life.

Michael is taking me to the remains

of one of the 900 available in the city.

The Caracalla baths,

named after one of the more obscure Roman emperors,

is a world away from my modest tub.

I mean, stating the bleeding obvious. They are massive.

But look at it. I mean, look at this.

Do you want to know where we are right now in the baths complex?

Um, no... Yes, I do. I do!

-No, I don't, I have absolutely no interest(!)

-OK.

We have just emerged into the hot room.

-The caldarium.

-Imagine it...

-Which was a round... I see.

All of this was part of the original fabric.

We're standing in the hottest part of it.

Right under here is where they're going to be having to create all that heat

to feed the hot room, the caldarium.

This place also contained warm and cold rooms, a swimming pool,

two massive gymnasia and libraries,

and even its own bakery.

It also had some of the classical world's finest works of art.

-So, who could come here?

-Well, I mean, the numbers are huge.

Something like 10,000 people.

This was a baths that could serve

pretty much most stratas of Roman society.

So, if you were an Ancient Roman walking along

through this network of marble-adorned rooms,

you must just have thought you were it.

While Michael heads down to explore the underground secrets

that supported the luxury of the baths above,

I'm bunking off to indulge myself in a bit of that pampering.

'Dr Mark Bradley is an expert in Roman hygiene.

'We're going to enjoy a luxury Roman spa together.'

Every Roman would attend the bathhouse on a daily basis.

Very democratic.

You come in here naked, you know, so without all your trappings.

This is something that everybody could participate in.

Take off your fine toga, you take off your purple stripes,

and you put them aside and everybody is the same.

But I suppose that maybe things did occasionally stray.

Any evidence of that?

In fact, a lot of the writers who write about baths

focus not on the democratic nature of the baths

or on the technical ingenuity of baths,

but on all the really bad things that went on in baths.

-Go on, Mark, tell us.

-We're talking political conspiracies,

we're talking...orgies.

Orgies. Now we're getting there. I see.

Well, I mean, it would happen, you know,

-in the sultry atmosphere of a...

-Yes.

-..of a caldarium.

-Maybe not the caldarium. Maybe the tepidarium.

-Maybe the tepidarium.

Yeah, yeah.

The baths experience concluded with a massage.

It involved lots of pummelling and scraping with a strigil.

Before soap was invented,

it was an effective way of removing sweat and dirt.

For the Romans, daily bathing helped set them apart from the barbarians.

-It looks a little bit like a scene from a torture chamber.

-Just a bit.

But all these things are designed to sculpt and polish your body.

Extraordinary level.

I mean, right down to cell level, really, of cleanliness.

The Romans must have found it almost unbearable

to come into contact with any other race.

-I mean, it must have been a real challenge to their senses.

-Yeah.

But this is exactly how, as a Roman, you differentiate yourself

from people who don't engage in this cleansing process.

Um... Walnuts. What are they for?

These are not there to nibble on if you get peckish.

So, this would be heated to a very high temperature.

And then they would be placed on your forearms or your legs

where you wanted to singe the hairs away

to make your skin smooth and pliant.

'But all this luxurious pampering came at a price.'

Wow!

Michael's exploring the powerhouse of the Caracalla baths

with our underground expert, Adriano.

And this is as massive and as big as what is above ground.

MICHAEL LAUGHS

It's incredible.

It feels like we're in a kind of roundabout.

It's exactly what it is.

It was the roundabout at the main entrance

to control everything that was delivered here.

-So, what kind of things would have been coming here?

-Mainly wood.

-Ten tonnes of wood per day.

-Wow.

All that wood fuelled a battery of furnaces,

serviced by an underground army.

These were the stairs to one of the places where the fire was made.

We are just below the pools of the caldarium.

There were 49 ovens all around underground.

And on these steps, thousands of slaves walked up and down

bringing wood and a lot of heat.

There were thousands of slaves working here every day.

It was really something very close to hell.

Around a third of Rome's one million residents were slaves.

In a preindustrial era, it was an efficient,

if utterly brutal way of organising the economy.

Caracalla was a microcosm of Rome itself.

It was an underground city that made what was above ground work.

Because without this running, we would have not had the baths on top.

'Even though built on the shoulders of slaves,

'the baths were still one of the pillars of Roman civilisation.'

It represents a sort of bookmark in our evolution, really, doesn't it?

I mean, this is certainly where cleanliness, and thereby health...

-Yeah.

-..becomes a feature of daily lives.

-That's definitely right.

So, just like aqueducts... and sewers,

every Roman colony, every part of the empire,

was expected to have a bathhouse.

This is something... This is Roman civilisation.

-And look at it. It's lovely.

-It is. It is indeed.

All this luxury

powered by an invisible subterranean world of slaves and furnaces,

connected by 6km of corridors on three separate levels.

One for the furnaces, another for the water

and a third for the sewage.

In a metropolis with up to 900 public baths,

and a million people, all that waste had to go somewhere.

And nowhere was there more of a sewage problem

than the centre of Roman public life, the Forum.

Originally Rome's marketplace,

it became one of the most famous meeting places in history.

To clean up the Forum, the Romans built the Cloaca Maxima,

the big drain, right underneath it.

And, I'm afraid, that's where we're heading next.

I've never been in a sewer before. I hear it's great.

The rules are, don't touch your face with the outer layer of gloves.

Take that off, use the inner layer

if you need to touch your face or anything like that.

Yeah, I'm rather intrigued.

Little bit more scared of what lies ahead.

A lot of instructions about how to touch my face

or avoid touching my face,

what I have to remove if I want to touch my face.

I won't be touching my face.

'This is probably the oldest

'continuously working sewer in the world.

'That's 2,500 years of excrement.'

I can smell it's a sewer.

Down I go into the underworld.

OK.

Down we go. It's... Blimey, look at this.

Wow, I wasn't expecting this at all.

Good Lord! It's huge.

I mean, it's absolutely staggering, the majesty of this.

There's a famous story that it was big enough

for you to drive a cart down full of hay.

The monumentality is simply breathtaking.

'Part of the sewer even has a base made from travertine marble.

'I've seen worse kitchen floors.'

Here, look at these colossal great blocks of basalt,

beautiful high vaulting.

But just to complete the slightly Halloween aspect of it down here,

see how these curious sort of festoons of who knows what

have obligingly gathered just to add to the atmosphere.

And the way the stones are bleached with millennia of stuff

-that I really don't want to know...

-Let's leave it at "stuff".

-Yeah!

It's every bit as unpleasant as the aqueduct was pleasant.

We're helping the archaeologists working here

to map the oldest section of this enclosed sewer.

These vaulted ceilings date from around 600 BC.

London didn't get a proper sewage system until the Victorian era.

The Romans engineered this 2,500 years earlier.

So, Xander, come and have a look at this.

This is an absolute privilege to see this.

This layer, this is our cement, our pozzolana, making this waterproof.

-But can you see that emerging just behind it?

-What is it?

That was a wooden stake, part of the support system

for when they put up this waterproof cement wall.

-That piece of wood, that's 2,000 years old.

-Look at that.

Pozzolana still doing its job. Amazing to see down here.

As the city expanded, so did its invisible world of reeking tunnels.

Each of them, including the Cloaca Maxima,

drained into the River Tiber.

The map also reveals more invisible Roman genius.

The sewers were part of an integrated network

of arteries and veins.

The aqueducts connected to the baths

and both then connected to the sewers.

They'd use the overflows from the aqueducts

to flow into the drain and across the streets in Rome

to push all the crap that was on the streets into the drain system

to get it back out to the Tiber again.

And that wasn't all.

This sewer was designed to deal with much more than effluent.

The area of land here, this is marshy, this is below sea level.

It floods lots.

So, this thing was constructed

right from the very beginning in the sixth century BC

to be big enough to take the floodwaters from the Tiber

when the water levels rose, so that Rome didn't flood.

That's extraordinary.

That helped reduce the number of mosquitoes that were around

and, as a result, the amount of malaria that was around.

So, this space, you know, disgusting and terrible as it is,

really is one of the absolute pillars on which Rome was built.

But you've got over a million inhabitants, Rome at its peak.

-That's a lot of poo.

-Yeah. 50,000 kilograms of excrement a day.

-Good God.

-But the funny thing is,

the one thing that this drain might not have been dealing with

in as much quantity as you might expect is pee, urine.

They were harvesting that stuff

and using it as laundry detergent in the city's laundries.

Marvellous.

'You think that's marvellous,

'the Romans even cleaned their teeth with urine.

'Mouthwatering.'

Well, I feel I really am truly in the viscera of Rome.

There's even a waterfall down there. It's pure slurry coming down.

You just don't want to look too carefully

where you're putting your feet.

Going down into the sewer has been quite an eye-opener.

Something I certainly won't forget in a hurry.

A horrific experience being down there, but that whole idea

that they built it so big so early on in Rome's history,

because it was going to be a way of taking the surge waters of the Tiber

and thus keeping the land of the Forum and central Rome from flooding,

I mean, that kind of foresight really took me by surprise.

-And architecturally magnificent as well.

-Yeah.

I mean, a very extreme experience, as you say.

And, actually, once you've got the smell of it out of your nostrils...

-It took a while.

-Yeah.

-I wondered why people were avoiding us in the streets of Rome.

-Yeah.

-And still, you'll notice.

-You're not avoiding me, are you?

Well, I'm not. Contractually, I can't!

No, but I think as an experience, it was way off the scale.

It was like visiting the underworld, it really was.

Our descent into invisible Rome has given me a unique insight

into how the Romans built and organised their metropolis.

But the underworld can also reveal things

about the spiritual life of the Eternal City.

This place has been the crucible of Christianity for over 2,000 years.

The evidence is everywhere on the surface.

But in the grounds of the Barberini Palace,

underneath the Italian Army's Officers' Club,

there's evidence of a mysterious religious cult

that once rivalled Christianity.

All its temples were built underground,

a symbol of the cave at the centre of the cult's founding myth.

-So, yes, if we take a right...

-OK.

Ah!

Wow! That's not what I was expecting at all.

Welcome to the cult of Mithras.

Are you about to initiate me?

How extraordinary. We've come through a...

I thought this might be a sort of bag check or something.

'The cult of Mithras disappeared in the fifth century AD.'

-And this is the altar?

-Yeah, absolutely. We can get up close.

'Frescoes like these are now the best clues we have

'to what it was all about.'

-That is Mithras there.

-Absolutely. Looking like Superman.

I bet Superman wishes he had those little sparkly bits

on the lining of his cape.

They never thought about it in time.

-This is the key image, if you like, of this cult, this religion.

-I see.

'Mithras' dress suggests the cult had its origins in the Middle East.

'And what's perhaps most striking

'are its similarities to Christianity.'

I notice up in the top right-hand corner

there's someone with what appears to be a halo,

somebody who looks like they've escaped from a Christian image.

Yeah, there is a lot of overlap.

People talk about Mithraism and Christianity.

They're both developing in Rome at around the same time.

There's a kind of key date

in the Roman pagan religious world in Mithraism and in Christianity.

It all overlaps. And that's December 25th.

December 25th, for the Romans, was the birthday of the sun.

In Mithraism, the sun had a huge part to play.

And the early Christian writers are really quite keen to point out

-that Mithraism is a dubious, devilish copy of Christianity.

-I see.

Close enough for them to be worried.

Ultimately, Christianity won out.

But there was a time when Mithraism was hugely popular,

especially with the soldiers and the poor right across the Roman Empire.

Another shrine under Rome opera's workshops

is one of the 35 underground Mithras temples

that have been found in Rome alone.

A further 400 are have been uncovered throughout the empire.

One as far north as Edinburgh.

Before the triumph of Christianity,

Mithraism was one of hundreds of Roman cults.

People talk about the religions of Rome, and that's absolutely crucial,

because there are tons of gods and it's an open-ended thing, you know.

So that people could just pick and mix, really?

Every time the Romans conquered somebody new,

they sort of invited their gods in, join the Roman party, if you like.

Rome and Roman religion does a brilliant job

of just incorporating them all, as long as, at the end of the day,

the emperor got their ultimate loyalty.

And do you know what? It's rather like...

We've looked at the incredible Roman concrete, Roman cement.

This is a kind of social cement as well,

that there's rigidity where it's required,

and flexibility where it's required.

And here, it's allowing certain freedoms,

but knowing where the structure needs to be

to support the weight as well.

I think that's a really nice way of thinking about it.

Our journey beneath the ancient metropolis

has given me a real sense of what it meant to be a Roman.

From the bread and circuses of the Coliseum

to the ritualised bathing and the tolerance of different religions

in its underground temples.

But for me, invisible Rome would be incomplete

without exploring its most iconic underground space.

I've always wanted to visit the catacombs,

the place where millions of Romans were laid to rest.

-You want to head straight ahead.

-Straight on down.

Wow.

So, what we need to do is get along here.

-THEY CHUCKLE

-This is incredible.

Between Emperor Augustus and Emperor Constantine -

about three-and-a-half centuries -

-there would have been between about 10 and 14 million people needing burying.

-Right.

-That's a lot of people to bury.

-That is a lot.

And one of the reasons that you end up

with such an enormous number of catacombs is simply space.

And it's one of these things, it's a fantastic reuse of materials.

So, the quarry that we were in the other day,

lots of quarries around Rome,

-excavating, finding the pozzolana to make...

-Yeah.

But when they're done with the quarry, it's just an empty space.

Oh, so this is also a quarry? I mean, first and foremost a quarry.

Absolutely, started life as a quarry.

-So, this is our old friend tufo again.

-This is tufo, yeah.

This is the natural rock on which Rome is built.

Each of these, obviously, was a sarcophagus, really, wasn't it?

The body would just have been laid there.

We can't think of any big coffin like we would nowadays.

But these were sealed in.

Yeah, sometimes with a clay bit on the front with a name.

-And so who exactly was buried here? Sounds a silly question.

-No, no, no.

The story that's normally told, if you say "catacomb",

you hear "Christian", don't you, Christian catacombs?

And there were lots and lots of Christians buried in the catacombs,

but that's not the full story.

I want to take you somewhere to show you positive proof of that.

Here...

-See something pretty special.

-This is incredible.

-Wow. Look at this. This is plaster.

-Yeah.

-And then decorated.

-And look at that, the menorah.

-Yeah.

-So, this is a Jewish catacomb.

The Jews were using catacombs for burial

from about the first century AD.

Yeah, I had no idea that there were Jewish catacombs.

You think of catacombs very much as Christian.

But this isn't just a single catacomb.

It's a whole complex stretching for nearly a kilometre.

The final challenge for the scanning team.

The scan reveals how the passages relate to each other.

It also shows the wonderfully detailed images

in the more intricately decorated vaults.

Archaeologists can now study these frescoes in minute detail

without harming them.

-Oh, my goodness. Look at that.

-Yeah.

Whoa.

We've come to what feels like almost the end of the catacombs.

No, it goes on and on after this as well.

We've walked for miles and we've come upon this.

This is a really expensive place to be buried, this one,

-and beautifully, beautifully decorated.

-Staggering.

Look, there's a pigeon over there, there's peacocks over here.

Sheep there.

And then this is probably an athlete being crowned

with a garland over his head by the lady to his right.

But incredibly well preserved.

And all these kinds of images

we would normally associate much more with Roman pagans,

-you know, the Romans.

-Oh, I see. This is a pagan...

So, we've seen much more kind of Christian locali,

we've been into Jewish cubiculums.

And now we're into a much more pagan space.

This catacomb is one of 70 known to exist around Rome.

Their tunnels stretch round the city for miles.

The final resting place of so many ordinary Romans

who built and ran this remarkable metropolis.

A fitting underground space to finish our adventure.

It's been an incredible privilege, hasn't it,

to see not just the muscles of Rome, but its arteries, its lungs

and its intestines and colon as well!

And it gives you a real sense

of just how this city of a million people was able to function.

I think, also, we've seen

the extraordinary and complete fluke confluence of circumstances

that allowed Rome just to make its stance

and say, right, here we are, we're going to take over the world.

It's that perfect storm, isn't it,

the Roman genius that turns that pozzolana into cement and concrete

that builds the Pantheon,

that pushes the boundaries of what's possible with architecture

and create spaces and places on a scale never dreamed before.

All of this came out of invisible Rome.

The honeycombs of quarries that made Rome's building revolution.

The aqueducts and sewers that supplied and cleansed it.

The spaces that nurtured it spiritually.

And, finally, the places that received its dead.

Our journey through invisible Rome has opened my eyes

to so many new secrets in this, my favourite city.

I won't be going back down that sewer in a hurry, though.