Red Rock Island, visible on San Francisco Bay Ferry’s Vallejo route, is the only privately owned island in the San Francisco Bay. It can be found just south of the Richmond-San Rafael Bridge. Northbound ferries destined for Vallejo come closest to the island, though you can also see it on southbound trips off the port side of the ferry on clear days.
Because Red Rock Island was established as a triangulation point by surveyors in 1850, it hosts three different counties. Four acres of the island are in Contra Costa County, 1.5 acres are in San Francisco County, and less than a tenth of an acre is in Marin County. The Contra Costa portion of the island is within Richmond city limits, so the cities of San Francisco and Richmond technically share a land border on Red Rock Island.
So how much does a private island in the San Francisco Bay go for? According to news reports, in 2012 the owners listed the island for $22 million, but later reduced the price to $9 million. Before the island could be sold, the owner passed away and no sale was made. Today, it is reportedly owned by his son who lives in Alaska.