5 million barrels of oil—794,936,475 liters, equivalent to about 318 olympic swimming pools or about 714,000 inflatable kiddie pools—that’s the official estimate of the Deepwater Horizon oil spill’s size. And until earlier this week, we had no clue what had happened to 2 million barrels of it.

714,000 of these and we’ve accounted for all the missing oil!

How, you might wonder, do you lose that much oil? With everyone and their mother’s attention on the spill back in 2010, it seems like we should have had this in the bag. But (as marine scientists and pirates know all too well) the sea is a harsh and fickle mistress. Complex currents transporting oil all over the Gulf, bacteria gobbling up some of the oil, and the inherent difficulty of seeing anything going on in the deep ocean have made it difficult for scientists to determine what happened to the missing oil.

But now, 4 years after the spill, scientists from the University of California at Santa Barbara have emerged from the deep to let us know—some of the missing oil has been found! Dr. David Valentine and his group of researchers looked for the presence of oil in over 3000 samples of mud and sand from the seafloor surrounding the Macondo well, and found a 3200 km^2 area contaminated by oil. That, folks, is a “bathtub ring” of oil about the size of the state of Rhode Island.

From Valentine et al. 2014: “hot” colors show areas with higher oil concentrations, while “cool” colors are areas with less oil. The star shows the location of the Macondo well.

Because it’s been a while since the spill occurred, this oil has degraded to a point where researchers can no longer use chemical signals to confirm that the oily seafloor is the result of the BP oil spill. To get around this problem, Dr. Valentine’s team used other indirect methods to tie the presence of oil back to the Macondo well. They found that oil concentrations decreased deeper in the sediment, which indicates that the oil was deposited very recently. In addition, his team found a clear high-oil anomaly in the area surrounding the site of the Macondo well and used computer modeling to confirm that the variations in oil concentrations across the seafloor fit with what we know of the oil’s dispersion path.

Researchers aren’t yet sure exactly how all this oil ended up at the bottom of the ocean—usually, most oil isn’t dense enough to sink to the seafloor. Dr. Valentine guesses that oil-loving bacteria gobbled up a lot of this oil, creating larger clumps of oil-laden bacteria. These then sank to the seafloor, latching onto more oil particles on their way down. Regardless of the “how”, scientists are now using their new knowledge to determine the extent of ecological damage to organisms such as deep sea corals in the contaminated area. BP, so far, has refused to take official responsibility for the oil–but the evidence, we might say, is oil-erwhelming.

Link to research paper: http://www.pnas.org/content/early/2014/10/23/1414873111.full.pdf+html  (it’s open access woooo!)