The Kowloon Walled City was located just outside Hong Kong, China during British rule. A former watchpost to protect the area against pirates, it was occupied by Japan during World War II and subsequently taken over by squatters after Japan’s surrender. Neither Britain nor China wanted responsibility for it, so it became its own lawless city.
Its population flourished for decades, with residents building labyrinthine corridors above the street level, which was clogged with trash. The buildings grew so tall that sunlight couldn’t reach the bottom levels and the entire city had to be illuminated with fluorescent lights. It was a place where brothels, casinos, opium dens, cocaine parlors, food courts serving dog meat and secret factories ran unmolested by authorities. It was finally torn down in 1993 after a mutual decision was made by British and Chinese authorities, who had finally grown wary of the unsanitary, anarchic city and its out-of-control population.
And if that isn’t enough, here is more abandonment! I do so love derelict structures.