Hashima is an island which is located in the ocean about 19 kilometers southwest from Nagasaki port.
It is called Gunkanjima (battleship island).
That place prospered from undersea coal mining from the Meiji to the Showa era through most of the 20th Century.
It was just 160 meters in width and 480m long, and 5200 people lived there at its peak.
Some of the earliest ferroconcrete high-rise buildings were built there for the first time in Japan.
And also schools, hospitals, movie theaters, and stores, etc were there.
But it was closed in 1974 and nobody lives there now. It is disintegrating.
Now you can visit there by tour.
Gunkanjima was chosen as World Heritage site in 2015 and it is a popular ruined island.
http://www.gunkanjima-concierge.com/en/index.html