Oceanographers announced today the discovery of a wispy oil plume at least 22 miles long and 1.2 miles wide floating beneath the surface of the Gulf of Mexico, a sign that plenty of the oil from BP’s Deepwater Horizon leak remains in the environment. It’s the first conclusive proof that a deep-sea plume from the leak exists, which at least partially explains what happened to the oil in the three months since the Deepwater Horizon rig exploded. It also casts doubt on the federal government’s statement earlier this month that most of the oil has dispersed or disappeared.

But the new study is merely a rough snapshot of what is happening in the depths. Wide disagreement persists among scientists who study the Gulf and oil spills, and they say it could take generations to fully understand the leak’s scope. The best minds in marine science and geology can’t say yet how bad it will be.

Woods Hole Oceanographic Institution researchers Richard Camilli and Chris Reddy followed the plume starting about three miles from the wellhead of the deep-sea Macondo well. Using autonomous submersibles, they took samples for 22 miles, until the approach of Hurricane Alex forced them to turn back.

As of now, they don’t know how much oil (of the estimated 4.9 million gallons leaked) is in the plume, and they can’t be certain how diffuse it is until they analyze more water samples, Reddy said. The researchers say the levels of dissolved oxygen within the plume had not dropped to levels that would suggest bacteria were breaking down the oil in significant volumes.