Crews from the National Park Service are dumping gallons of hydrogen peroxide into the Lincoln Memorial Reflecting Pool to combat algae that has grown in the pool after its multimillion-dollar makeover.

The reflecting pool has historically dealt with algae growth issues because of the hot and muggy Washington, D.C., climate in the summer, the pool’s shallow, relatively stagnant build, and its direct exposure to sunlight. The pool turned a bright green hue this week due to algal buildup, marking a sharp contrast from the “American Flag Blue” that President Donald Trump had the bottom of the pool coated with.