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 on end, scientists said Thursday in the first conclusive evidence of an underwater plume of oil from the disaster.

The plume consists of droplets too small for the 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 cold, 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 the oil in the spill was mostly ‘gone,’ and it is gone in the sense that you can’t see it. But […]