THE DALLES, Oregon On the banks of the windswept Columbia River, Google is working on a secret weapon in its quest to dominate the next generation of Internet computing. But it is hard to keep a secret when it is as big as two football fields, with twin cooling towers protruding four stories into the sky. The towers, looming like an information-age nuclear plant, mark the site of what may soon be one of the world's most powerful supercomputers, helping to supply the ever-greater horsepower needed to process billions of search queries a day and a growing repertory of other...