An oil spill from a leaking underwater well grew to cover 1,900 square miles of the Gulf of Mexico on Monday as the U.S. Coast Guard scrambled to keep the slick from reaching the fragile Gulf Coast shoreline.

The well, 5,000 feet under the ocean surface off Louisiana’s coast, is leaking about 1,000 barrels of oil a day. The spill, which the U.S. Coast Guard has called “very serious,” has put the coasts of Louisiana, Texas, Mississippi and Florida on alert for potential oil contamination.