Scattered around San Francisco are a total of 177 large cisterns buried beneath the streets. Their presence is visible at many intersections in the shape of large brick circles embedded in the pavement. These cisterns hold the city’s emergency water supply, to be used should the city’s domestic water supply fail in the event of a nasty fire.

San Francisco is the only city in the world to have such a system. The cisterns form an integral part of the city’s Auxiliary Water Supply System that consist—aside from the cisterns—water reservoirs, pump stations, suction connections and fireboats, and was designed to help firefighters deal with emergencies.

A circle of bricks marking the position of a subterranean cistern. Photo: christeli_sf/Flickr