1. THE DEAD ZONE. (Photo courtesy of NASA.) There are now many Dead Zones around the world -- parts of the ocean that can become so oxygen starved that they are almost sterile.

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Every summer, 7,000 square miles of the Gulf of Mexico becomes a Dead Zone. What causes this phenomenon? Oxygen in the Gulf's Dead Zone is consumed by the decay of massive algae blooms, which are fertilized by tons of fertilizer, sewage and animal waste that washes into the Gulf from polluted rivers. Most creatures -- except for throngs of jellyfish which thrive in polluted waters -- either flee the Dead Zone or suffocate. For more information about jellyfish in the Dead Zone, see Lily Whiteman's article, The "Blobs of Summer" which appeared in OnEarth -- the magazine of the Natural Resources Defense Council.