San Francisco Bay is the artificial and flat Treasure Island, site of the 1939 Golden Gate International Exposition. From the Second World War until the 1990s, both islands served as military bases and are now being redeveloped. Isolated in the center of the Bay is Alcatraz, the site of the famous federal penitentiary. The federal prison on Alcatraz Island no longer functions, but the complex is a popular tourist site. Despite its name, Mare Island in the northern part of the bay is a peninsula rather than an island. San Francisco Bay is thought to represent a down-warping of the Earth's crust between
Alcatraz Citadel coves and bay. In 1882, the citadel was enlarged to provide more accommodation for the families of the military officers working on Alcatraz. Between 1873 and 1895, 32 Native Americans were imprisoned at the citadel on Alcatraz, including 19 Hopi men held in captivity there between January and August 1895 after being transferred from Fort Defiance. The island continued to develop in the 1880s and in 1898 the population of Alcatraz increased dramatically from 26 to over 450 due to the Spanish–American War and placed a demand for new buildings. The original barracks evolved into Building 64 in 1905. The