Making subterranean sexy, the annual World Tunnel Congress in San Francisco drew 2,000 engineers, developers, scientists, project owners and governmental reps to what was billed as the largest gathering of tunnel experts in history. They've been talking dirt and drills, soil and shale. And commiserating about the unimaginable pressures -- think seismic, financial, political -- that they labor under. And all 2,000 of them appear to be deeply passionate about the underground.