San Francisco Bay, 50 mi (80 km) long and from 3 to 13 mi (4.8–21 km) wide, W Calif.; entered through the Golden Gate, a strait between two peninsulas. The bay is as deep as 100 ft (30 m) in spots, with a channel 50 ft (15 m) deep maintained through the sandbar off the Golden Gate. San Francisco is on the southern peninsula; on the northern peninsula are the residential suburbs of Marin co., while on the eastern shore of the crescent-shaped bay are such industrial cities as Alameda, Oakland, Berkeley, and Richmond. The Santa Clara Valley, part of a great depression paralleling the coast, is the landward extension of the bay. Angel Island, Alcatraz, and Yerba Buena Island are in the bay. With San Pablo Bay and Suisun Bay, the natural harbor of San Francisco Bay is one of the best in the world. On those secondary bays and on Carquinez Strait, which connects them, are the cities of Vallejo, Benicia, Martinez, and Pittsburg. The Bay Area is served by a network of bridges, most important of which are the Golden Gate Bridge, the San Francisco–Oakland Bay Bridge, the Richmond–San Rafael Bridge, and the San Mateo–Hayward Bridge. The Trans-Bay Tube, a tunnel 3.5 mi (5.6 km) long between San Francisco and Oakland, is one of the longest underwater rapid transit tubes in the world. The tunnel was especially constructed to absorb earthquake tremors. Several U.S. navy facilities are located in the region. The English navigator Sir Francis Drake visited the bay in 1579; the Spanish explored it more fully in the late 18th cent.