You’ve probably seen the large, brick circles in the pavement at various intersections in San Francisco. These patterns aren’t art installations, leftover bricks from turn-of-the-century house foundations or former streetcar turnaround locations.
These circular patterns identify underground water cisterns that function as emergency water sources in case a major fire breaks out.
There are reportedly 170 to 200 underground cisterns scattered around the city, some of which hold up to 90,000 gallons of water. After massive fires dating back to the mid-1800s destroyed large swaths of the city, followed by the devastating 1906 earthquake-related ones, city planners decided to construct the cisterns for an auxiliary supply. (The one above is at the corner of Dolores & 28th Street, near my home. The cistern below it holds 75,000 gallons of water.) Today, the cisterns are part of our overall emergency water supply that includes water reservoirs, fire boats, suction connections and pump stations.
Here’s a photo of an actual underground cistern (photo courtesy of Robin Scheswohl/San Francisco Public Utilities Commission):
Fire fighters can drop their hoses into the cisterns and tap the water supply if needed.
Curious where the cistern closest to your home is? Here’s a link to the SF Fire Department’s list of cistern locations.