If you’ve been to San Francisco, you’ve probably noticed the mysterious brick circles at many of the intersections. Just what are these, exactly? We set out to discover the reason behind these odd formations and, as it turns out, they’re more than just decorations. 

The purpose of the mysterious brick circles dates back to the 19th century. They marked the locations of huge underground tanks of water that were used to fight fires. There are more than 170 of these water tanks, or cisterns, scattered around the city. According to KQED, some can hold as much water as two swimming pools. 

An ancient Roman Cistern

So why were the cisterns first built? Back in the mid-1800s, San Francisco wasn’t the thriving metropolitan area it is today. In 1848, the city was home to fewer than a thousand people. (Can you imagine San Francisco with so few residents?) By the following year, however, gold was discovered, and the city experienced a huge influx of new residents. By 1849, the city had at least 20,000 residents — that’s 20 times more than it had the previous year. (And we thought the tech boom was drawing a lot of people to Silicon Valley.)

The huge rise in population spurred a building frenzy to meet the new demand for housing. Most of the new homes were built out of wood. During the 1850s, the city experienced six big fires, which were later known as the Great Fires. The city knew it had to act to prevent this kind of devastation from happening again, but there wasn’t a feasible way to lay out water pipes. So, city leaders built 16 cisterns around San Francisco to store water. The first was built in Portsmouth Square and it could hold 12,000 gallons of water.

Over the years, more and more cisterns were built, but the city also began constructing fire hydrants and water pipes. By the end of the 19th century, the city had enough faith in its new firefighting system that the cisterns were no longer needed. 

Photo by Robin Scheswohl

Then, a natural disaster happened that changed people’s minds. When a devastating earthquake struck on the morning of April 18, 1906, much of the city was destroyed - 3,000 people died and 200,000 no longer had homes. Much of the destruction wasn’t even caused by the quake itself, but by the fires that followed. The quake broke many of the water pipes in the city, so most of the fire hydrants - when they were needed most - didn’t work. The cisterns, however, were intact, and they helped firefighters save many neighborhoods around the city. Over the next decade, the city began building and repairing dozens of cisterns, having realized the true value of these underground water tanks.

So turns out these cisterns aren’t just a relic of the past. They’re still there today, ranging from 75,000 to 200,000 gallons in size, according to UpOut. 

The city also recently finished building 30 new ones. They were placed in areas that previously didn’t have any cisterns, or in places where they would be crucial in stopping fires from spreading. Each one cost approximately $1 million to build.

Although the cisterns haven’t been used since the big 1906 earthquake, they’re down there, ready to save San Francisco if the time comes. 

Want to go searching for all these brick circles and cisterns underneath? There’s a handy map to show you where each one is right here.