"If these plumes are associated with the oil slicks on the surface, then it's not just incidental gas common everywhere in the Mississippi River Delta," said Ian MacDonald, a Florida State University oceanography professor.
Oil and gas plumes found at site of 13-year-old leak in Gulf →
"These are oil slicks," MacDonald said. "On a calm day, it's mousse. It produces genuinely thick oil that nobody would mistake for sheen."
Company in settlement talks over money for 13-year oil leak →