U.S. Forest Service geologist Gordon Grant calls it a “geologic gift.” University of Oregon earth scientist Leif Karlstrom described it as “a big water tower” at the top of the mountains. The two researchers were describing a massive storehouse of water buried below Oregon’s Central Cascade Mountains that could define the state’s future in a world with ever-growing thirst. In a study published earlier this month, Karlstrom and Grant estimated there is at least 81 cubic kilometers — or three times the volume of Lake Mead, the United States’ largest reservoir — buried in the volcanic rocks from Mount…..