theastralcity:

The Kowloon Walled City in Hong Kong: Being slightly smaller than an NFL stadium, the structure was built of 350 smaller interconnected buildings and hosted, at it’s peak, a population density of 5 million people per square mile.

To put those numbers in perspective, this would be like taking the entire population of metro Philadelphia, the 4th largest in the US, and putting it in 1 square mile instead of 1,744.

The area was also largely ungoverned and unregulated.  Factories, apartments, schools, temples, churches, shops, cafes, hotels and almost anything else one could imagine were housed within the structure that never had a full blueprint of it done. Buildings were built onto buildings, expanded, rebuilt, and re-purposed as needed without a central authority of any kind.

Within the structure, natural light was almost non-existent, and an unknown number of miles of jury-rigged wires provided electricity to everything.  Water constantly dripped down to the lower levels from both rain and leaking pipes, while garbage filled every passage.  A constant yellow haze filled the structure and there were never any government safety inspections.

The Kowloon Walled City was demolished in the early 1990s as part of the deal that returned Hong Kong to the Chinese from the British. The entire area is now a park.

For a documentary shot inside of the Kowloon Walled City, check here:

http://www.youtube.com/watch?v=Lby9P3ms11w