At a time when our cities are thought of as single, giant, non-polluting neighborhoods, and when rooftop gardens threaten to replace supermarkets, what remains of a half a century of development and expansion during which concrete was always the easy answer?

The vestiges of our cities will be staircases, parking lots, road junctions, building foundations, and train stations—large, bulky structures, urban bunkers standing awkwardly next to green spaces and eco-friendly housing.