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NEW ORLEANS  For more than three months, Gulf Coast residents and federal officials have asked where the oil spill was headed and how much damage it would deliver.

Now, a new, equally baffling question looms: Where has the oil gone?

The amount of surface oil that has bubbled up from the leaking well at the site of the Deepwater Horizon rig sinking has rapidly shrunk in size since the well was capped 11 days ago, according to the Coast Guard.

Recent flyovers of the spill area spotted only one sizeable oil deposit in the region, down considerably from the large pools of thick, reddish oil that washed into Louisiana's coastal marshes and beaches along the Gulf of Mexico.

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"What we're trying to figure out is: Where is all the oil at?" said retired Coast Guard admiral Thad Allen, the oil response's federal overseer. "There's still a lot of oil that's unaccounted for."

Federal scientists estimate that 126 million to 218 million gallons have spilled into the Gulf since the start of the spill. About 80 million gallons of that has been skimmed, burned off or captured in containment efforts — leaving at least 40 million gallons of crude unaccounted for.

Oil that climbed up 5,000 feet from the site of the well and then traveled more than 40 miles to near shorelines could have evaporated or been gobbled up by tiny microbes in the Gulf that feast on oil, said Aixin Hou, an environmental and pollution assistant professor at Louisiana State University's School of the Coast and Environment.

The Gulf's searing summer heat could also speed up the biodegradation process. Microbes have been known to eat as much as 50% to 80% of oil patches in a few weeks during experiments, she said.

Allen said no one should breathe easy yet. "Less oil on the surface does not mean that there isn't oil beneath the surface, however, or that our beaches and marshes aren't still at risk."

Federal scientists are still trying to determine how much oil may be lingering underwater. The underwater oil is floating through the water column, not embedded on the seafloor, said Jane Lubchenco, head of the National Oceanic and Atmospheric Administration.

The lack of surface oil is being closely followed by the teams of fishers who have been recruited to clean up the oil as part of BP's Vessels of Opportunity program. Many of them have relied on the program for income ever since the spill forced their fishing grounds to close.

Eddie Adams, a Port Sulphur, La., charter boat captain hired in the cleanup effort, said the bayous and marshes around Venice, La., haven't been hit with a rush of fresh oil in several weeks, but the oil still continues to sprout up sporadically.

"I'm telling you, it's not over," he said. "Every time I think it's going away, it pops back up again."