Marine scientists said an underwater plume of drifting oil from the BP Plc disaster stretched at least 35 kilometres in the Gulf of Mexico in June and proved the need for rethinking clean-up operations after deepwater drilling accidents.

The study by scientists from Woods Hole Oceanographic Institution and other groups would likely become a “major part of the case” that the US government is developing against parties responsible for the spill, said Steve Murawski of the National Oceanic & Atmospheric Administration (NOAA).

The plume may have been even longer, but the measuring work was interrupted by the approach of Hurricane Alex in June. It was 200 metres high and up to 2 kilometres wide.

BP and its subcontractors have spent billions of dollars cleaning up the months-long spill from the April 20 explosion of the Deepwater Horizon drilling rig operating at 1.5 kilometres below the ocean surface.

But most of the efforts have been focussed on surface oil.

The study “established that oil can exist down there ... but there’s no technology for clean up at this depth,” lead scientist on the study Richard Camilli of Woods Hole told the German Press Agency, dpa.

The scientists discovered the plume of oil at 1.1 kilometres beneath the ocean surface in mid-June - an almost accidental byproduct of their work in determining the amount of oil that was flowing out of the ruptured wellhead.

As a Woods Hole team raised and lowered a remotely operated vehicle to take the measurements, it noticed an unusual band of turgid water that the vehicle passed through every time, Mr. Camilli said. That finding prompted them to measure the length, height and width of the plume.

The report is not the first to identify underwater plumes.

Researchers at the University of South Florida in July conclusively linked vast underwater plumes to the BP disaster.

The oil gusher was finally blocked earlier this month with a cement plug in the damaged well head. It could be early September until the final seal is finished, federal officials said Thursday.

The spill, which Woods Hole scientists called the “largest offshore oil spill in history,” presented a unique opportunity for scientists to study the behaviour and after—effects of the underwater plumes.

“We would be scientifically negligent if we did not respond to this uninvited guest,” said Christopher Reddy, one of the lead researchers of the study.

Much of their interest focused on the movement of the plume, about 10 kilometres a day in mid-June, and the behaviour of microbes at that ocean depth.

Over the past months, scientists have worried that oil-eating bacteria would consume massive amounts of oxygen in their oil feast and create lethal zones for marine wildlife.

But the Woods Hole researchers found that deep-sea microbes were degrading the plume relatively slowly, meaning the plume had persisted for some time and could still be drifting in the Gulf. The low temperatures at 1.1 km underwater make microbes about 10 times slower than those at the surface, said researcher Benjamin Van Mooy.

Those findings could challenge federal government estimates that most of the 5 million barrels of oil estimated to have leaked has already been cleaned up or been broken down by bacteria. They also show that hydrocarbons can move into deep marine ecosystems.

It is not known what has happened to the plume, but the scientists intend to go back and take added measurements. The plume’s border was defined and measured by comparing concentrations of oil inside the plume to the lesser background concentrations outside the plume.