Just off the coast of Louisiana and Texas where the Mississippi River empties, the ocean is dying. The cyclical event known as the dead zone occurs every year, but scientists predict that this year's could be one of the largest in recorded history.

Annual spring rains wash the nutrients used in fertilizers and sewage into the Mississippi. That fresh water, less dense than ocean water, sits on top of the ocean, preventing oxygen from mixing through the water column. Eventually those freshwater nutrients can spur a burst of algal growth, which consumes oxygen as the plants decompose.

The resulting patch of low-oxygen waters leads to a condition called hypoxia, where animals in the area suffocate and die. Scientists estimate that this