China's antics in the South China Sea are not only for direct military purposes. There is an abundance of valuable mineral deposits such a manganese nodules as described in this 2013 report:
`Submersible taps vast mineral deposits in South China Sea'
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They're also exploring the Pacific seabed for minerals: "The mineral potential of the deep ocean seabed was underscored on July 3 [2011] when a group of Japanese specialists announced the results of a seabed survey of 78 sites in the Pacific containing rare earth minerals that are critical to a wide range high-technology products for both civilian and military use. They estimated that a one square-kilometer area in one hot spot alone held a cache of rare-earth oxides equivalent to one-fifth of current global demand in a market where China produces around 97 percent of the supply and has sent prices sky rocketing by imposing increasingly tight export quotas."
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