From K-QED.

Hey guys, Olivia Alan Price here, and I'm standing at an intersection in San Francisco's

Mission District.

I'm out here because Bay Curious Listener, Matthew Cross Cross noticed something unusual about many of the manhole covers in San Francisco.

A lot of them are surrounded by a big brick circle inlaid in the pavement. It's probably 25 feet or so in diameter.

Now these circles are actually a symbol. What lies beneath each one is a huge

underground tank of water. A cistern, so big that the water inside could fill

two backyard swimming pools, and there are more than 170 of them all over San Francisco.

Our listener, Matthew Cross, wants to know,

what are these cisterns doing underneath San Francisco streets?

You're listening to Bay Curious. This week a trip underground and back in time to learn why these

cisterns are here.

Support for Bay Curious is brought to you by Sierra Nevada Brewing Company, still family owned, operated, and argued over.

Explore their brews wherever fine beverages are sold and taste how trailblazing runs in the family.

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Reporter Sarah Craig went to the other side of town to meet up with a cistern

expert Scott killed all.

Oh, what's this one say? This one says cistern. I meet Scott at the corner of a busy intersection in Chinatown,

where he shows me San Francisco's very first sister.

So it's right in the middle of the street,

so it's kind of awkward to stand on top of it.

Yeah let's go to this corner right here.

...