The invisible oil plume from the Deepwater Horizon oil spill in the Gulf of Mexico off the coast of Louisiana will be long-lasting, says a study released Thursday. ((Charlie Neibergall/Associated Press))

A 35-kilometre-long invisible mist of oil is meandering far below the surface of the Gulf of Mexico, where it will probably loiter for months or more, scientists reported Thursday in the first conclusive evidence of an underwater plume from the BP spill.

The most worrisome part is the slow pace at which the oil is breaking down in the cold, 4 C water, making it a long-lasting but unseen threat to vulnerable marine life, experts said.

Earlier this month, top federal officials declared the oil in the spill was mostly "gone," and it is gone — in the sense that you can't see it. But the chemical ingredients of the oil persist a kilometre beneath the surface, researchers found.

And the oil is degrading at one-tenth the pace at which it breaks down at the surface. That means "the plumes could stick around for quite a while," said study co-author Ben Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts, which led the research published online in the journal Science.

Florida State University scientist Ian MacDonald, in testimony before Congress on Thursday, said the gas and oil "imprint of the BP discharge will be detectable in the marine environment for the rest of my life."

The underwater oil was measured close to BP's blown-out well, which is about 64 kilometres off the Louisiana coast. The plume started about five kilometres from the well and extended more than 32 kilometres to the southwest. The oil droplets are odourless and too small to be seen by the human eye. If you swam through the plume, you wouldn't notice it.

"There's no visible evidence of oil in the samples; they look like clear water," study chief author Richard Camilli said. 

The scientists used complex instruments — including a special underwater mass spectrometer — to detect the chemical signature of the oil that spewed from the BP well after it ruptured April 20.

The equipment was carried into the deep by submersible devices. With more than 57,000 individual measurements, the scientists mapped a huge plume in late June. The components of oil were detected in a flow that was more than one-and-a-half kilometre wide and more than 198 metres from top to bottom.

Federal officials said there are signs that the plume has started to break into smaller ones since the Woods Hole research cruise ended. But scientists said that wouldn't lessen the overall harm from the oil.

The oil is at depths of 900 to 1,200 metres, far below the environment of the most popular Gulf fish like red snapper, tuna and mackerel, but it is not harmless. These depths are where small fish and crustaceans live. And one of the biggest migrations on Earth involves small fish that go from deep water to more shallow areas, taking nutrients from the ocean depths up to larger fish and mammals.

Those smaller creatures could be harmed by going through the oil, said Larry McKinney, director of Texas A&M University's Gulf of Mexico research center in Corpus Christi.