Alcatraz Island (urban island in San Francisco Bay)

Alcatraz is a rocky island in San Francisco Bay that lies within the city's jurisdiction and is best known for the former federal penitentiary that operated there from 1934 to 1963. Geologically it is a resistant rock outcrop; ecologically it supports dense seabird colonies and a mix of native and introduced plants unusual for an island so close to an urban shoreline. The island's isolation within the bay has made it a prominent example of island biogeography and of an offshore landform preserved amid urban development.