Dead Zones are places were pollution, primarily fertilizers, at first causes a boom in algae and other aquatic plant life. The algae then dies releasing massive amounts of carbon dioxide as it decays. The shift from oxygen in the water to carbon dioxide kills water life and creates a zone where nothing can live. Besides the obvious ecological damage done, massive economic damage occurs also because the area is no longer productive to commercial fishing and tourism.
A new study has mapped dead zones and declares there are four hundred five such areas. The zones are found primarily close to coasts were runoff and river currents stall after entering the ocean. The reason for an increase both numerically and spatially from sixty or so in the 1960s is because of increased industrialization and heavy farming around the world.
Right now it seems dead zones are a byproduct of progress. Feeding more people requires fertilizer and thus runoff into the oceans is almost bound to increase. Hopefully we can find a way to farm for such a populous world while preventing such harmful happenings.