There are an estimated 50 "Dead Zones" in the world's oceans. Dead zones can be natural events, but most occur when polluted water drains out of cities and off farms and ends up concentrated in part of the ocean. The pollution triggers a chain of events that sucks the oxygen out of this water. The biggest Dead Zone in the Western Hemisphere develops every year at this time in the waters of the Gulf of Mexico, near the mouth of the Mississippi River. NPR's John Nielsen reports.