California, Gulf of, or Sea of Cortés, arm of the Pacific Ocean, c.700 mi (1,130 km) long and 50 to 130 mi (80–209 km) wide, NW Mexico; separates Baja California from the Mexican mainland. The gulf is part of a depression in the earth's surface that extends inland to the Coachella Valley, S Calif. The Imperial Valley and the Salton Sea, once part of the gulf, have been cut off from it by the growth of the Colorado River delta. The gulf deepens from north to south; its greatest depth is c.8,500 ft (2,590 m). The coastline is irregular, with numerous islands; Tiburon, inhabited by aboriginal tribes, is the largest. Storms and tidal currents hinder navigation in the gulf. Once a rich commercial and sport fishing ground, the gulf now suffers from overfishing. The region is a developing tourist center; La Paz, Guaymas, and Mazatlán are major cities. The area was first explored in 1538 by the Spaniard Francisco de Ulloa.