BP's "top kill" effort has halted the flow of oil from the Mississippi Canyon 252 well, according to U.S. Coast Guard Admiral Thad Allen, national incident commander for the oil spill response. BP has yet to comment, however, noting only that "there are no significant events to report at this time," according to a statement on its Web site.

There's still a lot that could go wrong: the mud now flowing from the leak points could end up hollowing out more of the blowout preventer, ultimately making the spill worse. Or the cement that will be used to cap the well now that the mud has muscled the oil back beneath the seafloor could fail. One thing to watch for is the fountains of mud spewing from the leak sites in the blowout preventer and riser pipe to die down as BP brings the pumping down—that may have already happened.

The Coast Guard, for one, is "cautiously optimistic" about this effort to end the oil spill that has gushed between 12,000 and 25,000 barrels per day since April 20, according to a new estimate from the U.S. Geological Survey. That makes this spill at least as large as the Exxon-Valdez spill in 1989—the previous worst spill in U.S. history. And compare that to the previous estimate of 5,000 barrels per day from the Coast Guard and BP; airborne spectrometer measurements show that as much as 11 million gallons of oil are spread across the surface of the sea alone, not counting plumes beneath the surface. "The spill in the Gulf, which is just heartbreaking, only underscores the necessity of seeking alternative fuel sources," President Obama said during a tour of Fremont, Calif.-based Solyndra, a thin-film solar cell manufacturer backed by a federal loan guarantee on May 26.

Obama will visit the Gulf again on May 28 and ordered today an extension of a moratorium on new offshore drilling in all U.S. waters until the end of the year as well as cancelled planned lease sales. Of course, there are already multiple deep-water wells in the Gulf of Mexico alone, including ones such as Shell's Perdido at roughly 2,450 meters beneath the waves that are nearly twice as deep as the BP blowout. In fact, roughly 9 percent of all U.S. oil production comes from such "ultra-deep water" wells, according to the U.S. Energy Information Administration.

Unfortunately, for the oil already spilled in the Gulf, the U.S. National Oceanic and Atmospheric Administration also anticipates an "active to extremely active" hurricane season this year, with between eight and 14 hurricanes. That could drive oil deep into the wetlands of the Gulf Coast, where it will be almost impossible to remove. And, of course, hurricanes pose their own set of challenges for offshore oil and gas infrastructure, including the risk of a catastrophic leak from a ruptured pipeline as a result of strong underwater currents. At least that's the conclusion of a new study from the Office of Naval Research in Geophysical Research Letters. And that means we won't truly be done with oil spills until we're done with oil.

Image: The mobile offshore drilling unit Q4000 holds position directly over the damaged Deepwater Horizon blowout preventer as crews work to plug the wellhead using a technique known as "top kill," May 26, 2010. The procedure is intended to stem the flow of oil and gas and ultimately kill the well by injecting heavy drilling fluids through the blow out preventer on the seabed down into the well. A nearby vessel sprays sea water near the surface of Q4000 to keep oil and fumes from interfering with operations. U.S. Coast Guard photo by Petty Officer 3rd Class Patrick Kelley.