The Gulf of Mexico's undersea oil plume is no more. For nearly a month, scientists sampling the site of a deepwater plume stretching southwest from BP PLC's failed well in the Gulf have been foiled. Their sensors have gone silent. Where once a vibrant -- if diffuse -- cloud of oil stretched for miles, 3,600 feet below the surface, there is now only ocean, and what seems to be the debris of a bacterial feeding frenzy. "For the last three weeks, we haven't been able to detect the deepwater plume at all," said Terry Hazen, a microbiologist and oil spill expert at Lawrence Berkeley National Laboratory who has had a clutch of researchers monitoring the Gulf since late May. The disappearance is backed up by government sampling data. The plume is simply gone. And Hazen knows why.