Its seeming ubiquity masks its growing scarcity. More than 10% of humanity lacks access to it while 90% of natural disasters are linked to it. Wars are fought over it. Women in poor countries spend their days hauling it. Major cities—Sâo Paulo, Capetown—have nearly run out of it.

The water crisis that experts have been warning about for decades has arrived. A warming climate and growing population mean a dwindling supply of fresh water. A scientist, activist and entrepreneur are among those on the front lines of the efforts to provide clean water.