Hashima Island aka Battleship Island. Former coal mining facility owned by Mitsubishi Motors, it was once the most densely populated place on earth, packing over 13K people into each square kilometre. Operated from 1887 until 1974, after which the coal industry fell into decline. With their jobs gone and no other reason to stay in this mini urban nightmare, almost overnight the entire population fled back to the mainland, leaving most of their stuff behind to rot.
端島(軍艦島), Hashima Island, Gunkan-jima (Battleship Island), Japan, fifteen kilometers away from the infamous Nagasaki. This is amazing inspiration for a short story and I can't wait to incorporate it into one.