Kowloon Walled City
“With the Triads weakened, the Walled City began to grow. Square buildings folded up into one another as thousands of modifications were made, virtually none by architects or engineers, until the entire City became monolithic. Labyrinthine corridors ran through the City, some former streets (at the ground level, and often clogged up with refuse), and some running through upper floors, through and between buildings. The streets were illuminated by fluorescent lights, as sunlight rarely reached the lower levels.”
Source: Wikipedia