Nature: Following the Deepwater Horizon oil spill in 2010, the US government concluded that one-fourth of the oil was unaccounted for. Scientists are now reporting that up to one-third of the spilled oil may have mixed with sediments and settled to the bottom of the Gulf of Mexico. A series of reports presented at the Gulf of Mexico Oil Spill and Ecosystem Science Conference show that surface material was deposited at 10 times the normal rate during and following the spill. That increased rate appears to have been caused by the oil creating clumps of plankton and sediment, which fell to the sea floor and formed a darker-than-normal layer. Observations during the spill recorded layers of water that would normally be full of plankton instead being clear—except for strings of material settling to the bottom. Why that occurred isn’t certain, though one report found that weathered oil may cause clumps to form more easily than clean oil. Another report suggests that the clumping material may have picked up even more oil as it reached lower levels before it settled and mixed with the sea-floor sediments. Although oil concentrations found in samples taken from the bottom of the Gulf have been low, the total volume of oil on the sea floor could be large if it was widely spread.