Seawater has been converted into drinkable freshwater for decades in the Middle East and in the Caribbean. Only a few major seawater desalination plants exist in the U.S.; the largest operates in Tampa, Fla., and a project twice the size is being developed in Carlsbad, Calif. But that number could grow quickly as millions of people move to coastal communities, which often have insufficient groundwater. “Almost 20 desalination plants are proposed for California alone,” says Tom Pankratz, a desalination consultant in Houston. Installations are being considered in Texas and Georgia, as are more in Florida.