United States Penitentiary, Alcatraz Island, also known simply as Alcatraz or The Rock, was a maximum security federal prison on Alcatraz Island, 1.25 miles (2.01 km) off the coast of San Francisco, California, United States. The site of a fort since the 1850s, the main prison building was built in 1910–12 as a U.S. Army military prison.

Alcatraz Island is a small island 1.25 miles (2.01 km) offshore from San Francisco, California, United States. The island was developed in the mid-19th century with facilities for a lighthouse, a military fortification, and a military prison. In 1934, the island was converted into a federal prison, Alcatraz Federal Penitentiary. The strong currents around the island and cold water temperatures made escape nearly impossible, and the prison became one of the most notorious in American history. The prison closed in 1963, and the island is now a major tourist attraction.