On a fiery hot day in late June, tourists filled the cafes and hotel rooms along Spain’s Mediterranean coast, including in Torrevieja, a small city of tightly stacked apartment blocks running along a curved beach. The seasonal population surge in this dry, sun-baked region might strain water resources were it not for a set of buildings overlooking a pink-tinged lagoon nearby. These low-slung structures house a vast network of pipes, pumps and tanks in a plant that performs a kind of alchemy crucial to the economy of this part of Spain: drawing huge volumes of water from the sea, removing the salt and creating more than 60 million gallons of fresh water a day.