Kowloon Walled City was the densest place in the world, ever.
By its peak in the 1990s, the 6.5 acre Kowloon Walled City was home to at least 33,000 people (with estimates of up to 50,000). That’s a population density of at least 3.2 million per square mile. For New York City to get that dense, every man, woman, and child living in Texas would have to move to Manhattan.
To put it another way, think about living in a 1,200 square foot home. Then imagine yourself living with 9 other people. Then imagine that your building is only one unit of a twelve-story building, and every other unit is as full as yours. Then imagine hundreds those buildings crammed together in a space the size of four football fields.
http://99percentinvisible.prx.org/2012/11/19/99-invisible-66-kowloon-walled-city/