In present-day China, the ongoing plan is to move 250 million people from farms into its developing megacities.
That might boggle your mind, but the most heavily populated country in the world tends to face mind-boggling issues.
Between the 1950s and 1994, tens of thousands of immigrants constructed a towering community 12 stories high across a 6.4-acre lot in Hong Kong.
It was called Kowloon Walled City.
With a population of 33,000 squeezed into the tiny lot, at its peak the city was 119 times denser than present-day New York City.
Scroll down to see the city in all its claustrophobic glory.
By 1986, the Walled City had caught the attention of photographer Greg Girard. Girard would spend the next four years in and out of the city, capturing daily life inside its teetering walls.
The Lego-like city was built over decades, as residents simply stacked rooms one on top of another. The end result 'looked formidable,' Girard says, 'but who knows?'
Kowloon offered just about every business imaginable, for better or worse. At night, schools and salons were converted into strip clubs and gambling halls. Trafficked drugs -- mostly opium -- made frequent appearances.
Like many of the Walled City's dentists, Wong could not practice anywhere else in Hong Kong. This drew hordes of working-class citizens to visit the city just to receive affordable healthcare and services.
Stacked housing blocks meant virtually no sunlight could pierce through. Even during the day, Girard says, 'it was nighttime all the time in there.'
The one place of respite from the dampness was on the roof, though Girard says this was the most unsafe of all. 'There were a lot of additional things sticking out and spaces between buildings that had been combined.'
In-house manufacturing was a huge part of the Walled City's infrastructure. Dog meat butchers, entrepreneurs, and noodle makers enjoyed zero oversight inside the walls.
Hui Tuy Choy opened his noodle factory in 1965, totally free to ignore health, fire, and labour codes.
Sanitation was of minimal importance, Girard says. 'It was an intensely difficult place to function, with no laws governing health or safety.'
Law enforcement typically only intervened for serious crimes, Girard says, though rumours were always swirling that Hong Kong's government preferred to turn a blind eye.
One law that was consistently enforced: the Walled City could be no higher than 13 or 14 stories high. Otherwise, low-flying aircraft would have trouble meeting the nearby runway.
Despite its seedy reputation, the Walled City offered a sense of togetherness to thousands of people who had no community, Girard says.
'Its physical reality kind of belied this community that it was,' he says, adding that people's attitudes toward him changed around 1990 when they learned the structure was to be knocked down.
People began living increasingly quiet, traditional lives. Though he entered as a threatening outsider, Girard eventually left having formed genuine relationships.
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