BP's latest strategy to stanch the spill is known as "top kill," which involves pumping a heavy drilling fluid into the well lines before cementing it shut:
[BP executive Doug Suttles] said the biggest risk is the possibility that the drilling mud would be forced from the wellhead into the water instead of forcing the flow of oil and natural gas back down into the wellbore. If that occurs, Suttles said the company could then deploy what it calls the "junk shot," using tire shards, golf balls and other odd-sized debris to force it back. "That is one of the options we would have available," Suttles said. "If we saw the right conditions and felt like that would be the right next step." He said BP decided to use top kill prior to the junk shot for fear that the junk shot might clog the lines and eliminate the top kill option altogether.
If that fails, BP will try placing another containment device over the leak. Andrew Revkin thinks they shouldn't get to that point:
To my mind, if the “top kill” procedure being prepared for [Wednesday] fails, Obama must step forward far more forcefully and publicly engage an oil-well SWAT team drawing on the country’s leading lights in hydraulics, deep-ocean engineering and geology, from the Pentagon outward.