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Chava Kronenberg is on a manhole hunt.

“Oh! Wow! There it is! We have a winner!” she exclaims near the intersection of Geary Boulevard and 5th Avenue in San Francisco.

Kronenberg is a volunteer helping a local artist with a project. She’s biking around the city with her partner for the day, Lucas Keijning. They’ve got a list of the potential locations of a specific kind of manhole. Their assignment is to find as many of these manholes as they can, take a picture, and grab the GPS coordinates using a smartphone app.

When they find one in the middle of a crosswalk, Keijning darts out into traffic and snaps a picture of it.

The manholes they’re hunting look just like any other, but these ones are each surrounded by a ring of bricks about 20 feet across.

“A lot of people have noticed the brick circles around San Francisco,” says local artist Scott Kildall.

Kildall is the one who sent Kronenberg and Keijning out out on their mission. He’s working on a project to make people more aware of city infrastructure. As far as he knows, no one has ever created a complete map of all of these brick rings. So that’s part of his project.

“And once you point them out … you can’t not see them in San Francisco,” Kildall says. “Everywhere you look seems to be these brick circles. And I, like many other people, have wondered, ‘What the hell is underneath those brick circles?’”

What the hell is under there?

“It's a concrete vault, underneath the ground,” says Ken Lombardi, Assistant Deputy Chief with the San Francisco Fire Department.

The vaults are called cisterns – and they’re full of water.

“This is our emergency water supply,” Lombardi explains. “As long as we pull that manhole cover, there'll be 75,000 gallons of water for us.”

The cisterns are totally unconnected from the rest of the water system; there are no pipes running in or out. Almost 200 of them are scattered around San Francisco. Lombardi says this is the only city that has them. To understand why, we need to go back to the gold rush.

The birth of the cisterns

“In those days, San Francisco had several problems,” says San Francisco author Robert Graysmith. Graysmith wrote a book called Black Fire, about six major fires that ravaged San Francisco between 1849 and 1851.

And he’s right. Back then, the city had problems. For starters, it was basically a tinder box. The buildings were all made of wood, old boat parts, canvas – whatever was lying around. There was also no plumbing, and no official fire department. Instead, troupes of volunteer firefighters formed.

“If you were a fireman, you were the elite,” says Graysmith. “Because these volunteers – they were handsome, sexy. They had tight leggings and shiny boots and red jackets. But they risked their lives and ... they did everything themselves.”

The volunteer companies developed intense rivalries. They would race to be the first to the fire, and sometimes get in fist fights along the way. Just getting somewhere wasn’t easy back then. The unpaved streets were often full of mud.

“Literally, to try to cross the street back in 1850, you were taking your life in your hands,” says Graysmith. “There had been scenes where a guy’s driving his wagon with all the horses, and the whole thing gets sucked right down. He's still cracking the whip, and literally they have to lasso people and pull them onto the planks built along the road.”

The volunteer firefighters had to haul 2,000-pound water tanks through these muddy streets, and then fight a raging fire. Even after all that work, sometimes the water tank would run dry before the fire went out.

Some of the fire chiefs got together and begged the city for help. They asked for underground water storage tanks to be built around San Francisco. And the city built about a dozen – the first San Francisco cisterns.

The system modernizes

The fire epidemic stopped after 1851. By the late 1860s, San Francisco had both an official fire department and underground plumbing. The city kept some of the cisterns full, but basically they were old news.

“They were a laughing stock of the city,” says artist Scott Kildall. “People were wondering, ‘Why are we spending any money filling up these banks of water that we're never going to use?’”

And then, the 1906 earthquake struck. Fires broke out and the water mains burst.

“And so the fire department couldn't put out the actual fires all over the city,” Kildall explains. “And at that point, they tapped into these old cisterns that they had been diligently filling up every year, and put out some of the fires in Telegraph Hill and Rincon Hill [to] help save vital portions of the city.”

Despite these few victories, the 1906 fire raged for three days, destroying huge portions of the city. Later, a national fire board declared that if enough water had been available, it most likely could have been put out on the first day. In 1908, the city passed a $5 million bond to fund a backup water supply system, including separate pipes for fire fighting, and about 150 new cisterns.

These systems are still up and running today but, “the reality is that most people don't actually notice the cisterns until you point them out to them,” says Kildall. So that’s what Kildall is trying to do with his mapping project – point them out.

“What I hope people take away from the waterworks project is that there's an amazing circulatory system that was underneath their feet. And it's something we don't think about very often, or we take for granted – that water just flows out of the tap.”

Take a tour of the cistern map here. For more on the cistern mapping day, click here.

To learn more about how the cisterns work, watch this.

This story originally aired in January of 2016.