At the northern edge of the Geysers, the world's largest geothermal complex, which sprawls over nearly 40 square miles north of Santa Rosa, Calif., Houston-based power company Calpine is conducting an experiment. On the surface, not much sets the project apart from the 18 ridge-top power plants and dozens of other drilling platforms here, most of which Calpine owns. The site, high on a sun-baked hillside, is home to a 110-foot drill rig surrounded by a couple of trailers and a mass of machinery. But two miles underground, something big is happening.