Gulf of Mexico dead zone is

The largest dead zone ever recorded in the U.S. has appeared at the mouth of the Mississippi River. According to scientists, it's primarily caused by fertilizer and sewage that wash off farmland in the river's watershed and eventually make their way to the sea. Scientists announced this month the dead zone measures nearly 9,000 square miles - about the size of New Jersey. Underwater video recorded Tuesday afternoon shows the transition from life to death as green fades to black. It becomes so dark, divers need flashlights to find their way around. The abyss stretches over an enormous portion of the Gulf. This is the largest one we've ever measured. And the northern Gulf of Mexico dead zone is the second largest human-caused dead zone in the ocean," said Nancy Rabalais, the nation's foremost expert on dead zones. She's been measuring oxygen levels in the Gulf since 1985. Dead zones happen when agricultural runoff sends nitrogen-rich fertilizer downstream into the sea. The fertilizer feeds harmful amounts of algae at the surface that eventually die and sink to the bottom. Bacteria feast on the dead algae, removing oxygen from the water. "The solution lies upstream in the watershed with better agricultural management practices - a switch to crops that have deeper roots and don't need as much fertilizer," Rabalais said.