Two separate team of scientists have calculated a new estimate of the oil leaked into the Gulf of Mexico since April 20, and even their most conservative estimate makes it the largest spill in U.S. history. According to their findings, between 504,000 and more than 1 million gallons of oil per day have spilled into the gulf, far larger than the original estimate of about 200,000 gallons, the Associated Press reports.

The conservative estimate puts the total amount of oil at almost 19 million gallons. The 1989 Exxon Valdez spill in Alaska was 11 million gallons and had been the largest in U.S. history. The largest spill in the history of the Gulf of Mexico was in 1979 and released 140 million gallons of oil.

The latest attempt to stop the leak has shown signs of success. The heavy drilling fluids injected into the well, the first part of a procedure called “top kill,” has slowed down the flow of oil and gas. If this first step continues to work, BP will inject cement into the well to cap it, but officials cautioned that it is still too early to declare success.

And in Washington, the head of the Minerals Management Service, the agency in charge of drilling oversight, stepped down on Thursday. The agency’s director, Elizabeth Birnbaum, has only held the job since July of last year, but her agency is under heavy criticism for a long history of lax regulation and close ties to the oil industry.

The government has also stopped work on the 33 wells being drilled in the Gulf of Mexico and suspended planned drilling off the coasts of Alaska and Virginia.