Last Updated at 1:09 p.m. EDT
Oil from a blown-out well is forming huge underwater plumes as much as 10 miles long below the visible slick in the Gulf of Mexico, scientists said as BP wrestled for a third day Sunday with its latest contraption for slowing the nearly month-old gusher.
BP, the largest oil and gas producer in the U.S., has been unable to thread a tube into the leak to suck the crude to a tanker. Engineers remotely steering robot submersibles were trying again Sunday to fit the tube into a breach nearly a mile below the surface, BP said.
Special Section: Disaster in the Gulf
Oil Spill by the Numbers
Oil has been spewing since the rig Deepwater Horizon exploded April 20, killing 11 people and sinking two days later. The government shortly afterward estimated the spill at 210,000 gallons - or 5,000 barrels - a day, a figure that has since been questioned by some scientists who fear it could be far more. BP executives have stood by the estimate while acknowledging there's no way to know for sure.
The company has been casting about for ways to contain the leak since it was discovered several days after the blast. First robot submarines were unable to get valves to work on machinery at the well head called the blowout preventer. Then the company failed to capture the oil with a 100-ton box after icelike crystals formed in it.
BP has also been burning small amounts of floating oil and spraying chemical dispersants above and below the surface.
A relief well, considered the permanent solution the leak, is still being drilled and is months away from completion. Until then, the company is also considering a smaller containment dome or trying to clog the leak with golf balls and rubber.
Word of huge submerged oil plumes, meanwhile, raised the specter of more damage to the ecologically rich Gulf. It also adds to questions about when large amounts of crude might hit shore. So far, tar balls have been sporadically washing up on beaches in several states, but oil hasn't come ashore in big quantities.
"It's just a matter of time ... and the first significant amount of oil is going to show up around the U.S," said Hans Graber, director of the University of Miami's satellite sensing facility, who has been tracking the oil slick.
Researchers from the National Institute for Undersea Science and Technology said Saturday they had detected the underwater oil plumes at depths between just beneath the surface to more than 4,000 feet.
Three or four large plumes have been found, at least one that is 10 miles long and a mile wide, said Samantha Joye, a marine science professor at the University of Georgia.
Researchers Vernon Asper and Arne Dierks said in Web posts that the plumes were "perhaps due to the deep injection of dispersants which BP has stated that they are conducting." BP has won government approval to use chemicals on the oil near where it is gushing to break it up before it rises to the surface.
The researchers were also testing the effects of large amounts of subsea oil on oxygen levels in the water. The oil can deplete oxygen in the water, harming plankton and other tiny creatures that serve as food for a wide variety of sea creatures.
Oxygen levels in some areas have dropped 30 percent, and should continue to drop, Joye said.
"It could take years, possibly decades, for the system to recover from an infusion of this quantity of oil and gas," Joye said. "We've never seen anything like this before. It's impossible to fathom the impact."
Joye's lab was waiting for the research boat to return so a team of scientists can test about 75 water samples and 100 sediment samples gathered during the voyage. Researchers plan to go back out in about a month and sample the same areas to see if oil and oxygen levels have worsened.
The latest effort to contain the leak, inserting a mile-long pipe into the largest of two leaks, hit a snag Saturday. BP chief operating officer Doug Suttles said one piece of equipment, called the framework, had to be brought to the surface and adjusted to fit with the tube.
The framework holds a pipe and stopper. If it works, the tube could capture more than three-quarters of the leak. BP also must contend with a smaller leak that's farther away.
One expert said BP's latest idea seems to have the best chance for success so far. Inserting a pipe into the oil gusher would be easy at the surface, said Ed Overton, a LSU professor of environmental studies. But using robots in 5,000 feet of water with oil rushing out of the pipe makes things much more difficult.
"It's something like threading the eye of a needle. But that can be tough to do up here. And you can imagine how hard it would be to do it down there with a robot," Overton said.
© 2010 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.