A team of University of Georgia scientists said Tuesday that they had traced a 15-mile-long undersea concentration of oil and gas near the BP leak, adding to evidence that the biggest spill in U.S. history isn't just sloshing around on the surface.

Researchers, who had just returned from a two-week expedition aboard F.G. Walton Smith, said the oily undersea zone, three miles wide and 600 feet thick at its core, contained levels of methane gas 10,000 times greater than the norm.

“I’ve never seen concentrations of methane this high” in the Gulf of Mexico, said marine sciences professor Samantha Joye.

Hectic activity by oil-consuming microbes has depleted oxygen levels in and near the petroleum concentration, which diminishes with distance from the blown-out BP well.

“The entire water column has dispersed oil,” said Joye, whose sail was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration. “The system as a whole has been substantially perturbed by this event.”

In a news briefing and in blog posts during the voyage, Joye described the oily area, which forms about 600 feet above the seafloor, as irregular and highly diffuse, like a mist of olive oil.

“There’s really no way to get the oil out of the water,” she said. “The stuff is going to stay in there and circulate around and have a long-term impact… It’s going to be months or years before we realize the full consequences of the spill.”

Although BP has released large amounts of chemical dispersants at the well head, Joye said that given the force with which much the oil and gas is spurting into the ocean nearly a mile beneath the surface, it would have spread through the water column even without the help of dispersants.

NOAA had criticized as premature earlier reports of a "plume" of oil and gas by the University of Georgia team. But the agency on Tuesday separately confirmed the presence of undersea oil from the spill.

For more detailed information on the environmental effects of undersea oil, click here to read the full report by Los Angeles Times reporters Bettina Boxall and Alana Semuels.

-- Bettina Boxall