The 2010 Deepwater Horizon spill left oily residue on the seafloor that scientists liken to a "bathtub ring"—if the tub were big enough to fit Rhode Island. Researchers who tested thousands of sediment samples from more than 500 locations in the Gulf of Mexico believe around 84 million gallons of oil—out of the 172 million gallons of oil spewed out by the BP rig—ended up on the seafloor, CBS reports. The oil, much of which never reached the surface, was deposited in the "bathtub ring" over 1,250 square miles and in a "fallout plume" of oil particles that sank into sediment, according to the study, which was published in the Proceedings of the National Academy of Sciences.
"I think people have been curious about what happened to the oil in the deep Gulf of Mexico," the lead researcher tells the New Orleans Times-Picayune. "Now we have some handle on this question," he says, explaining that the study will help determine the damage to deep-sea coral and seafloor ecosystems. BP disputes the findings. In a statement to NBC, the company says the researchers "grossly overstated" the amount of oil from its Macondo well found on the seafloor because a lack of "rigorous chemical fingerprinting" caused them to mix up residue from the Deepwater spill with natural seepage. (Read more Deepwater Horizon stories.)