But for a stretch of Namibia's coastal waters, they can also be caused by plumes of hydrogen sulfide gas in a phenomenon scarcely seen anywhere else in the world.
A combination of scant oxygen in the water and a large amount of decaying organic material makes that part of southwestern Africa’s shores prone to the world’s largest “belches” of the gas most commonly associated with the smell of rotten eggs.
As tiny single-celled diatoms fall to the ocean floor off Namibia, they are decomposed by a type of bacteria in the muddy sediment. That process produces huge amounts of hydrogen sulfide gas.
When various influences cause the gas to rise, a chemical reaction removes oxygen from the water, suffocating large numbers of fish and other marine creatures.
But the deaths are a boon to birds that feed on their carcasses.
Namibians are also known to feast on the lobsters that crawl onto land in an attempt to escape the oxygen-deprived water.
The image to the upper right of the Namibian coast was captured by NASA’s Aqua satellite on February 29, 2010.
The milky green colors in the ocean indicate high concentrations of sulfur and low concentrations of oxygen. The sulfur reflects sunlight, appearing pale green.
Full story and image: NASA