“For decades, desalination has been the only reason that many places, from the Caribbean to the Emirates, have been habitable. But it’s always been a solution of last resort, for one big reason…

Up on land, engineers would literally boil the ocean, creating steam that would become drinking water and, on its way, drive some power-generating turbines to pay back a bit of its cost. It was so energy intensive that in the 1960s, some proposed using nuclear power to do it. The world’s largest desalination plant, in Ras al-Khair, Saudi Arabia, produces much of its water through evaporation.

Around the year 2000, reverse-osmosis changed everything, says Pankratz. In this process, water is forced across a plastic membrane with holes so tiny only water molecules fit through, leaving behind salt and other impurities. This process requires about half the energy, making it a credible option for Trinidad, which in 2002 got a plant that now produces 40 million gallons of water a day, and Israel, which got one in 2005 that now produces 85 million gallons of water daily. Many more plants followed, and this is now the standard way to desalinate water…

Oslo-based Flocean, Netherlands-based Waterise and Bay Area-based OceanWell are among the companies that seized on this idea of desalination—then submerged it to a depth of at least 400 meters.

The principle is easy to grasp: Instead of expending huge amounts of energy to pump seawater onto land, and then pressurize it inside a plant, why not take advantage of the ocean’s extreme natural pressure? At depth, seawater naturally wants to cross a desalination membrane, so long as the fresh water on the other side of it is being pumped to the surface. The result is a net energy savings of up to 40%.”

From Wall Street Journal.