Scientists have already reported finding what they called a 1,235-square-mile "bathtub ring" of oil on the floor of the Gulf of Mexico left over from the huge 2010 BP oil spill.

Now it appears this ring is part of a washroom set: A different team of scientists has found that up 10 million gallons of oil have created what can be called only a "bath mat" beneath the sediment of the gulf's floor.

First the ring. David Valentine and colleagues from the University of California at Santa Barbara wrote in the Proceedings of the National Academy of Sciences in October that about 10 million gallons of the spilled oil settled on the gulf's floor. Its size: about the size of the state of Rhode Island.

But what about the rest? As much as 200 million gallons of oil were spilled after the Deepwater Horizon oil rig, owned by BP and Anadarko Petroleum Corp., exploded off the coast of New Orleans, killing 11 workers on the rig, injuring 17 more, and allowing oil to gush into the gulf for nearly three months.

All that oil has been hard to find. But a team of scientists led by Jeff Chanton found between 6 million and 10 million gallons buried in the sediment at the bottom of the gulf about 60 miles southeast of the Mississippi Delta. Chanton is a professor of oceanography at Florida State University.

Oil on the surface of the Gulf of Mexico at the site of the Deepwater Horizon oil spill in the Gulf of Mexico in 2010. REUTERS/Sean Gardner/Files Chanton and his colleagues, writing in the journal Environmental Science & Technology, say they determined how oil caused particles in the gulf to accrete, or clump together, then sink all the way to the floor of a body of water.

Their secret weapon, they wrote, was carbon 14, a radioactive isotope often used to date ancient artifacts. In this case, though, the FSU team used carbon 14 as an "inverse tracer" to find the oil. It works this way: Oil doesn't contain carbon 14, so any sediment that did contain the isotope would become evident in their search. What didn't show carbon 14 was oil.

Then Chanton worked with Tingting Zhao, an associate professor of geography at FSU, to create a map of the areas without carbon 14 on the floor of the gulf. From that information he was able to estimate the amount of oil that made up this "bath mat."

The "bathtub ring" and "bath mat" metaphors may be lighthearted, and at first, Chanton said, he wondered whether the accretion of the oil and its sinking to the floor of the gulf might be benign, if not necessarily beneficial, to the aquatic ecosystem. After all, he told FSU's news department, the water was clarified and the oil had separated from the water.

But in the long term, he said, it was a problem because the "mat" of oil removes oxygen from the materials that make up the floor. That, in turn, makes it harder for bacteria to attack the oil and make it decompose.

"This is going to affect the gulf for years to come," Chanton said. "Fish will likely ingest contaminants because worms ingest the sediment, and fish eat the worms. It's a conduit for contamination into the food web."