The "gunkanjima" island in Nagasaki, named because it looks like a certain class of battleship, is actually named
Hashima. It was a coal mine in operation into the mid-70's, featuring at times "forced labor" (slaves) from Korea. It's tiny, but was home to 5,000 people (miners and their families), who worked a mine more than a kilometer deep and so spread out that it connected to shafts from other islands.