Five years ago this week, the Deepwater Horizon oil rig exploded, killing 11 workers and setting off the worst oil spill in US history. The images are unforgettable: The Gulf of Mexico on fire. Pelicans emerging from the water entirely covered in thick, black oil. Planes flying overhead, spraying more than a million gallons of an oil-dispersing chemical called Corexit in an attempt to control the spill.