The chemical dispersant being dumped into the Gulf by the hundreds of thousands of gallons is called Corexit 9500, manufactured by a publicly traded outfit called Nalco. Here's a Nalco corporate video demonstrating how Corexit works.
Notice the color of the post-dispersant water at 1:45. The water quality went from clear with a sheen of oil to cloudy and gray following the dispersant. Lovely. The droplets of oil that are formed in the process appear to be what's causing the gigantic plumes of oil under the surface of the Gulf.
Meanwhile, no one knows the level of harm these chemicals will do at this volume. No one. If they do, they're not saying.
The reasoning behind using the chemical is that it's "less toxic" than the oil. But the oil isn't disappearing. It's just being spread around. So they're not making the oil vanish from our plane of existence like magical chocolate milk. The oil is still there, but now it's mixed with hundreds of thousands (if not millions) of gallons of Corexit 9500, which is a total question mark in terms of its long-term impact. In other words: oil, now with extra poison!
Remember what Shannyn Moore tweeted:
During the 1989 Exxon spill Alaskans called Corexit 9527 (dispersent) HIDES IT 9527.
Again, the oil doesn't go away. Even the stupid corporate video shows the disgusting water with the oil all spread all around. That's not good.