BP was running a crucial test on Thursday on its ruptured Gulf of Mexico oil well that could stanch the flow of crude that has polluted the ocean and shoreline since April.

BP began the process on Wednesday night, which could stretch up to 48 hours. The British energy giant began the tests after getting the green light from top U.S. government officials who had delayed the plan by 24 hours on concerns the process could irreparably damage the well.