Something new is happening on Planet Earth, big enough to be seen from space.
Hot spots buzzing with the energy of millions of people.
For the first time in human history,
more of us live in cities than in the country.
But these are cities on a different scale.
In just 50 years, we've seen the birth, the growth
and now the dominance of the megacity.
Sprawling, seething, noisy,
polluted, crammed with 10 million,
15, sometimes even 30 million people.
These cities are complicated, fragile places,
constantly on the edge.
These are places overcrowded in squalor.
But these are also the most exciting places on the Earth...
..brimming with optimism and fun and energy.
Wahey! Arrgh!
Love them or loathe them, fear them or embrace them,
the megacities are the human future of the planet.
'They are also Man's biggest and most dangerous social experiment yet.
'How do you begin to protect and control a city
'of 20 million people?
'I've been off to look and learn, going to an evasive driving school on the edge of Mexico City
'where kidnapping is a huge social problem, not just for the rich, but for the middle classes too.
'I've signed up as a rather wrinkled volunteer in London's Police Riot Control Academy.'
There's nothing quite like being hailed with bricks and petrol bombs to make you see things differently.
'And I've been disaster training in Tokyo, the world's most advanced metropolis.'
This is, um...not funny.
So far, our planet has 21 megacities,
home to at least ten million people each.
Every one of them struggles with unprecedented problems of crime and social control.
Not new, but new in scale.
And these great cities are peculiarly vulnerable to natural threats as well.
Why is this? It's because from Ancient Rome's Pompeii under its volcano
to San Francisco on its fault line,
many of mankind's greatest cities have grown where they did not because of human stupidity,
but because fault lines produce rich, natural soil
and vulnerable bays are very handy for trade.
Take Tokyo -
two major rivers running through it and built on fabulously mineral-rich land,
which is why it is where it is,
but this natural richness is no accident.
It comes from underground.
Tokyo's 33 million residents are living right on top of three
of the world's most unstable geological fault lines.
Or Dhaka,
a haphazard, unregulated city, built on a fertile flood plain.
Great farming, great fishing, which is why it's there.
But in consequence, its 13 million people now face catastrophic flooding,
rising sea levels and disease.
London, not only one of the oldest megacities, but the only one in Europe
and then only if we take in all of its suburbs, is well protected from the sea with a hi-tech barrier.
But as a global city, one of the world's most mixed metropolises,
its 13 million people are particularly vulnerable to terrorist attack.
My journey, however, begins in Mexico City,
home to more than 20 million people
and a city built on an ancient, now hidden lake...
..surrounded by mountains, prone to severe earthquakes and flooding,
but most urgently drowning under a tidal wave of crime.
Almost everywhere has got some kind of problem with crime.
But in Mexico City, it's at a different level.
Large, extremely violent drug gangs.
Trafficking, prostitution and kidnapping.
According to one local crime survey,
there are something like 500 kidnaps in Mexico every month.
In fact, Mexico is the kidnap capital of the world.
And it's not just the rich who are targets.
Kidnappers will go for anyone they think has got a bank account
or for their children or grandmother.
In this city, they snatch at cash points, in the middle of traffic even, in taxis.
Why is this? Partly because of the example of the drug cartels.
Partly because the millions in poverty live jammed up against the better off,
their noses rubbed in middle-class success.
Something which is obvious when you see the city from above.
From your perspective, literally up here,
things like kidnapping, is that because there's so many rich and so many poor very close together?
It's definitely due to the contrast. Yeah. The contrast in social classes.
You can see from one hill to the other huge economic differences.
In an area where you'll find on one hill one house,
on the other one you'll find 40 or 50 families living.
'It isn't just the super wealthy who are living in DIY fortresses.
'Even in the lower middle class suburbs, residents have built their own gated communities
'and paid for their own armed guards to protect themselves against thieves and kidnappers.
'Throw in a police force with an ineffective and often corrupt reputation
'and it isn't hard to see why Mexico City's crime rate is out of control.
'Around here it seems it's often pointless to call the police,
'but there are people who are willing to help for a price.
'Tom Cseh is a former United States Air Force Special Agent.
'But he's become Mexico City's self-styled kidnap guru.'
Here in the Mexico City area, 80% of kidnaps happen in the morning
when the victim is on their way from their residence to work or to school.
Why is that? Because most of us follow some sort of routine in the morning.
The bad guys do not go after the very well-to-do
who mostly have their own security, either armoured cars or bodyguards or whatever.
'Tom's most over-subscribed service is his evasive driving course.
'He regularly teaches Mexico City's middle-class mums how to beat kidnappers at their own game.
'Can he, however, do the same for an ageing British hack
'who has no ambitions to be Jeremy Clarkson? Lesson number one...'
You're driving down the road and maybe some bad individuals pull up alongside of you.
They will tell you "pull over" in Spanish. You hit the brake.
They're going to drive on at least a car length.
You're going to come to a complete stop and make a U-turn. It's a very safe manoeuvre.
'A very safe manoeuvre? Well, up to a point.
'Now, I am an impatient driver, but this is not quite my normal style outside the local supermarket.
'The 180-degree, gangster-avoiding handbrake turn!'
God!
SHOUTING
'Now, that didn't seem too bad, but I'm not finished yet. Now Tom wants me to go on to the attack.'
OK, the next manoeuvre is what we call "the surgical" or "the PIT and turn".
This is very commonly used by US police departments in the United States to stop a fleeing felon.
This is also useful in Mexico because if the bad guys pull up alongside of you,
you know exactly where you need to hit that car to knock them off the road.
I'll learn how to knock people off the road?
They're going to pull up, threaten us. OK. And then you're going to hit them.
It's that area right between the rear tyre and the rear fender.
TYRES SCREECH
CLATTER Sorry! You're all right. Yeah.
Sorry.
Now, wait a minute. Don't film that! LAUGHTER
'And Tom's got one more lesson for me.'
OK, so this is your typical blocked highway here.
The bad guys are blocking you. You're coming down a one-way street. You cannot reverse out.
The only option is to go forward and if you want...
This kind of thing happens in Mexico City? It happens in Mexico City, yes. OK.
So the bad guys would never expect that you are going to ram their car.
'Yet driving into another car proves surprisingly difficult.
'Even in London, it's not exactly instinctive.'
SHOUTING
Oh... 'Missed!'
Sorry.
'Take two.'
Esta es la calle de Morelos!
Good. Right on.
That was worryingly enjoyable.
Thanks, bad guys! LAUGHTER
Most of Mexico City's kidnappers are, of course, simply desperate opportunists.
The alienating, unfair and cramped atmosphere of the megacity
encourages some very violent individuals.
But what happens when parts of the megacity kick off en masse?
Cities are places where millions of people are crammed together
and sometimes they are places where millions of very angry people are crammed together
and it doesn't take much to light the touchpaper.
CHANTING
When a riot does kick off, streets turn from shopping arcades and open-air cafes
into narrow, high-stakes battlefields and it happens the world over.
Go! ALL: Ten, nine, eight, seven, six,
five, four, three, two, one!
'So every self-respecting megacity needs to have a defence force on stand-by
'and I'm reporting for basic training with London's.
'These men aren't crack troops. They're ordinary bobbies from the London Metropolitan Police Force.'
Five, four, three, two, one!
'Yet all the kit that's needed to combat a riot in a metropolis
'quickly turns these perfectly pleasant bobbies and me
'into what could be seen as faceless storm troopers.'
I've always wanted to look well built. This is the cheating way.
And you'll be feeling quite warm now. I'm already feeling hot, yeah.
God, it's a formidable outfit.
Three, two, one, go!
'The suits are fire-proof, sweat-proof and knife-proof.
'The stab vest alone weighs about eight kilos, so they're not the easiest things to get around in
'and that's without the added weight of a nine-kilogram polycarbonate shield.'
Come on, keep going, keep going!
I thought I was quite fit.
But that was hard.
It's the weight of all this stuff.
'Next we're taught how to treat an ordinary street as a war zone.'
902, forward! Forward!
'I feel a bit like I've been thrown back in time.'
What really strikes me about this is we have a very, very sophisticated, modern city here,
and all these megacities, these huge metropolises are sophisticated and modern,
and in the end, we are behaving just in the same way that Julius Caesar's troops behaved.
Short shields back, short shields back!
It's exactly the same tactics you'll read from 2,000 years ago.
Go! With long shields linking and short shields moving forward and back.
Short shields, let's head for the rear. Head for the rear.
'In the modern megacity riot, though, it's not arrows and spears you need protection against.
'As darkness falls, the training is stepped up
'and more realistic riot conditions take over.
'We're up against an ugly, angry mob.
'They're actually off-duty coppers playing at being fascist or anarchist thugs.
'They're taking the game seriously enough.
'In a riot, the familiar, everyday fabric of the city takes on a different look.
'Paving slabs and bottles and bricks and waste bins become weapons.'
REPORTER: Dozens of police and demonstrators were hurt by missiles
including bottles, stones and scaffolding poles.
Cars were overturned and set alight.
'Riots are caused by all sorts of different factors -
'oppressive politics or racial tension, religious extremism.
'But some scientists have discovered what they think is another rather simpler trigger -
'heat.
'An increase in temperature causes a higher serotonin release in the brain
'which in turn leads to increased aggression.
'So here's a rather bizarre claim.
'27 to 32 degrees Centigrade is apparently optimum rioting weather.
'Back at my riot and apparently, we're winning.
'After taking a battering and absorbing everything they can unleash on us,
'we can advance and bring the crowd under control.'
There's nothing quite like being hailed with bricks and petrol bombs to make you see things differently.
My admiration for these guys is pretty high just at the moment, but you'd expect that.
It's just the petrol talking.
Escucha! Jesucristo...
'Back in Mexico City, it isn't just the police force that needs to be kitted out.
'This is an exceptionally dangerous city where it pays to wear the right thing at the wrong end of a gun.
'Miguel Caballero, named after its owner, is a stylish boutique
'selling a rather special line in men's clothing for the Mexico City gent about town.'
This is like an ordinary fashion collection.
That is the idea. There you go.
Now we have a shirt, now we have tops.
That is all the sweaters. Nice jerseys. That is totally new. It's from last week.
The only thing different about them is that they'll stop you being gunned down? Yeah. That's amazing.
We have to guarantee discretion, fashion, comfort.
These are the guns you protect against? Yeah. Wow!
That is high protection. These are serious machine guns.
'Made of reinforced Kevlar, the exact composition of Miguel's bullet-proof clothing is a secret.'
What kind of people are coming in for this? What kind of people are asking for it?
Politicians. Politicians, yeah, obviously. CEOs.
CEOs of the companies, president of the companies.
Gobernadors. Yeah. Basically, rich people who think they're targets? Yeah.
Can I try on this? Totally.
Yeah, that's a bit more, um...
for an evening out. You're taking someone out and you think you might be machine-gunned, so you wear...
If you are on any weekend and you want to maintain the casual wear, that is the perfect idea.
Yeah, it's big protection.
Let's have a little look in the mirror.
The idea is all the time to maintain your discretion. You wouldn't know I was bullet-proof protected.
So how much would this cost me?
It's around 900 US dollars to 4,900 US dollars.
'Strange fact - Mexico City has just got one legal gun shop.
'100 miles away, though, over the border with the United States, there are 7,000 of them.
'You don't need to be an economic genius to work out what's going on. Poor old Mexico.
'Miguel is showing me just how much protection his casual wear offers.'
We're now here at the police shooting range.
And I've got the jacket.
However, I've been told that BBC Health & Safety won't allow ME to be actually shot,
so I'd like to say sorry to those people watching who were desperate to see Andrew Marr take one.
'However, Oscar, one of Miguel's employees, has kindly agreed to take one on my behalf.'
And then we're going to go through into the shooting range and you'll see what happens next.
We'll all see what happens next.
Let's go through. Please.
So here's the gun.
Now, one bullet. One bullet only.
We always give the opportunity to the victim to choose the bullet.
You want to choose the bullet? OK. This is gruesome.
There's your bullet. All right, mate.
Uno!
GUNSHOT
Wow!
Are you all right? Yeah. You feel OK? Yeah.
Nothing there. God!
And there it is.
That's amazing.
It's hot. It's hot.
Oh, it's hot, yeah. Still hot.
And you're not in pain? It's not sore? No, nothing.
Wow! I think I need one of these. Next time I'm in Downing Street, I'll take one of these with me.
If you want, I can shoot you? That, not this.
But who do you turn to if you are one of the millions of metropolis denizens
who doesn't have a thousand dollars for a special kind of jacket?
Tepito is one of the toughest, most down-at-heel areas of Mexico City.
Here, criminals and victims and their friends and families
have turned to an altogether different type of protection.
She is big, she's popular and she goes by the name of Santa Muerte,
or Saint Death.
What people say about Santa Muerte is that she is there for the people at the bottom -
the hard guys, people who have done terrible things and their victims.
There's an official Catholic shrine just 20 yards away.
It's been extended, it's a kind of rival "come and look at me" shrine
on behalf of the Catholic Church who hate this stuff.
It's bigger, it's more impressive.
And there's nobody there.
Dona Queta, known locally as the Queen of Tepito, has made caring for the shrine her life's work.
TRANSLATOR: This is very important for a mother like her,
for a mother like a lot of mothers that have maybe a kid in the jail or a kid that is doing drugs.
And they are concerned.
They want to put their faith in something, somebody that is going to help them.
It's also for people that are going to lose their houses and they need money for the rent.
They don't have money and they don't see how they are going to get it. They come and make their praise.
It's hope for everybody.
Why are people leaving cigarettes? And what else are they leaving? There's sweets and fruit and...
DONA QUETA SPEAKS IN SPANISH
TRANSLATOR: It's an offering. You put food, you put cigarettes.
They bring this stuff every day.
At lunchtime, people will bring more food, and dinnertime, the same.
Basically, what you like the best is what you would offer. I see.
I get it.
'This man, who doesn't want to appear on telly,
'has worshipped Santa Muerte for ten years and he swears by her.'
I've just been told that his brother had been kidnapped
and day after day, he'd come here and pray to Santa Muerte,
begging her to get his brother freed.
And not only was his brother freed,
his brother was freed at the moment the senior kidnapper was himself murdered.
And not only that, the kidnapper's body was brought past the family house on the way to be buried.
And that, he said, is what Santa Muerte can do.
Mexico City - where you wear a bullet-proof jacket to the corner shop,
where mothers and murderers worship at the shrine of Saint Death.
So what on earth do people do for fun here?
Well, it seems that that can be pretty violent as well.
On any given night of the week,
50,000 Mexicans flock to the city's ten huge arenas
to lose themselves in their favourite sport.
My guides tonight are die-hard fans Alejandro and Francisco.
We're here for the "lucha libre" or "free wrestling",
which is half recognisable sport and half nightmarish pantomime.
And it seems the crowd can't get enough of it.
MUSIC: "We Will Rock You"
For the residents of Mexico City,
watching 250-pound men pile-drive each other into the floor
seems to be perfect therapy after an average day in the megacity.
Do the crowd get almost as aggressive as the ring? Yeah, that's correct.
Most of the people use the sport to yell, to take out all of the stress of the...
Get all their emotions out. Yeah.
There's a lot of craziness in the city, so you come out here to scream, to shout, to throw beer.
Because of the craziness in the city, it's a release? Yeah.
CHEERING
'With good guys and bad guys to cheer and to boo...'
Boo!
'..I'm again reminded of life in one of the first great metropolises.'
We've always loved a bit of communal violence - human beings.
And around me here people are using the same gestures
that they used to use when they were watching the gladiators
or the Christians being eaten by lions in Ancient Rome.
'Well, the Royal Ballet it ain't.
'But the atmosphere is certainly infectious. It's raw, loud, unzipped,
'somewhere between pantomime and sumo.
'And over the course of the evening, I can see why it is so popular.'
No-one's being hurt here tonight, I think.
But these are really important events
because they allow people to let off steam.
They're like kettles.
And the more stress there is out in the city,
the louder the whistle.
JEERING
If the residents of Mexico City flock to arenas
to vent their pent-up frustrations,
on the other side of the world, in Tokyo,
one small band has decided to turn the megacity itself into a giant arena.
To be more accurate, an extraordinary, vast race track.
After a lot of persuasion, they've agreed to meet us at a secret rendezvous point
somewhere off Tokyo's main airport highway.
Well, every Saturday, me and my friends get together.
We run the highway, you know, who's fastest. Not really a race, but yeah, a race.
TYRES SCREECH
Meet the Hashiriya...
..Tokyo's street racers who regularly risk life and limb
to drive at ludicrous speeds through Tokyo's state-of-the-art megacity road network.
Here, you can do anything, touge, you know, up mountains
and do the winding stuff.
You can go towards the ocean and do drifting.
You can go to Shuto Expressway, you can do circuit roads and stuff like that.
During the week, these guys have ordinary jobs. They go to work. They toe the line.
They're model citizens.
But come Saturday night, the Hashiriya tear up Tokyo's rulebook.
All that matters is driving as fast as you possibly can.
You can go 200mph for 300km.
The fastest I've been is, like, 320, 323.
Some of the guys have run 340.
'It's a deadly game, obviously, but for these Tokyo men it's one way of escaping the daily pressures
'of life in the metropolis.'
You feel free. You get out all this stress and stuff.
All these people, all this crowd.
A few boy racers letting off steam and breaking the rules isn't going to threaten the mighty metropolis.
But we can never afford to underestimate the level of damage
a few single-minded individuals are capable of unleashing.
Over the past 10 years, the work of a handful of suicidal terrorists
in cities like Mumbai, Madrid, London
and, of course, New York have sent shock waves around the world
and the message for the future of the megacities is now the bleak one - be prepared.
We're told that London, like many other megacities, is still a prime terrorist target
and we're told it's not a question of if the attack happens, but when.
So if, God forbid, you are caught up in such an attack and the world goes dark
and the buildings around you collapse and you're left, trapped,
who do you turn to?
Well, you turn to the specially-trained men and women
who are waiting all round the clock, all round the year, for that phone call.
This is the Urban Search and Rescue Unit, one of more than 21 specialist teams dotted around the UK
which were set up in direct response to the 9/11 attacks and the threat of more.
These two key members of the team are Darcy and Lucy. With two years' training under the collar,
they're sent into the rubble of collapsed buildings to sniff out survivors.
It is dangerous work and the dogs have to wear protective boots
as they scramble over the inhospitable landscape.
If there is a dead body in the wreckage, they're trained to ignore it.
That grisly task falls to a different dog team.
'So what do you do if someone's trapped in an air pocket under 50 tonnes of concrete?
'Using a specialist drill,
'the team has to first bore into the concrete to make a hole big enough for them to insert
'a high-tech camera.'
You get a good colour picture.
We can see that no one is just the other side of this wall. We can now demolish this quickly.
Speed is of the essence. Absolutely.
'There's a good reason why they use reinforced concrete in buildings -
'it's tough, it's very tough. I think this might shake my fillings out.
'It can take hours, even days working around the clock to knock through.'
We need to break all that out. That's physically quite hard work.
'Even when the teams break into the buildings, there's often more concrete to smash through
'before the dogs can pinpoint the exact location of the bodies.'
I'm just going to do something, mate, to make it easier.
'Dirty, cold and grim as the work might be, it's comforting to know
'that this team at least are prepared for the unthinkable.'
As the cities get bigger and more complicated and the threats become greater,
so the response has to be cleverer, too.
A few years ago, places like this didn't exist.
And yet, although it is a sophisticated and thought-through response to the next attack,
the next collapse of a building, at one level it's also reassuringly basic.
In the end, it's down to smell and muscle and a certain amount of courage.
But the human damage that can be inflicted is only one side of it.
All around the world, we're constantly reminded about the devastation
that can be wreaked on the most modern-seeming city by good old-fashioned nature.
Earthquakes. Floods.
Typhoons. By choosing to build on some of the Earth's most geographically unstable locations,
the metropolises are at times more exposed to the extremes of nature than less populated parts.
When they chose to settle near coastlines, waterways and fertile river valleys -
important for a city's survival - our ancestors could never have known they were laying their foundations
on slow, but remorseless time bombs.
More than half the world's 21 megacities of 10 million people
are in positions that leave them vulnerable to earthquakes.
And most at risk of all of them is Tokyo,
which lies on a complex and menacing web of geographical fault lines.
Scientists have told us the chances of a major earthquake not hitting Tokyo at some point were zero.
And they've been proved chillingly right.
The epicentre of the recent Sendai earthquake was hundreds of miles from the city centre,
but it was a devastating Scale 9 and the resulting tsunami wreaked apocalyptic havoc
on the immediate areas, killing more than 18,000 people. Many people are still missing.
And Tokyo didn't escape as powerful aftershock tremors hit the city.
So in the face of this onslaught, just how prepared was the most advanced, most efficient metropolis?
SPEAKS IN JAPANESE
Regular earthquake practice drills required by every Tokyo school were suddenly a reality.
Thousands of schoolchildren were safely evacuated to open ground.
'So focused are the people of Tokyo on arming themselves against the forces of nature,
'they've set up so-called life learning centres around the city
'where residents can experience pretty much every extreme that nature's likely to throw at them,
'from the typhoons which dump down thousands of gallons of rain water in Japan every year
'to the full force of an earthquake.
'This robotic platform is designed to mimic all the different magnitude levels an earthquake can unleash.'
Hold. Hold onto this. ..Protect your head.
Turn the gas off, right. Please open the door.
'I filmed this some months before the Sendai earthquake
'and the aftershock of a magnitude of 6.4, which was felt in Tokyo.'
This is going to be number seven. See what happens.
And it really is shaking quite a lot!
And I'm underneath...underneath...
Ah! Bloody hell!
It's quite something! Oh!
This is more than a tremor.
This is quite scary.
Jesus Christ!
This is...not funny. At all.
And it's stopped.
I crawl out... My goodness me.
Cupboards have fallen down. Off with the gas.
Check that door. Is it going to open?
Thank goodness. Yes, it is.
If this was for real,
in a city of 30 million people, I'm on the ground floor, one person with some padded furniture.
And the shock has been... I'm still actually moving.
You know, inside my head. It's really quite something.
It's like being seasick or very drunk.
Just imagine what would happen to 30 million people, many of them many floors up. Absolutely terrifying.
Well, as we know, it did happen.
However, Tokyo has nearly 3,000 really high buildings - more than 30 storeys.
Every single one is built to survive a high-level earthquake. That is impressive engineering.
And it largely worked.
Despite the strength of the tremors that shook buildings to their core,
there was relatively little structural damage and just 7 deaths in the city itself.
The earthquake threat to this densely-built megacity is so great, the response is on a huge scale.
This is the city's Disaster Management Centre which is, of course, earthquake-proof.
Totally self-contained, it's got its own independent power supply
and communications to the outside world.
And this is the man at the heart of the operation, Mr Toshiyuki Shikata,
Tokyo's Security Counsellor.
This is where the people who run the metropolis come to implement their highly-planned response.
All the information that we need we can see.
On the centre screen sometimes I have a conversation with the Prime Minister.
Vast amounts of supplies are stockpiled. This time, luckily, they weren't needed.
Cans, pickles, 4.09 million.
And blankets, 890,000.
So the authorities in Tokyo, indeed the authorities in Japan, were pretty well prepared,
but the earthquake showed just how quickly a highly modern infrastructure can collapse
because it took 10 days before the authorities finally got supplies like these
flowing into the disaster zones. The stakes are very high as this death meter in Tokyo shows.
We estimate
7.3 as it hit... on this point.
In that case, it is...
almost 6,400 people will be killed.
As we discovered, an earthquake can be far more lethal than that,
but again Tokyo is, perhaps, the best prepared city in Japan and probably in the world.
At the heart of the megacity's survival masterplan
is a building which dwarfs the entire city.
Still under construction,
the Sky Tree is 634 metres high,
currently the second-tallest structure in the world.
If, or rather when, a disaster strikes again,
this broadcasting tower will be crucial to keeping communications running.
But how do you build a tower two-thirds of a kilometre high
which is strong enough to stand up to the most powerful earthquakes?
Engineers looked to Japan's past and they've rather ingeniously borrowed
from a tradition that goes back 1,000 years - pagoda temple architecture.
Just like the temples,
the Sky Tree is built out of two independent elements.
In the Sky Tree's case, there's a central concrete shaft
and a surrounding steel structure. And when an earthquake hits the building, both move separately,
in theory, cancelling each other out.
And in practice because when the massive aftershock struck Tokyo,
this anti-seismic design did prove itself.
There was no damage to the structure of the building at all and the 500 people working on it were uninjured.
Tokyo's planning and super technology helped them be prepared.
This time, it survived the worst of the immediate effects of the earthquake.
But the full power of nature means that the world's most advanced metropolis still faces
an unpredictable future.
If you live in one of the poorer megacities and disaster strikes, the odds aren't strongly in your favour.
On the other side of the planet, just like Tokyo, Mexico City's land has become its enemy.
Sitting in a massive geographical bowl, surrounded by mountains and volcanoes,
and built on a network of tributaries from an ancient lake,
it's vulnerable to both earthquakes and, more crucially, flooding.
More than seven million people occupy the shanty towns that sprawl across the hillsides.
Most are haphazardly constructed without proper foundations.
Factor in the endless building work that's cleared and concreted over vital tree and vegetation cover
and you have a very dangerous place to be living in.
It's the 30th October, 2009, in the middle of the afternoon.
It's the end of a month which has seen the highest rainfall here since records began.
And over the next three or four hours,
a downfall of biblical proportions takes place.
This is one part of the city, like so much of it, which is completely unplanned and absolutely crammed.
Migrant families simply arrive here, search around for a little scrap of land they can call their own
and start to build a house with their own hands.
Now as a result, when the flood comes, thousands and thousands of houses are swept away.
The Jimenez family built their house here, on the edge of the creek.
And, as you can see, it's gone.
'The Jimenez family have since built themselves another house, but it's a precarious existence
'and the threat of flooding is never far away.'
Could you ask them how high the water came?
SHE TRANSLATES: One metre and 20 centimetres. Where you see the mark.
How did they all get out?
They climbed that wall and then the next wall and after that there's a school
where they found shelter. All the water came in this direction and they needed to get out.
We are living through a time of more and more extreme weather conditions
and when that kind of stress hits, it hits megacities first
and it seems to hit the poorest first because they're the people in the most fragile parts.
That's exactly what's happened here.
There's an annual rainfall of more than 27 inches in Mexico City. Quite a lot.
'And yet, rather oddly, this city has never got to grips with dealing with so much water.
'And the problem lies right here in the network of underground drains.'
Amazing.
Fantastic, no? Yeah.
Underground river.
So when the rains come, how high up does the water go?
It fills the full pipe. It fills the pipe.
What's the water coming down? Just the drain? The overflow of the lake.
Oh, I see. OK.
'The system goes right back to the 18th century, way before Mexico City's population exploded.
'It was designed to capture fresh rain water from the floods,
'but now it can't cope.'
This is mostly sewage coming down from the city where about a million people live.
A million people's sewage? Yes.
'Water expert Valente Souza is trying to solve the contamination problem.'
You see, Andrew, this line is the last of this... of this rainy season's level.
It comes all the way... Above us. Exactly.
At this very point. And we walk 150 feet down
into another collector that drains the rainfall and now the sewage.
We don't have our masks now, but it smells... The methane is strong. Really bad.
And this is water... Rainfall. That could be used by the city of Mexico. Absolutely.
If they did things more sensibly.
'It really is a case of water, water everywhere, but not a lot to drink.
'Mexico City's hidden underground system is old and buckling
'and, frankly, failing to keep pace with its rapid growth.'
But let's go back to the megacity whose technological know-how
does appear more than up to these natural struggles.
'Built to protect against major typhoon flooding,
'this extraordinary underground drain complex lies 50 metres below the surface
'on the outskirts of Tokyo.
'And it's taken more than 18 years to build.
'Five giant concrete silos,
'65 metres high,
'are connected to pumps powerful enough to shift 200 tonnes of water every second.
'And the whole subterranean system spans almost four miles.
'It is a thing of wonder.'
I think if the pharaohs of Ancient Egypt had built cathedrals,
this is sort of what they would have come up with.
But it's an interesting commentary on the modern city.
This isn't about religion or the spiritual or the worship of great men.
This is...civil engineering.
Just what we have to do to keep the show on the road.
3,000 miles away, the megacity of Dhaka in Bangladesh
is in another world compared to Tokyo's mighty technological planning.
'A lot of Dhaka lies just a few metres above sea level.'
When the annual monsoons hit the city, it's swamped by more than six feet of rain.
If sea levels rise as much as some scientists say they will,
there's a real chance the entire city will have to be evacuated.
Where would these people go?
And when you consider that 85% of its 13 million population lives in slum conditions,
without proper drainage,
where a single toilet may be shared by as many as 400 people,
then Dhaka, quite obviously, faces another huge ongoing problem.
'Disease.'
Well, here they are.
This is, uh, a toilet.
It's very straightforward. You do your business here.
Actually, this is quite a posh toilet for a slum.
But the important thing is what happens next.
If you come here...
The human sewage, the crap, goes directly down there, underneath the toilet,
and joins the rest of it, and as the water rises
the sewage goes back into the water system.
And if you wonder what kills most urban people,
it isn't starvation, it's not even natural disaster.
It's crap in the water system.
Dhaka's constantly waterlogged state means it's impossible to keep dirty and clean water separate.
A single gram of human faeces can contain ten million viruses and a million bacteria
with far-reaching consequences for many of us.
Among other things,
the crammed and dirty slums of the megacities are the perfect breeding ground
for the next global pandemic.
Right at the start it will seem like nothing at all.
A cough. A child sneezing in a shack over there.
But it could be the beginning of a catastrophe that affects you,
me, everyone we know.
'There's no high-tech solution here. The answers lie with individuals like Runa Khan,
'a charity worker who is fighting an endless battle to keep Dhaka's slums free of disease.'
Epidemics are a big problem in the slums.
10 members of the family living in one little room.
There are no toilets, no drinking water. If you see the river,
this is the primary source of drinking water and bathing water.
You put your fingers in, you come out with six fingers. It's one of the most polluted rivers in the world.
Here, there is a prevalence of healthcare, there are hospitals,
but the sheer number and volume of people inhibits the population from accessing this healthcare.
'Today Runa's visiting a patient who's been ill for quite some time.'
'The infection spread in the slum is a lot more dramatic than in other parts of the country.
'It's very difficult to predict or to take care of.
'Luckily, this turns out to be a bug which can be treated and contained, but any one of these cases
'could turn out to be the start of an epidemic that reaches far beyond Dhaka, even around the world.
'You start seeing two cases, five cases, ten cases.
'And in a place where you have more than 2,000 people per square kilometre, you are scared
'because it can't be controlled.
'This is a very big challenge, not only for small organisations like us, but the government itself.'
The outbreak of the next global pandemic poses a very real, very urgent threat
to all the world's metropolises.
The sheer size and frequency of world air traffic
creates a perfect storm for the spread of global disease.
There are more than 35,000 air routes around the world,
and every year one and a half billion people are carried on 40 million flights.
Air travel is turning the planet into one giant metropolis.
It looks like a pretty ordinary office, but this is the United Kingdom's first line of defence
against the outbreak of deadly diseases.
The Health Protection Agency has teams of scientists on a state of constant alert.
They're part of a worldwide network. Professor Maria Zambon is Director of the Centre for Infections.
Diseases never sleep. It is like very fine-grained detective work
where not only are you piecing together different pieces of information,
you're also tracking things.
Bugs and germs can evolve very quickly and scientists are trying to prevent the next super disease
taking us all by surprise.
A nine-to-five day doesn't do it. You need to work round the clock
to develop the data to make sure that you have the answers.
The solutions don't end with boffins in laboratories.
Dealing with our next global epidemic takes huge resources.
Stockpiles of vaccines in huge secret warehouses are just one of the things that it takes
to keep our world of megacities from disaster.
'So terrorism, rioting,
'kidnap, earthquake,
'floods
'and plague - it's not an entirely cheerful outlook.
'It might even be called doom-laden.
'However...'
as we've seen, some excellent science, superb engineering and simple forethought
does mean we've developed defences.
Death and taxes remain inevitable.
Catastrophe doesn't.
Life in the megacities is dangerous and exciting,
but then it always has been.
'Next time I'm going to be finding out how transport has shaped the metropolis.'
Oh, my God! Oh, I'm sorry.
That was both...great fun and really, really hard work.
'I'll ask if there are different ways of feeding the hungry cities.'
This makes me feel...gaseous.
And horrible. Just look at this!
And I'll meet the smelly heroes dealing with all the stuff metropolises don't want.'
Unimaginable, what he's lowered himself into.
Subtitles by Subtext for Red Bee Media Ltd - 2011
Email subtitling@bbc.co.uk