Scientists are finding enormous oil plumes in the deep waters of the Gulf of Mexico, including one as large as 10 miles long, 3 miles wide and 300 feet thick. The discovery is fresh evidence that the leak from the broken undersea well could be substantially worse than estimates that the government and BP have given.

The plumes are depleting the oxygen dissolved in the gulf, worrying scientists, who fear that the oxygen level could eventually fall so low as to kill off much of the sea life near the plumes.

Dr. Joye said the oxygen had already dropped 30 percent near some of the plumes in the month that the broken oil well had been flowing. “If you keep those kinds of rates up, you could draw the oxygen down to very low levels that are dangerous to animals in a couple of months,” she said Saturday. “That is alarming.”

Scientists studying video of the gushing oil well have tentatively calculated that it could be flowing at a rate of 25,000 to 80,000 barrels of oil a day. But the government, working from satellite images of the ocean surface, has calculated a flow rate of only 5,000 barrels a day.

The undersea plumes may go a long way toward explaining the discrepancy, suggesting that much of the oil emerging from the well could be lingering far below the sea surface.

working,” Doug Suttles, BP’s chief operating officer for exploration and production, said Saturday after flying over the area above the oil well. “The oil in the immediate vicinity of the well and the ships and rigs working in the area is diminished from previous observations.”

Many scientists had hoped the dispersants would cause oil droplets to spread so widely that they would be less of a problem in any one place. If it turns out that is not happening, the strategy of using the chemicals could come under greater scrutiny. Dispersants have never been used in an oil leak of this magnitude a mile under the ocean, and their effects at such depth are largely unknown.

Much about the situation below the water remains unclear, and the scientists stressed that their results were preliminary. Interviewed on Saturday by satellite phone, one researcher aboard the Pelican, Vernon Asper of the University of Southern Mississippi, said the shallowest oil plume the group had detected was at about 2,300 feet, while the deepest was near the seafloor at about 4,200 feet.

Dr. Joye said the findings about declining oxygen levels were especially worrisome, since oxygen is so slow to move from the surface of the ocean to the bottom. She suspects that oil-eating bacteria are consuming the oxygen at a feverish clip as they work to break down the undersea plumes.

While the oxygen depletion so far is not enough to kill off sea life, the possibility looms that oxygen levels could fall so low as to create large dead zones, especially at the seafloor. “That’s the big worry,” said Ray Highsmith, head of the Mississippi center that sponsored the mission, known as the National Institute for Undersea Science and Technology.

“This is a new type of event, and it’s critically important that we really understand it, because of the incredible number of oil platforms not only in the Gulf of Mexico but all over the world now,” Dr. Highsmith said. “We need to know what these events are like, and what their outcomes can be, and what can be done to deal with the next one.”