The Kowloon Walled City (historic reports)
These are historic reports about the Walled City of Kowloon, which was once one of the most densely populated areas in the world, as people literally used up every square inch that was given for hosting more and more people in a small chinese enclave that has originally been a chinese military fort. Things got really messy there over the time, as there was no government and no regulation whatsoever except for the chinese mafia. All of this does no longer exist, as it was eventually evacuated, wiped and replaced with a nice park in 1993/94.
tl;dr: Enjoy a trip into one of the most fucked up things you will ever experience. Watch the first clip for a short glimpse into this mess, and / or the second one for a deeper look into things (German with English CC).
Reference: https://en.wikipedia.org/wiki/Kowloon_Walled_City