One of the largest urban parks in the world, Golden Gate National Recreation Area (GGNRA) welcomes over 17 million visitors a year. The park is as diverse as it is expansive; it contains attractions such as Alcatraz Island, Crissy Field, the Marin Headlands and Rancho Corral de Tierra. GGNRA also includes significant historical and natural resources and houses the largest museum collection in the National Park Service. Over half of North American avian species and nearly one third of California's plant species are found in the park.