Hashima Island (Gunkanjima)

Hashima, universally known as Gunkanjima or 'Battleship Island' for its warship-like silhouette, is one of the most extraordinary places in Japan: a tiny abandoned coal-mining island in the sea off Nagasaki. Mitsubishi began undersea coal mining here in the 1880s, and to house the workforce the island was packed with concrete apartment blocks — including Japan's first large reinforced-concrete building (1916). At its peak in 1959 some 5,259 people lived on the 6.3-hectare rock, making it the most densely populated place on earth, with schools, shops, a cinema, and even rooftop gardens. When Japan switched from coal to oil the mine closed in 1974 and the island was abandoned almost overnight; today its crumbling towers and empty stairwells form a haunting, post-apocalyptic ruin. Inscribed in 2015 as part of the 'Sites of Japan's Meiji Industrial Revolution,' it carries a difficult history too — wartime Korean and Chinese labourers were among those who worked the mine. You can only visit by licensed boat tour from Nagasaki Port: a fixed walking course with three viewing platforms (you cannot enter the buildings), with landings cancelled in rough seas.