Clean drinkable water is rare and precious. Of the waters that cover 70 percent of the Earth’s surface, roughly 97 percent is the saltwater of the oceans; just three percent is freshwater. Of that, only 1.2 percent can be used as drinking water; the rest is locked up in glaciers, ice caps, and permafrost, or stored too far beneath the surface to be retrieved. Much of the accessible freshwater has become polluted.

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