For the cities that are learning from Chicago’s example, Bernstein argues, recent storms furnish an important lesson: Bottling rainstorms is really hard. “If I was one of these other cities that have been convinced a deep tunnel system would make the difference, I’d be pretty angry by now,ȁ he says, “because putting big money and expertise behind the large structured system ended up delaying by decades action that might have [been] taken earlier to figure out how to make more use of the surface of the region."

While engineers’ penchant for megaprojects endures, some American cities are preaching deterrence. If Chicago built a bathtub, Philadelphia is trying to transform itself into a sponge with park space, street trees, and permeable pavement. The city is spending $2.4 billion to implement the nation’s largest green infrastructure plan, an experiment that positions it as the anti-Chicago. The city thinks keeping water out of the system will save billions of dollars compared to a rejected tunnel proposal—and that green initiatives will produce positive externalities, like improving air quality and creating verdant streets.