20. Hashima Island (Japan)
Hashima Island could once claim the title of the most densely-populated spot on the planet. The thousands of Japanese workers, as well as forced laborers from China and Korea who toiled in the island’s deep mine shafts during WWII, are long gone. When Japan shifted to petroleum as its main energy source instead of coal, the island was no longer economically viable, and hence deserted. Most of the massive concrete buildings are still standing, though, which gives this island the aura of a floating ghost town, or an old battleship.
