“Ocean deserts” or “dead zones” are oxygen-starved (or “hypoxic”) areas of the ocean. They can occur naturally, or be caused by an excess of nitrogen from agricultural fertilizers, sewage effluent and/or emissions from factories, trucks and automobiles. The nitrogen acts as a nutrient that, in turn, triggers an explosion of algae or plankton, which in turn deplete the water’s oxygen.

According to the Ocean Conservancy, a dead zone in the Gulf of Mexico—where the Mississippi River dumps untold gallons of polluted water every second—has expanded to over 18,000 square kilometers in the last decade. Many other such dead zones have also undergone rapid expansion in recent years.