Kowloon “Walled City” in Hong Kong was the most densely populated land in history, containing 33,000 residents within 6.5 acres (recorded in 1987). Based on this survey, the Walled City had a population density of approximately 1,255,000/km2. For comparison, Hong Kong as a whole (being one of the most densely populated areas on earth) had a population density of about 6,700/km2 as of July 2009.
For years Kowloon was controlled by drug dealers, and so outsiders would not dare step foot inside its borders without good reason. The city is infamous for rampant gang activity, prostitution, illegal businesses, and amateur dentists lining its walls.