There's no doubt oil has concentrated on the shallow seafloor, close to the Gulf coastlines—and will remain there for some time, experts say.

"There is oil on some of the shelf areas that mixes in with the sand and tar balls continue to wash up," said Larry McKinney, director of the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University in Corpus Christi.

"So the long-term chronic effect of that is something we have to be concerned about."

But getting a handle on the status of the remote, deep seafloor is proving far more difficult, with research expeditions turning up different results.

(See "Toxic Oil Found