Although BP officials won't know for sure until later this afternoon at the earliest, at least one BP spokesman says it appears drilling mud is now spewing from the ruptured pipe and the oil flow has been significantly slowed. This "Top Kill" procedure that started Wednesday would pump mud into the failed blowout preventers' choke and kill valves. If the mud can pump in at a higher pressure than the oil coming out, it should overcome the flow of oil. If the oil is stopped, or slowed enough, workers couldthen pump in concrete to permanently cut the flow.

Even if the underwater view show mainly mud spewing out, it won't be until later today when workers can assess whether or not oil is still coming to the surface. That will be the proof of the procedure working or not. Engineers are monitoring pressure readings to determine how much oil is escaping.