The first time I went on one of Joel Pomerantz’s tours of San Francisco, we hiked up to one of the few remaining aboveground portions of Islais Creek, part of the city’s old freshwater system. The creek used to run through the Mission and out into the bay; now it runs far out of sight, under a feeder road that leading to an interstate on-ramp.

It was the dry season, so the creek didn’t look like much — kind of like a very small, agitated puddle. But the water coming from it was ice cold and clear. I felt like I was being let in on an amazing mystery.

Pomerantz is how I found out that a hideous fountain downtown is one of the last places where the city’s underground streams bursts to the surface. He’s how I found out that the Wiggle, a popular bike route leading from the center of the city to the ocean, was an old stream bed. A passionate amateur, Pomerantz spent more time in archives and carrying out field research than a lot of academics I knew.

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Now, much of that quasi-secret knowledge is available in the form of a map, called Seep City. Today, Seep City is a hold-in-your-hand kind of paper map; a book, which is a collaboration with the illustrator Emily Underwood, is also in the works, for summer release.

The map is fascinating for a lot of reasons, but especially because it has the same open-ended quality that I’ve noticed in Pomerantz’s tours. As Pomerantz explained it, in an email:

NO LABELS

Yeah, yeah. The map is hard to find your location on. It has no specific labels (except one sewer treatment plant!)

Two reasons:

First, it’s amazing how people who see a map will instantly decide what function it’s good for, look up that one thing, and set the thing aside. With this map, you can’t. You simply have to explore a little. I’ve witnessed fun aha! moments when people slow and look closely. That rubber-necking, by itself, justifies the decision.

At any rate, if you love maps, San Francisco, or both, Seep City is well worth a closer look.