Scientists have found vast underwater plumes of oil, one 10 miles (16km) long and a mile wide, in the Gulf of Mexico, following last month's rig disaster.
Researchers from the National Institute for Undersea Science and Technology say they have detected several sprawling oil slicks lurking just beneath the surface of the sea and at depths of 4,000ft (1,200m).
The scientists said the chemical dispersants BP has been dumping underwater may be preventing the oil from rising to the top of the ocean.
The oil giant has said the chemicals, which it began deploying on Friday, have already resulted in less oil surfacing.
Were they just trying to sweep the oil under the carpet?