Guinea, Gulf of (gĭnˈē) [key], large open arm of the Atlantic Ocean formed by the great bend of the coast of W Africa. It extends from the western coast of Côte d'Ivoire to the Gabon estuary and is bounded on the south by the equator. The bights of Benin and Biafra belong to the gulf. The exploitation of major offshore oil deposits began in the late 1990s. There are also metal ore deposits. Islands in the gulf include Bioko (formerly Fernando Po), São Tomé, and Principe.