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.