Engineers for BP oil have started pumping heavy mud into the leaking undersea well head in the Gulf of Mexico. They say everything is going as planned in the company's risky attempt to halt the spewing of millions of gallons of oil. Chief executive Tony Hayward said it would be at least 24 hours before officials know whether the attempt has been successful. The company wants to eventually inject cement into the well to permanently seal it. The stakes are high - with 160 kilometers of Louisiana's shoreline already affected.