This island in Nagasaki Bay was bought by the Mitsubishi Corp. in 1890 as a platform for mining coal under the sea. Workers, who lived on the island, eventually required high rises which now combine to make the island look like a battleship. Legend has it that the U.S. navy fired on the island in WWII thinking it was an enemy ship.
In 1959 the population on the island rose to 83,476.2 inhabitants per km², supposedly the highest population density ever recorded anywhere. By 1974 petroleum had taken over coal as Japan's fuel of choice and Mitsubishi shut down the mine, giving everyone a limited time to evacuate. The result is a ghost island that's remained relatively untouched to this day.
Saiga Yuji has outstanding
photo sets both before and after the island was evacuated.
(Most sites will say you can't visit the island, but as recently as Spring '06, you could take a boat tour around the island.)