"Gunkanjima was once just a small reef but, following the discovery of coal in 1810, was turned into mining facility during the industrialisation of Japan. It gave rise to its own population of workers and inhabitants who were all densely-packed into a self-contained metropolis.
The 15-acre island was populated between 1887 and 1974, reaching its peak in 1959 with 5,259 inhabitants. However, as petroleum replaced coal during the 1960s, Japan’s mines were hit by closures which eventually reached Gunkanjima.
Within a matter of days of the mines closing in 1974, the workers and their families deserted the island, leaving their possessions, which still lay where they were left.”
Photos and information from here.