Yesterday, BP started an ambitious effort to stop the Deepwater Horizon oil spill with a "top kill"—pumping drilling "mud" into the well at a fast enough rate that it backs up the flow of oil behind the broken blowout preventer and starts to form a plug. Today, the Coast Guard announced that this effort appears to be working. It's too soon to call it an unqualified success, but things are looking good and we may get the leak officially stopped up in the next day or so. That doesn't solve the clean-up issue, but it's a relief to know that something is working.
I've seen a lot of really bad explanations of the top kill process in the news. Frankly, I wasn't entirely clear on how it worked until I read this description on the Oil Drum Blog. It likened the leak to a stopped up sewer line. If you have tree roots growing through your sewer, you can still turn the sink on, and liquid can get through. But if you up the volume—flush the toilet or run the washing machine—everything gets backed up and flows back into your basement. Essentially, the broken blowout preventer is the tree roots, and the drilling mud was used to increase the volume of liquid enough that it couldn't get through the break.
Also, for the record, drilling mud isn't mud like you'll find in your backyard. There's been a lot of confusion on that point, and a lot of the news reports I've seen haven't cleared it up much. Drilling mud is an engineered lubricant. Clay is often one ingredient, but there are many types of drilling fluid that get called "mud" and they're usually a mixture of clays, water and various chemicals. Amusingly—at least for anyone who saw "There Will Be Blood"—drilling mud is supposed to look a lot like a chocolate malt.