text: The note makes sense to me.  SF has a system of emergency cisterns built after the 1906 earthquake.  They're fairly large, and marked by brick circles/rectangles embedded in the street above.  Some have been mapped; most are still unmapped.
  
  Here's an article I found that has photos of this particular cistern under construction: http://www.sfchronicle.com/bayarea/article/Auxiliary-water-system-protects-S-F-against-6574933.php#photo-8796904 
  
  The aerial imagery doesn't appear to be up to date enough to see it yet.