


Quick answer: The brick circles show the outlines of underground cisterns of water that can be used to fight fires even when the city's regular water supply is broken.

The details…

First, I'm impressed that so many people recognized San Francisco, the City by the Bay, from the pictures I showed. I guess it's not a completely wild guess, given where I work and those hills in the background, but still—that's impressive deduction!



[ brick circles ]




[ brick circles San Francisco ]

There are a number of sources that give various versions of the answer. Let's talk about those versions for a moment.



The SF fire department claims there are 172 cisterns on this page—but in other SFFD pages, they say the number of cisterns is 177. Go figure. I imagine there's a master map someplace with all of the cisterns (see below), but bear in mind that the city is building more cisterns even now (see the Fire Commission meeting minutes from 2011) so the number COULD be varying!

Are the cisterns pressurized? Many readers gave solutions that commented on the cisterns being pressurized. They could be, but keeping large underground tanks pressurized for more than 100 years would be an engineering challenge. (Especially in a place like San Francisco, which has its fair share of earthquakes.) What's more, since the cisterns were built in response to the great earthquake and fire of 1906, the engineers realized that they couldn't rely on having power or intact water lines throughout the entire city. This led to the design of the cisterns as more-or-less stand-alone reservoirs. As this page from the SF fire department illustrates, there are actually 4 different water supplies for fire fighting in San Francisco.



And finally,


(4) Cisterns—these are the big tanks of water (usually around 75,000 gallons each) that are under key street intersections. They are unpressurized, but the water can be pumped out through the green-topped hydrants. On the site SFFD says "…the cisterns have no connection to either the Water Department or high pressure supply system. They are under continual inspection by the Fire Department and are kept full by the Bureau of Engineering and Water Supply." That is, they go around every so often and pour water in them, usually from the low-pressure system.



From SFgov2.org report on the Auxiliary Water Supply System Study, January 23, 2009.

For some truly beautiful photos of the cisterns, see: untappedcities.com/2012/06/29/cisterns/

Search Lessons: READ CAREFULLY! Several readers passed along information that the cisterns are part of the pressurized system. I looked at all of the references that readers gave, and I can see why you might make that inference. But if you read carefully, in most cases you'll see it's not actually implied… but merely a co-occurrence of a discussion about the "pressurized system" next to the discussion about cisterns. Careful not to overread. Pay attention to where and when the topic changes. (And, of course, always look for a non-copied confirmation in a second source.)



Search on!


Answer: What, why, where are these stone circles? | SearchReSearch
