San Francisco Bay is an estuary, where salt water and fresh water mix to form a rich and unique ecosystem that benefits fish, wildlife and people.
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The largest bays have developed through plate tectonics. As the super-continent Pangaea broke up along curved and indented fault lines, the continents moved apart and left large bays; these include the Gulf of Guinea, the Gulf of Mexico, and the Bay of Bengal, which is the world's largest bay.