San Francisco Bay, an arm of the Pacific Ocean in California and one of the world's finest deepwater harbors. It is nearly landlocked, being connected to the Pacific only by the narrow strait called the Golden Gate. Including San Pablo Bay, its northern arm, San Francisco Bay is about 50 miles (80 km) long and up to 13 miles (21 km) wide. Maximum depth in the bay proper is about 140 feet (43 m), in the Golden Gate about 350 feet (107 m). Ports accommodating large oceangoing vessels are located at San Francisco, Oakland, and Richmond. There are several islands in the bay, including Alcatraz and Yerba Buena.
The Bay Area—especially the eastern side, called East Bay—is a rapidly growing commercial and industrial region. Five major bridges, including the Golden Gate and San Francisco-Oakland Bay bridges, span the bay. The Transbay Tube, a subway tunnel beneath the bay, is between San Francisco and Oakland.
San Francisco Bay may have been entered by Sir Francis Drake in 1579, but most scholars believe he entered another bay, farther north. Actual discovery of San Francisco Bay is attributed to a Spanish land expedition under Don Gaspar de Portolá in 1769 Six years later Don Juan Manuel Ayala sailed into the bay through the Golden Gate.
