And now a team of marine scientists from the University of Georgia (UGA) thinks they know where a large portion of the oil from the BP spill has gone.
Last month, the National Oceanic and Atmospheric Administration (NOAA) and the Department of the Interior (DOI) estimated about one-third of the oil released by the BP spill had been contained by recovery operations, and that another third had naturally evaporated, dissolved or been dispersed into microscopic droplets.
The report said the remaining amount "is either on or just below the surface as residue and weathered tarballs, has washed ashore or been collected from the shore, or is buried in sand and sediments. Dispersed and residual oil remain in the system until they degrade through a number of natural processes. Early indications are that the oil is degrading quickly."
Millions of liters of toxic dispersants were also used during containment operations, in a controversial move to break up the BP spill.
But the UGA team, lead by Professor Samantha Joye, believes much of the oil has instead settled on the bottom of the Gulf of Mexico in an inches-thick layer.
Dr. Joye has been keeping a blog on the team's findings. In a recent entry, she talks about how core samples her team took about 16 nautical miles from the wellhead, and discovering "something we had not seen before: a layer of flocculent [fluffy or woolly in appearance], sedimented oil that was [centimeters] thick." When oil naturally seeps from the ocean floor, she says, the recovered core samples will be saturated, "oil-stained from top to bottom." But at the site visited by the researchers, Dr. Joye says the oil "obviously came from the top (down from the water column) not the bottom (up from a deep reservoir). What we found today is not a natural seep."
"We call it 'oil aggregate snow'," she writes, "because it settled down to the water column to the seafloor just like snow falls from the sky to the ground."
The UGA team is still determining the thickness of the oil on the Gulf of Mexico's floor, and is examining other sites near the wellhead. A photo of one recent core sample, shown on Dr. Joye's blog, appears to show a two-inch-thick layer of oil lying on top of the normal sediment.
According to the report, dissolved oxygen levels in the Gulf of Mexico affected by subsurface oil have dropped by about 20% from their long-term average levels. But "[s]cientists from agencies involved in the report attribute the lower dissolved oxygen levels to microbes using oxygen to consume the oil from the BP Deepwater Horizon oil spill."
The report says the dissolved oxygen levels were measured within 60 miles of the well head and were not low enough to create "dead zones," or areas where the oxygen level is so low it can't support most aquatic life.