"Up to 4 million barrels (167 million gallons), the vast majority of the spill, remains unaccounted for in government statistics. Some of it has, most likely, been cleaned up by nature. Other amounts may be gone from the water, but they could have taken on a second life as contaminants in the air, or in landfills around the Gulf Coast.
And some oil is still out there -- probably mixed with chemical dispersants. Some scientists have described it floating in underwater clouds, which one compared to a toxic fog."