“So if you think about it, that's about the cost of a really nice new home,” says Miller. “It’s about the size of a new home, too. It can be a nice bunker for somebody."

But why does the city need these underground tanks to fight fires? For that question, Scott Kildall is the one to ask.

He’s an artist who has lived in San Francisco for years, and he designed a map with the locations of each cistern. This mapping process turned him into a sort of cistern history buff.

“I’m not a historian,” he says, “but I’ve become a historian about cisterns.”

Kildall begins the story in 1848, when San Francisco was a bunch of tents, housing a little less than a thousand people. Then, in 1849, gold was discovered and thousands rushed into the city. In just one year, the city grew to about 25 times its size, to at least 20,000 residents.

Demand for housing skyrocketed and a building frenzy followed. Most homes were built out of wood, but the wood made for perfect kindling. In the 1850s, the city saw six big fires, which came to be known as the Great Fires.

The devastation from the fires spurred the city to act.

"People are like, 'Oh no, what are we going to do. We got to do something about this,' " Kildall says.

But at that time, he says, there was no way to lay out water mains and pipes. So, city leaders built 16 underground cisterns around San Francisco to store water for firefighting. The very first was a 12,000-gallon cistern in Portsmouth Square.

They also bought some fire engines pulled by horses, and set up a paid position for a fire chief. There was even an ordinance passed that required each family to have six buckets of water in their house -- just in case.

As the city grew, more cisterns were installed. But the city also started to install water pipes and hydrants. By the end of the 1800s, residents had full faith in their new water system, and the cisterns stopped being maintained.

But then came the massive 1906 earthquake.

It struck on the morning of April 18, and shook the city for about a minute. Windows were smashed and chimneys caved in. Even a train was thrown off its tracks. The damage was incredible. Three-quarters of the city was gone, 3,000 people died and about 200,000 were left homeless.

But most of the damage wasn’t from the earthquake itself -- it was from the three days of fires that followed. Since the quake broke a lot of the city’s water pipes, most fire hydrants quickly ran out of water or stopped working entirely.

But a lot of the old cisterns remained intact, and firefighters used them to save several San Francisco neighborhoods.

San Franciscans now realized their value, and they built and repaired over a hundred within the next several years.

"You can see that people didn’t think they’re actually useful until they were useful, and they said, 'Oh that’s a really good idea,' ” Kildall points out.

Kildall offered to take me on a tour of the cisterns in San Francisco's Chinatown. We visited about 10 cisterns, but not all of them had the brick rings. Kildall didn't know why, but Deputy Fire Chief Tony Rivera took a guess.

"The roads have been repaved so many times, there probably is an original brick circle somewhere deep under there," says Rivera.

Rivera is a fan of the red brick circle, although he says they come in two other designs: a double circle and a square. The designs aren't as important anymore for helping firefighters find the cisterns. Now, he says, they use GPS. So these days, the circles just serve as decoration.

What do you wonder about the Bay Area, its culture or people that you want KQED to investigate? Ask Bay Curious.

As for the cisterns underneath those designs, they haven't been used since the 1906 earthquake more than a century ago. But they are down there, just in case, ready to save the city when the next earthquake strikes.

As part of Kildall's mapping project, he designed a 3-D cistern model. You can 3-D print your own by downloading his file here.