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Ok, ready?

This is 'What If,'

and here's what would happen if everyone on Earth lived in one city.

To date, the greatest population density ever recorded

was in Hong Kong's Kowloon Walled City.

Before forced evictions took place in the early 1990s,

there were 33,000 people sharing this space,

which is about the same area as three football fields.

And that's equivalent to 1.2 million people per square kilometer.

You can understand why this walled city had to go.

But even if that's an extreme example,

our world is home to some pretty dense cities.

Manila is a city with roughly 43,000 people per square kilometer.

While the population of Manhattan is about 26,000 per square kilometer.

And while they're not ideal circumstances for claustrophobics,

most people can make it work.

It could...

but comfort would vary depending on the density.

Let's start with a fun city that has a pretty low population density.

New Orleans is roughly 784 people per square kilometer.

If you were to scale New Orleans to accommodate everyone in the world,

you'd get a city the size of the continental United States.

But if you were to do the same thing to a high population density city like Manila,

which, as we said, has a population density of 43,000,

then everyone in the world would have to fit into a city the size of the country of Uruguay.

It's hard to speculate on what life would be like if everyone in the world lived in a city this size.

In some ways, it might be more sustainable.

We'd save money on huge infrastructure projects like long highways and bridges.

Travel and transport costs would go down,

since all the materials we'd need would be going to the same place;

and that might make basic human necessities

like food and medicine more accessible.

Not to mention, having everything from work to home

to the supermarket and the shopping mall within walking distance,

or accessible by public transit, would reduce our environmental impact.

or accessible by public transit, would reduce our environmental impact. In fact, people in cities use half as much energy

In fact, people in cities use half as much energy

as people who don't – because they live in smaller spaces.

But when we think about urban living today, it's impossible to ignore the cost of rent.

With space at such a premium,

And what kind of quality of life could low-earners expect?

In Hong Kong, for example, an incredibly dense city

with the most expensive housing market in the world,

a 40 square meter (430 sq. ft.) apartment costs about 700,000 USD.

In some places, that kind of money could buy you a castle.

But with the world crammed into one city, your living quarters might look more like this.

The kind of geo-city we're envisioning raises a lot of social questions.

People tend to move to cities for better opportunities,

but, would there be enough to go around?

Would the high concentration of wealth inequality

that's often overlooked in today's world

breed more crime? Or even revolution?

Would there still be wars?

Or would proximity to one another help us to understand each other a little better,

and realize that we're not so different after all?

A truly global city is hard to imagine

because distance and diverse perspectives

have bred different approaches to urban planning.

Is there a one-size fits all approach to cities?

How can we make urban life sustainable and accommodating to all?

Luckily, the world will never live in just one city,

but these are questions we need to start taking seriously.

Because urban density is on the rise, and like it or not,

we're just gonna have to get more comfortable with each other.

Do you know someone who's claustrophobic?

Show them this video, and stay tuned for more 'What If.'