(CNN) – Dozens of ships are steaming toward the site of the oil spill in the Gulf today. In two days, they're expected to begin a "top kill" operation to plug the spewing well. Meanwhile, the debate over dispersants is raging. The EPA has ordered BP to switch to a less toxic product, but the oil giant is refusing. We've seen the damage that's been done on shore and on the ocean surface, but now we're bringing you exclusive pictures of the slick below sea level.