The wall is actually a pile of waste called phosphogypsum, and it encloses a huge pond of acidic water from the company’s Uncle Sam fertilizer complex on the Mississippi River. On Jan. 10, Mosaic, the company that owns the plant, sent a letter to state regulators advising them that the north slope of one of the stacks is moving, and the company is taking immediate action. Mosaic officials say they’ve been pumping process water from the pond, hoping to relieve pressure on the shifting slope. The stack holds an estimated 500 million gallons of acidic water.