It has now been more than 50 days since the BP oil spill began in the Gulf of Mexico, and there are still a million unanswered questions. How much oil is really out there? What will it take to bring it under control? When, if ever, will the wounds heal?
Here is a list of unknowns compiled by ABC News. You doubtless have many other questions, and we invite your comments below.
1. We don't know how much oil has escaped into the Gulf of Mexico.
How much is escaping daily? Is it 12,000-19,000 barrels per day, the lower-range estimate offered by the Interior Department last week? Or more like 25,000 barrels per day, as later suggested? Did BP's "cut and cap" effort successfully reduce the flow -- or let more oil out?
"This whole thing is frustrating," said Coast Guard Adm. Thad Allen, interviewed Tuesday by ABC's Diane Sawyer. The rate of escape was being estimated from BP's video of the well site, he said, and the estimates were unreliable.
2. We don't know how long the oil will keep coming.
Most engineers contacted by ABC News say they have confidence that BP's relief wells -- expected to be drilled by August -- will divert the flow from the blowout site if nothing else works before then.
"The size of the reservoir and its pressure are the two important variables," said Steven Wereley, a mechanical engineer from Purdue who has been working to estimate the size of the flow. "One important data point is the Gulf of Mexico Ixtoc well blowout in the 1970s ran unchecked for 10 months."
3. We don't know where the oil is really going.
Satellite images show oil on the surface of the Gulf -- but Paul Montagna, a professor of ecosystem studies at Texas A&M University in Corpus Christi, said there may be plenty of oil we never see. Video from the damaged well, he said, shows the oil escaping at such pressures that it may break into tiny blobs that stay deep in the water.
"There's no indication it's getting to the surface at all," he said. "It's affecting the entire food web now. It's not just oiling birds."
4. We don't know who's really in charge.
There have been many major accidents in modern U.S. history, and each time there is confusion over who takes charge of the recovery effort. Who's in charge in this case? BP? The White House? The Coast Guard?
"There are no easy answers," Allen said. "Some of the stuff is novel and being done for the first time."
A government flow chart, dated June 6, showed 13 federal agencies involved. Regulations put in place after the Exxon Valdez accident in 1989, Allen said, put the "responsible party" in charge of the cleanup effort. That was intended to make sure BP, not the taxpayers, are footing the bill -- but it also means the chain of command is far less clear.
President Obama was asked about it on Monday. "I don't sit around just talking to experts because this is a college seminar," he said in an interview with NBC News. "We talk to these folks because they potentially have the best answers, so I know whose ass to kick."
5. We don't know what the 2,600 vessels involved in the response effort are doing.
On Monday, BP sent a message via Twitter that "more than 2,600 vessels are now involved in the response effort" -- though the company later acknowledged that only 115 of those boats were actually skimmers, getting oil out of the water.
Allen, asked by Diane Sawyer why more civilian boats are not being used to corral floating oil, said it was a complicated process to find the right "vessels of opportunity" for the right parts of the job, train the people on board, get them to the right places -- and, he said, get BP to coordinate them.
6. We don't know the long-term consequences of the dispersant chemicals used to break up oil.
In May, the Environmental Protection Agency gave BP 24 hours to find a "less toxic" chemical than Corexit, the dispersant it was using, and 72 hours to begin using it. The EPA said testing had determined that the use of the dispersant Corexit had killed up to 25 percent of all organisms living at 500 feet below the surface in areas where the dispersant was used.
Doug Suttles, BP's chief operating officer, said the company tried. "Right now we cannot identify another product that is available that's better than Corexit."
7. We don't know what shorelines may be affected.
The National Center for Atmospheric Research did a computer simulation of the spill, which showed the oil would likely remain stagnant in the northern Gulf of Mexico for about two months -- but then get caught up in the Loop Current that sweeps around the southern tip of Florida and out into the Atlantic.
That's not much comfort to the people and businesses along the Gulf Coast. Summer vacationers are canceling beach trips by the thousands, but in some places the fear of oil may be worse than the slick itself.
8. We don't know if artificial barrier islands will do more harm than good.
This idea was brought up publicly by Louisiana Gov. Bobby Jindal, who wanted to build giant berms to protect the fragile wetlands along his state's southern coast. The federal government said it worried that they would disturb that ecology of the marshlands so important to the Gulf's coastal economy. Wetlands are a natural barrier for coastlines; why build a barrier to protect a barrier?
Jindal replied that something needed to be done quickly. Last Friday BP set up a $360 million escrow account to pay for them.
9. We don't know what a hurricane might do if it passes over the spill.
The odds of a direct hit are actually pretty small -- even if, as private and government forecasters say, 2010 turns out to be a worse-than-average hurricane season. But a storm passing over the slick could make a mess, both of the oil and of the cleanup efforts.
"It may change significantly where the oil is located," said Tony Barnston, lead forecaster at the International Research Institute for Climate and Society at Columbia University in Palisades, New York. "It would definitely spread it around both vertically and horizontally. It would break it up, it would mix things up."
Already, the weather has affected the surface slick: "Right now, the main spill has been pretty fragmented, and that's partly due to the almost daily, very intense thunderstorms hitting the area," said Hans Graber, professor of marine physics at the University of Miami.
10. We don't know enough about controlling oil spills.
When oil executives protest that major spills are rare, that's true enough -- but a major spill is a national crisis, even for people far away from the accident zone. The Exxon Valdez affected decisions about oil drilling for years.
And when the Deepwater Horizon caught fire, those in charge concede they were unprepared for the confusion, red tape and sheer difficulty of controlling the oil.
"I'm on the road a lot," said the Coast Guard's Allen. "It's not easy to go from town to town and see the level of frustration and anger of the American people out there."
John Wetenhall contributed reporting for this story.