This year's "dead zone" in the Gulf of Mexico is supposed to be a record-breaking 8,500 square miles off the coasts of Texas and Louisiana. Within that area (about the size of New Jersey), there won't be enough oxygen in the water to support life.
Spring runoffs from major US rivers pump massive amounts of farm chemicals and other contaminants into the Gulf waters. These give rise to huge algae blooms. As the algae die they sink to the bottom and undergo decomposition that strips oxygen from the water. Marine life that can't get away simply dies.
Scientists suspect this year's runoff was heavier than normal due to more intensive agriculture to produce biofuels. Great, just bloody lovely!