Today's large desalination plants, though, cost millions of dollars to build. Most use reverse osmosis, which forces seawater through salt-blocking membranes. The required electricity accounts for up to half of a plant's expenses, and the process leaves behind a supersalty, chemical-laced soup that can harm local ecosystems. Such facilities are typically powered by carbon-emitting fossil fuels; efforts have been made (especially in the Middle East, Asia and Africa) to use solar panels instead, but that also comes at a cost and does not address the toxic discharge.

So researchers are trying to use the sun's heat more directly to remove salt and other contaminants. The simplest option is to let water evaporate, leaving behind salts and chemicals, and then condense the vapor into clean water. Humans have used versions of this technique, called solar distillation, for hundreds of years. Today Saudi Arabian engineers plan to build a plant with giant mirrors that concentrate sunlight and superheat water within a steel-and-glass dome more than 50 meters across.

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