<p>For most of history, thirsty humans made do with what moisture fell from above: The sun warmed the salty seas, pure water evaporated into the air and then cooled and fell to the earth as precipitation. There it clung to glaciers, froze and thawed in lakes, was absorbed by plant roots, coursed through fractured bedrock, and seeped slowly through soil, into aquifers. Most of it returned to sea and sky all over again. There is as much of that water on the planet today as when the first amphibian flopped ashore; as much as when the ancient Greeks divined the future in the babble of brooks.</p>