According to an afternoon aerial observation, a stretch of oil about 50 yards wide and 15 miles long is about two miles south of the coast.

"If conditions do not change I would expect landfall by morning potentially impacting a large portion of the coast," said George Surry, a battalion chief with Gulf Shores Fire Rescue who was on the observation flight.

Despite the commonly held belief that oil always floats, both the Press-Register and Alabama officials have documented oil on the seafloor of state waters in the wake of the Deepwater Horizon spill.

The U.S. Coast Guard and scientists have also documented that oil sometimes sinks after major spills.

Scientists are unsure how quickly such oil would degrade underwater, how they would locate it on the bottom or how it could be cleaned up. A sample of the oil found by the Press-Register in late June was provided to Ed Overton, a Louisiana State University scientist, who has been analyzing oil for federal officials since the spill began.

Overton said he had received other reports of submerged oil but no other samples. After getting the Press-Register sample, he sent an e-mail about the newspaper's find to 14 scientists with the National Oceanic and Atmospheric Administration. He urged them to collect and send samples for study.

"This represents a very serious turn if we find that, as the oil weathers, it becomes more dense than the water or at lease dense enough for easy submersion with attachment to detritus," the e-mail read in part.

Overton said the biggest threat posed by the submerged oil was the inability to contain it.

Healthy oxygen levels of 4 parts per million or higher were found in the upper portion of the water column at each location. But beginning at about 15 feet above the seafloor, levels declined to as low as 0.1 parts per million. Scientists consider 2 parts per million to be the minimum required for most marine life.

The low oxygen areas threaten the creatures that live on or buried beneath the seafloor, such as clams and marine worms. The worms, for instance, are exceptionally abundant and provide a primary food source for snapper, grouper and other fish.