Researchers at the University of Georgia have said that about 75% of the oil from the ruptured BP well remains below the surface of the ocean.
It is also claimed that up to 79% of the estimated 4m barrels of oil that were released into the sea was not captured directly at the wellhead, and was still in the Gulf.
Dr Charles Hopkinson was involved in the research.
He said the findings contradict an official US government report into impact of the spill, which found that the vast majority of oil spilled had been evaporated, burned, skimmed or dispersed.
'We just reanalysed this report...and then we calculated how much oil is still likely to be out there and that is how we came up to 70% to 79% that must be out there,' said Dr Hopkinson.
'One major misconception is that oil that has dissolved into water is gone and therefore, harmless,' he told AFP.
'The oil is still out there and it will likely take years to completely degrade. We are still far from a complete understanding of what its impacts are,' he said.
