When the Deepwater Horizon oil rig burst in 2010, about 200 million gallons of crude oil flooded into the fragile ecosystem of the Gulf of Mexico. While much of that oil washed up on shore or was cleaned up, sometimes using controversial dispersants, there is still a lot of oil unaccounted for. A new study in Environmental Science and Technology reveals that a good portion of that missing oil—about 10 million gallons—is sitting on the floor of the Gulf, where it is wreaking havoc with the sea environment.

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The missing Gulf oil has eluded scientists, the US government, and BP clean-up crews for years. In order to find it, researchers took samples from 62 areas surrounding the spill, avoiding areas with natural oil seeps. To identify the oil, scientists looked for carbon-14; an isotope that appears in oil but not in the ocean seabed material. Using these samples, scientists mapped out an area impacted by the spill.

Related: There Are Still 2 Million Barrels of BP’s Deepwater Horizon Oil Hanging Out in the Gulf of Mexico

Normally, oil floats rather than sinking, so no one is exactly sure how the oil got to the bottom of the sea. One idea suggests that oil particles bonded together on the water’s surface until they became heavy enough to sink.

While the sunken oil may not command as much attention as the oil-slicked birds that we so frequently see in the media, it is still a major problem. Oil decomposes more slowly on the ocean floor because there is less oxygen there, and it is also extremely damaging to the ocean’s wildlife. “This is going to affect the Gulf for years to come,” said Jeff Chanton, the study’s lead researcher. “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.”

+ Environmental Science and Discovery

Via Discovery

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