By SETH BORENSTEIN A 22-mile-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, 40-degree 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 you can't see it.