Washington - Around two million barrels of oil from the BP spill off the US Gulf Coast in 2010 is believed to have settled on the ocean floor, according to a study.
The fate of two million of the nearly five million barrels that gushed into the open waters has remained a mystery until now, said the findings.
Researchers analyzed samples collected at more than 500 locations around the Macondo Well, where the leaked oil emerged, and found it had spread widely, settling down like dirt in a bathtub.
The oil was found to have spread as far as 3 200km2 from the site, and may have gone even further, the report said.
"Our analysis suggests the oil initially was suspended in deep waters and then settled to the underlying sea floor", said the study by the University of California, Santa Barbara; the Woods Hole Oceanographic Institution in Massachusetts; and the University of California, Irvine.
Researchers came to this conclusion by studying seafloor sediment cores for residual hopane, a hydrocarbon that comes from crude oil.
The National Wildlife Federation said earlier this year that scientific studies on 14 different types of creatures affected by the spill show that long lasting harm was done to dolphins, sea turtles, tuna, loons and other animals in the region.
In pleading guilty to the spill, BP agreed to pay the government $4.5bn to settle criminal charges in the case.
It also agreed in 2012 to settle damage claims by businesses and individuals for about $7.8bn.
Last month, a federal court judge in New Orleans concluded that BP acted with "gross negligence" ahead of the massive 2010 Gulf of Mexico oil spill, meaning BP may face billions of dollars in new fines.
The 20 April 2010 Deepwater Horizon drilling rig blowout, which killed 11 people, happened because BP's US subsidiaries, along with oil-services company Halliburton and rig owner Transocean, did not take adequate care in drilling a risky well, the court found.