Kowloon is the most crowded place on Earth (source: Business Insider)

Kowloon Walled City was a densely populated, ungoverned settlement in Kowloon, an area north of Hong Kong Island. What began as a Chinese military fort evolved into a squatters' village comprising a mass of 300 interconnected high-rise buildings. The city began as a low-rise squatter village during the early 20th century. After World War II, Hong Kong experienced a massive influx of Chinese immigrants. This led to a lack of housing in the city. In response, entrepreneurs and those with "squatter's rights" in Kowloon built high rise buildings on the space to capitalize on the housing demand.