BP has commenced its “top kill” attempt to choke off the oil spewing into the Gulf of Mexico by pumping heavy drilling mud and cement into the breached mile-deep well. If the operation succeeds, BP would then inject cement to seal the well shut. The procedure has previously worked above ground but has never been attempted at such deep sea levels. BP says it hopes to know by later today whether the top kill is succeeding. It’s estimated a 70 percent chance of success, while acknowledging the operation could result in even more breaches of the well, worsening the spill.