WASHINGTON – The oil is there, at least 22 miles of it. You just can’t see it.

A lot of the crude that spewed from BP’s ruptured well is still in the Gulf of Mexico, but it’s far below the surface and invisible. And it’s likely to linger for months, scientists said Thursday in confirming evidence of an underwater plume of oil from the disaster.

The plume consists of droplets too small for the unaided eye to see, more than a half-mile down, said researchers who mapped it with high-tech sensors.

Scientists fear it could be a threat to certain small fish and crustaceans deep in the ocean. They will have plenty of time to study it for answers.

In the 40-degree water, 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 Ben Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts, a co-author of the research, published online in the journal Science.

Earlier this month, top federal officials declared that oil from 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, researchers found.

“We absolutely should be concerned that this material is drifting around for who knows how long,” said Monty Graham, a scientist at the Dauphin Island Sea Lab in Alabama who was not involved in the new research. “They say months in the (research) paper, but more likely we’ll be able to track this stuff for years.”

The oil was measured close to BP’s blown-out well, about 40 miles off the Louisiana coast. The plume started three miles from the well and extended more than 20 miles to the southwest. But if you swam through the plume, you wouldn’t notice it.

“The water samples when we were right in the plume look like spring water,” study chief author Richard Camilli said Thursday. “You certainly didn’t see any oil droplets, and you certainly didn’t smell it.”

The scientists used complex instruments — including a special underwater mass spectrometer — to detect the chemical signature of the oil that poured from the BP well after the April 20 explosion of the Deepwater Horizon oil rig. The equipment was carried into the deep by submersible devices.

Using more than 57,000 measurements, the scientists mapped the plume in late June, when the well was still leaking. The components of oil were detected in a flow that measured more than a mile wide and more than 650 feet 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 trip ended. But scientists said that wouldn’t lessen the overall harm.

The oil is at depths of 3,000 to 4,000 feet, far below the environment of the most popular gulf fish like red snapper, tuna and mackerel. But 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 shallower areas, taking nutrients from the depths. Then they are eaten by large fish and mammals.

Those smaller fish could be harmed by going through the oil, said Larry McKinney, director of Texas A&M University’s Gulf of Mexico research center.

Some aspects of that deep region are so little known that “we might lose species that we don’t know now exist,” said Graham of the Dauphin Island lab.

“This is a highly sensitive ecosystem,” agreed Steve Murawski, chief fisheries scientist for the National Oceanic and Atmospheric Administration. “The animals down at 3,300 to 3,400 feet grow slowly.” The oil not only has toxic components but could cause genetic problems even at low concentrations, he said.

For much of the summer, the existence of underwater plumes of oil was the subject of a debate that at times pitted outside scientists against federal officials who played down the idea. Now federal officials say as much as 42 million gallons of oil may be lurking below the surface in droplets that are much smaller than the width of a human hair.

While federal officials prefer to describe the lurking oil as “an ephemeral cloud,” the Woods Hole scientists use the word “plume” repeatedly.

The study conclusively shows that a plume exists, that it came from the BP well and that it probably never got close to the surface of the Gulf of Mexico, Camilli said. It is probably even larger than 22 miles long, but scientists had to stop measuring because of Hurricane Alex.

Earlier this week a University of South Florida team reported oil in amounts that are toxic to critical plant plankton deep underwater, but the oil was not necessarily in plumes. Those findings have not been reviewed by other scientists or published.

The plume is probably still around, but moving west-southwest of the BP well site at about four miles a day, Camilli said.