Underneath a Bayview skateboard park there is a giant, empty box waiting for the next rain.
It is a part of the city's sewer system; boxes like it ring the city, ready to collect rainwater runoff to prevent the sewer pipes from backing up during periods of heavy rain.
But that does not always work.
"We had .7 inches of rain in 20 minutes, that's like pouring a cup or glass into a straw, can't go in fast enough an d when it does get in there it gets overwhelmed," San Francisco Public Utilities Commission Assistant General Manager Tommy Moala said, recalling an October storm.
Moala says the city has to make some tough choices about which sewer projects will get priority.
About 50 miles of San Francisco's 1,000 miles of sewer pipes date from the Gold Rush era.
The solid waste tanks at the treatment plants need seismic upgrades.
"We have an earthquake, we can't treat any of the solid waste here in the city, that's a major environmental issue," Moala said.
The PUC says high tide is now higher than when the rainwater boxes were built, so sometimes high tide fills the boxes, something they were never designed to deal with.
A typical box is 32-feet deep and 30-feet wide, running across several city blocks. It holds 45.1 million gallons; one way to think about that - if the box were filled every day for two weeks, it would be the equivalent of filling Candlestick Park.
But it still needs upgrading. The PUC is asking people to think about something they do not normally.
"You hit the flush button, it just disappears, we don't want to think about where it goes," Moala said.
The PUC says it will easily take billions to make all the necessary upgrades -- the question is, which ones will come first.
"Right now we have X amount of overflows. Do you want to cut that in half? There's an associated cost. Or do you want to make that zero, there's an associated cost," Moala said.
So how much will it cost to do the upgrades? To do the seismic upgrade of the solid waste treatment plant will be $1 billion alone.