A worker stands inside a cistern in San Francisco's Mission District. Robin Scheswohl/San Francisco Public Utilities Commission

Subterranean vats were an emergency response to the city being repeatedly and savagely burned to the ground.

Traipse around San Francisco long enough and you’ll notice weird patterns on the streets: bricks arranged in sprawling circles and rectangles, reminiscent of ancient stoneworks or satellite calibration symbols.

But it’s what lies beneath these curious road-tattoos that’s really interesting. Embedded in the earth are colossal cisterns that hold sometimes substantial artificial lakes. Are these lightless pools meant for the swimming pleasure of C.H.U.D.s? In reality, they’re a physical reminder that San Francisco historically has been a tinderbox, and it’s always good to have extra water lying around in case of a nasty fire.

San Francisco’s bad flare-ups kicked off in 1849 when the old Shades Hotel burned down. A few months later a ship went up in flames in the harbor and a fast-moving blaze reduced a hunk of the ‘burg to ashes. The city formed a fire department soon after, but a series of wind-aided “great fires” sprung up in the next several years that terrorized citizens and carbonized acres of buildings.

Robin Scheswohl/San Francisco Public Utilities Commission

One of the ways officials responded to these blazes was to build cisterns. These subterranean vitrines were designed as a last-resort source of agua for firefighting. San Francisco’s 19th-century cistern system was reinforced with more, larger cisterns after the Earthquake of 1906, whose subsequent firestorm killed roughly 3,000 and left much of the city’s land looking like a blasted moon. To date there are 170 to 200 of the tanks stashed around town—the city’s numbers vary quite a bit—functioning as emergency water sources apart from the water mains and suction stations pumping saltwater from the Bay.

Thanks to the work of Mapzen’s John Oram, everybody can explore this semihidden galaxy of cisterns. “I’d long been fascinated by the cisterns and their brick circles of mystery, as I think is everyone who moves to San Francisco,” says Oram, who, indeed, has long covered cistern-related topics on his local news site, Burrito Justice.

Robin Scheswohl/San Francisco Public Utilities Commission

Oram got the idea for his cistern map while helping his buddy Andrew Sullivan research the 44-O'Shaughnessy bus route for a radio segment. They both were struck by the vast amount of infrastructure, including underground structures to fight fires, created in the early 1900s under City Engineer Michael O'Shaughnessy.

“It began to dawn on us just how much Michael O'Shaughnessy had planned and built in San Francisco as City Engineer: tunnels, boulevards, the Auxiliary Water Supply System for the San Francisco Fire Department, and the system of new, bigger firefighting cisterns to supplement the ones built in the 1850s after the city nearly burned down six times in a year and a half,” Oram says.

After pulling data from a previous cistern project by San Francisco artist Scott Kildall, Oram used Mapzen’s Tangram Play tool to plot 170 buried, and often colorfully decorated, cisterns. Zooming in reveals their cross-streets and volumes, ranging from less than 10,000 gallons to a supersized 243,000-gallon one under downtown’s Civic Center.

“The cistern at Civic Center is huge!” says Oram. “It’s also amazing how you can readily see the original [smaller] ones built in the core of the city after the fires in 1850 and 1851.”

It’s also apparent where people resided when the city starting building bulked-up ones in the early 1900s—meaning, not in the western areas. “Few people lived out in Richmond and Sunset then,” he says. “It was mainly sand dunes until the 1920s and 1930s. Sunset really didn’t get developed until O'Shaughnessy built the Twin Peaks Tunnel,” which opened in 1918.

Make sure to check out the full-page map, where you can see new data appear as you zoom in.

Robin Scheswohl/San Francisco Public Utilities Commission

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