(CNN) -- Scientists at the Woods Hole Oceanographic Institution said they detected a plume of hydrocarbons in June that was at least 22 miles long and more than 3,000 feet below the surface of the Gulf of Mexico, a residue of the BP Deepwater Horizon oil spill.
According to the institution, the 1.2-mile-wide, 650-foot-high plume of trapped hydrocarbons provides at least a partial answer to recent questions asking where all the oil has gone as surface slicks shrink and disappear.
"These results indicate that efforts to book-keep where the oil went must now include this plume" in the Gulf, said Christopher Reddy, a Woods Hole marine geochemist and oil spill expert. He is one of the authors of the study, which appears in the Aug. 19 issue of the journal Science.
Researchers saw the plume over two weeks in June but were chased away by Hurricane Alex, Reddy told CNN Radio.
"I have no idea where those compounds are now," he said.
Another of the report's authors said the plume has probably moved elsewhere, noting that the BP-operated well has been capped for more than a month and that the plume was moving in a southwesterly direction at a rate of about 6.5 kilometers (4 miles) a day.
"(It's) extremely likely that the hydrocarbons in that plume have long moved elsewhere," report author Rich Camilli told CNN.
Reddy said that experts need more data before they can determine how much remains in Gulf.
Whether the plume's existence poses a significant threat to the Gulf is not yet clear, the researchers say. "We don't know how toxic it is," Reddy said in a statement, "and we don't know how it formed, or why. But knowing the size, shape, depth, and heading of this plume will be vital for answering many of these questions."
Camilli, also a Woods Hole scientist, said colder temperatures at the plume's extreme depths inhibited the degradation properties of oil.
Microbes act more slowly on the subsea oil than on surface oil because of lower temperatures, he said. If all other conditions were equal, microbes would eat up the plume's subsea oil about 10 times more slowly, Camilli said.
Meanwhile, Thad Allen, the government's point man for the oil disaster, responded Thursday on CNN to two recent studies that appeared to contradict the government's estimate that about 75 percent of the oil has been cleaned up.
Researchers at the University of South Florida have concluded that oil may have settled at the bottom of the Gulf farther east than previously suspected -- and at levels toxic to marine life. In addition, a team from Georgia Sea Grant and the University of Georgia released a report that estimates that 70 to 79 percent of the oil that gushed from the well "has not been recovered and remains a threat to the ecosystem," the university said in a release.
Allen said the government has determined the flow rate to have been about 53,000 barrels a day, or a total of 4.9 million barrels.
"The next question is, what happened to it?" he said. "There are certain things we know for certain. We produced almost 827,000 barrels that we collected and brought ashore." The government also knows how much oil was skimmed, how much was burned and how much was affected by dispersant use. When that is added up, it leaves 26 percent still in the water, Allen said.
"That's not a definitive statement, but that's a way to start a conversation about the oil," Allen said. "You can take a lot of different estimates and run that formula, but that's the one we're starting with ... other than the 26 percent, the rest can be accounted for some way. That 26 percent is going to end up on a beach or dealt with somehow."
CNN's Vivian Kuo contributed to this report