Kowloon Walled City
Kowloon Walled City is the most weird city district in the Earth history. Resided in Hong Kong for almost two centuries it was the most populated spot on our planet. Also being almost independent from Hong Kong government and police it was one of the most criminal and dirty. Drug dealers, pimps and hookers, illegal casinos and medical establishments were hiding inside labyrinth of buildings grown without any sense of order. It was finally demolished in 1993, but a lot of pictures could be found over the internet, here or in Wikipedia article.