An untested procedure to plug the blown-out oil well in the Gulf of Mexico seemed to be working, officials said Thursday, but new estimates from scientists showed the spill has already surpassed the Exxon Valdez as the worst in U.S. history. A team of scientists trying to determine how much oil has been flowing since the offshore rig Deepwater Horizon exploded April 20 and sank two days later found the rate was more than twice and possibly up to five times as high as previously thought. The fallout from the spill has stretched all the way to Washington, where the head of the federal agency that oversees offshore drilling resigned under pressure Thursday. Even using the most conservative estimate, the new numbers mean the leak has grown to nearly 19 million gallons over the past five weeks, surpassing the size of the 1989 Exxon Valdez disaster in Alaska, which at about 11 million gallons had been the nation's worst spill. Under the highest Gulf spill estimate, nearly 39 million gallons may have leaked, enough to fill 30 school gymnasiums. A researcher surveys oil floating on the surface of the Gulf of Mexico near the coast of La., Wednesday, May 26, 2010. Reporter Anderson Cooper is reflected in oil filled water during a tour of areas where oil has come ashore May 26, 2010 in Blind Bay, La. BP crews clean oil off of the beach at Port Fourchon, Louisiana, Monday, May 24, 2010. Oil was first spotted here on Wednesday, May 12.