Alcatraz Island, San Francisco Bay

Alcatraz Island, byname The Rock, rocky island in San Francisco Bay, California, U.S. The island occupies an area of 22 acres (9 hectares) and is located 1.5 miles (2 km) offshore. The small island was developed with facilities for a lighthouse, a military fortification, a military prison (1828), and a federal prison from 1934 until 1963.