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.