What are those big brick circles in the intersections all around the city?

A. Urban crop circles

B. Masonic fairy rings

C. Last ditch backups of the city’s fire-fighting system

D. Part of a city-wide connect the dots game

As much as I wish it were D, the answer is actually C. When I first moved to San Francisco, I noticed the brick circles here and there and just figured they were old roundabouts from the horse and buggy days. I learned what they really are a couple years ago on a walking tour of the city’s historical waterways called Thinkwalks. I promise that experience was as nerdy as it sounds. (On that same tour, I learned what those little spray painted dots on all the storm drains were too.)

The brick circles mark the locations of 177 underground cisterns, or artificial water reservoirs. The original few were built as early as 1860 and were made of brick. Those have since been refurbished to match the newer reinforced concrete ones built in the years following the 1906 earthquake. The brick circles are no longer part of the actual structures but remain as a sort of ornamental marking. You can also identify the cisterns by the two manhole covers at each site bearing the label, “CISTERN S.F.F.D.”

The 30’ markers are hard to miss when you’re driving over them, but a lot of people never bother to think about their actual purpose. They’re scattered around the city in a fairly lopsided arrangement, loosely correlating to the most densely built areas. The greatest concentration of cisterns can be found in the Financial District, with 31 in just a half mile radius, while the entirety of the Sunset boasts a grand total of seven. The Outer Richmond, in classically pointless San Francisco form, has squares instead of circles. Some of them don’t have any bricks at all, but you can tell there’s one nearby if you see a hydrant with a green cap.

So what exactly is the point of these oversized water jugs? It seems the city’s planners kept the hard lessons learned in 1906 close to heart. We currently have 44 fire stations, two independent water supply systems with a few thousand hydrants covering the city (each at a cost of up to $20,000, by the way), a mobile pumping vehicle with a mile long hose, two fireboats, 52 sea water suction points, and the cisterns — which act as an ultimate back up, holding an approximate total of 11 million gallons of water. Each one individually holds between 75,000 and 200,000 gallons. That’s roughly half of an olympic swimming pool per cistern.

While the cisterns haven’t been used to fight fires since 1906, this systematic redundancy has done some good in more recent times. After the low-pressure domestic water system was damaged in the ’89 Loma Prieta earthquake, the high-pressure auxiliary system kept the Marina from being burned off the peninsula. A few pipes cracked and some systems went down at that time, but the backups did the job and kept the fire from spreading too far. I happen to live up the street from a big square cistern in The Avenues, and even though it probably won’t save any of my flammable belongings anytime soon, it does make me feel better knowing it’s there.