In 2018 estimates by the United Nations, 81 cities around the world have been categorized as metropolitan areas with a minimum human population of five million. Tokyo stands out on top, with a population of almost 37.5 million. However, the studies were focused on a mix of city proper, metropolitan area and urban area.

Tokyo, the city with the highest population number, stuffs in as much as 16,121.8 people per square mile. That sounds like an awful lot, but long ago, there was a tiny settlement with a far higher density.

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Welcome to Kowloon Walled City.

Before it got shut down in 1994, Kowloon Walled City filled in only roughly 6.5 acres of land, yet it was "home" to 50,000 people. That would be five million per square mile!

Despite the nightmare, I've been imagining an alternate history in which the current population is still at 7.5 billion, and the greatest cities in our world--not just Tokyo, but others like Osaka, Beijing, New York, Chicago, Mexico City, Los Angeles and on and on and on--are identical in land area to our timeline but each one having a population density of five million people per square mile, making each one the macrocosm of the Kowloon Walled City nightmare.

But what point of departure would I need to turn this nightmare into a reality?

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    $\begingroup$ How the heck are these cities even still around in the present day? The Kowloon Walled City was so densely populated that it was highly vulnerable to a plague, starvation, or a tainted water supply. People would be fleeing these cities en masse for the countryside, and many of them would end up depopulated when a plague swept through a city that packed. Architects would figure out pretty quick that cities that are this densely packed don't work. $\endgroup$ Jun 23, 2021 at 0:42
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    $\begingroup$ The Kowloon City only existed because it was located in a legal no-man's land between mainland China and British Hong Kong. The primary reason people went there was because it was so lawless or else they had nowhere else to go, and an organized government couldn't enforce zoning standards or other laws. Note what happened to the Walled City: once China and Britain came to an agreement the Walled City was an embarrassment to both of them had to go, the city came down really fast. $\endgroup$ Jun 23, 2021 at 0:48
  • $\begingroup$ At that size they probably wouldn't be as dense a Kowloon as the size would allow for far more height so while population density would be they same or grater individually people would probably get more space. $\endgroup$
    – OT-64 SKOT
    Jun 23, 2021 at 0:50
  • $\begingroup$ Given how much cities are a product of their unique history, I suspect that the deviation needed for every city are going to be vastly different. As is this question is incredibly broad. Can you edit this question to focus on a specific city? $\endgroup$
    – sphennings
    Jun 23, 2021 at 0:56
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    $\begingroup$ @Trioxidane Point of departure. Perhaps the point of bifurcation would be more fitting here. $\endgroup$
    – Otkin
    Jun 23, 2021 at 17:37

1 Answer 1


1: Just create an area directly adjacent to an existing megacity where there is no law. Then the economics of drugs and no planning regs will do the rest.

This is how kowloon came to be. The area was technically under the jurisdiction of mainland China, but China had zero desire to police it and the UK could not police it as it was not under their jurisdiction. As a lawless mini state it became self-managing.

2: For environmental reasons make it illegal to build outside existing urban areas and then consider the possibility that the climate emergency will make Africa, central India, western China, and the southern US uninhabitable. The massive wave of immigration could result in a 2-4x population increase in the still habitable areas.

3: 2 is still not even close to enough, so we also need uncontrolled birth rates. The problem is always going to be that no current government would sit back and do nothing whilst witnessing a population explosion against the backdrop of climate emergency and no more space, so there needs to be a very compelling reason to still increase the birth rate. How about a virus which we think is going to kill 98% of the population in ten years but turns out to be harmless?

Then there is the matter of how - we need to make the malthusian catastrophe happen, so we need to remove a lot of the features of modern life which prevent it from happening.

  • No birth control
  • No TV
  • Stop teaching responsible parenting
  • No TV
  • Free houses for everyone
  • No TV
  • Polygamy encouraged
  • Early marriages encouraged
  • Having > 5 children gives you a lifetime income and you never have to work again.

Environmental pressure and massively increased birthrate is likely to be the key driver, but you need to get that birth rate up up up! In order for just London to have the correct density you will need 3,030,000,000. That's nearly half the world's current population in just London! Edit: Assuming other cities have the same ratio of population:area, that would make for a total global population of 2.4 trillion

Not to mention feeding them all, but you would at that point probably need to have people hooked up to cybernetic implants which convert solar energy directly into glucose and trace minerals - there is absolutely no way in hell that the planet can sustain that many people using any currently known or theoretically plausible agriculture method.

In fact, transhumanist tech might be the initiator of the second population boom - it bypasses the aforementioned malthusian disaster and allows once again for unchecked population growth. So I'll make that my 4: Transhumanist revolution.


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