The San Francisco Bay Area is a region of Northern California built around San Francisco Bay and anchored by the cities of San Francisco, Oakland, and San Jose. As defined by the Association of Bay Area Governments, it comprises nine counties bordering the bay and its connected estuaries: Alameda, Contra Costa, Marin, Napa County, California, San Mateo, Santa Clara, Solano, and Sonoma, plus San Francisco itself. Per the 2020 census the nine-county region held roughly 7.76 million people.
The Bay Area is the geographic setting for the Zodiac Killer case. Several of the attributed attacks occurred within it or in adjacent areas tied to the region — including the Lake Herman Road murders and Blue Rock Springs attack near Vallejo (in Solano County), the Lake Berryessa attack in Napa County, California, and the murder of Paul Lee Stine in San Francisco. Investigating agencies including the San Francisco Police Department, Vallejo Police Department, and Napa County authorities operate within this region.
The Bay Area is noted for its concentration of technology companies, research universities, and progressive politics, and its complex multi-county and multi-agency governance — a fragmentation often cited as a complicating factor in the cross-jurisdictional Zodiac investigation.
Note
This Wikipedia article is a general regional overview and does not itself discuss the Zodiac case; the connections above are drawn from existing Zodiex entities located within the region.