What is this metal pole in my backyard?
I really like &quot;what is this old thing left from a previous time&quot; questions.
This is just a guess, but here it is.

https://www.rockridgewindmills.com/photo-video-gallery/early-california-windmill-photos/#rockridgewindmills-1/0/Redwood_Windmill_Water_Tower.jpg
What you have left was once the center pole from a water pumping windmill.  You must imagine the windmill and the wood frame, and the water tank that was once filled by this windmill.  But a windmill and a water tank in the Bay area seems right, from when this was still an agricultural area.