Alcatraz Island military magazines and casemates

Alcatraz Island contains 19th-century U.S. Army fortifications that include underground magazines, casemates, and concrete defensive works constructed as part of San Francisco Bay's coastal defenses. These military-era bunkers and powder magazines predate the island's later use as a federal penitentiary and remain visible in the island's historic fabric. Tours run from the mainland allow visitors to see these preserved fortification features. The island's fortifications were consolidated into the larger military defense system of the bay in the late 1800s.