Java is a narrow volcanic island in Indonesia, about the same size as England, that holds more than 150 million people — by most measures the most crowded large island on earth. A spine of dozens of active and potentially active volcanoes runs its length; their eruptions are deadly, but their ash makes the soil among the most fertile in the world, feeding intensive wet-rice farming that has supported dense populations for over a thousand years. That fertility built a long succession of civilizations and made Java the political, cultural, and economic heart of the Indonesian world, anchored today by the sinking megacity of Jakarta.
Pack the entire population of Russia onto an island the size of England, and you have something close to Java. More than 150 million people live on this one narrow strip of volcanic land in Indonesia, which makes it, by most measures, the most crowded large island on earth. The reason lies underfoot. A line of dozens of active and potentially active volcanoes runs the length of the island, and they kill: ash clouds, mudflows, and collapse have buried towns within living memory. Yet the same volcanoes are why anyone is there at all. Their ash breaks down into soil so rich that in well-watered areas, farmers can sometimes pull two or even three rice harvests a year from it. Deadly mountains made the most fertile fields, the fields fed the densest crowds, and the crowds built one civilization after another on a single small island.