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From NPR News, it's ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Melissa Block.

Scientists on a research vessel in the Gulf of Mexico are discovering a substantial layer of oily sediment stretching for dozens of miles in all directions.

Their discovery suggests that a lot of oil from the ruptured BP well didn't simply evaporate or dissipate into the water, it settled to the sea floor, as NPR's Richard Harris reports.

RICHARD HARRIS: The Research Vessel Oceanus is on a quest to answer one of the biggest remaining questions about the BP oil-well blowout: What happened to the more than four million barrels of oil that gushed into the water?

Onboard, Professor Samantha Joye from the University of Georgia says she suddenly has a pretty good idea about where a lot of it ended up: It's showing up in samples of the sea floor, between the well site and the coast.

Ms. SAMANTHA JOYE (Professor, University of Georgia): I've collected literally hundreds of sediment cores from the Gulf of Mexico, including around this area. And I have never seen anything like this.

HARRIS: Speaking via satellite phone, Joye describes seeing layers of oily material, in some places more than two inches thick, covering the bottom of the sea floor.

Ms. JOYE: It's very fluffy and porous. And there are little tar balls in there that you can see that look like little microscopic cauliflower heads.

HARRIS: And it's very clearly a fresh layer. Right below it, she finds much more typical seafloor mud. And in that layer, she finds recently dead shrimp, worms and other invertebrates.

How did the oily sediment get there? Joye says it's possible that chemical dispersants might have sunk some oil. It's also likely that natural systems are playing an important role.

Ms. JOYE: The organisms that break down oil excrete mucus, copious amounts of mucus. So it's kind of like a slime highway from the surface to the bottom because eventually the slime gets heavy, and it sinks.

HARRIS: That sticky material can pick up oil particles as it sinks. Joye can't say with 100 percent certainty right now that the oily layer is from the BP blowout.

Ms. JOYE: We have to fingerprint it and link it to the Deepwater Horizon. But the sheer coverage here is leading us all to come to the conclusion that it has to be sedimented oil from the oil spill because it's all over the place.

HARRIS: So far, the research vessel has traveled in a large X across the gulf, within a few dozen miles of the well. They've taken eight sets of samples. And Joye says they all contain this layer, thin in some places, inches thick in others.

Eventually, scientists hope to collect enough samples to figure out how much oil is now settling to the seafloor.

Ms. JOYE: It's starting to sound like a tremendous amount of oil. And so we haven't even sampled close into the wellhead yet. That's on tomorrow's agenda.

HARRIS: Last month, another research group also reported finding oil on the sea floor. Researchers at the University of South Florida say they saw oil particles sprinkled on top of the mud.

These new findings strongly suggest that it didn't just drizzle oil. In some places, it was a blizzard.

David Hollander, from the University of South Florida, says the government's original attempt to figure out what happened to the oil toted up how much washed ashore, how much evaporated, how much might have stayed under the waves, but it didn't consider that oil could also end up on the seafloor.

Mr. DAVID HOLLANDER (University of South Florida): And so now the bottom really is turning out to be an important sink for the oil.

HARRIS: The ecological impacts of that depend on the depth of the ocean where it lies. Joye's findings so far have found oil in depths ranging from 300 to 4,000 feet. Hollander says those shallower waters in particular are potentially important, not just for life on the bottom, but for the entire marine ecosystem.

Mr. HOLLANDER: A lot of fish go down to the bottom and eat, and then come back up. And if all their food sources are derived from the bottom, then indeed you could have this impact.

HARRIS: Figuring all that out will probably take many years.

Richard Harris, NPR News.

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