Book excerpt: Have you ever wondered what lies beneath the streets of your city? Do you picture, in isolation, a series of train tunnels and pipes? Or perhaps the foundations of tall buildings that lie scattered, like icebergs, beneath the surface? As our cities grow up, out, and down, it is time we better understood how the different layers of these complex urban environments relate to one another. Underground Urbanism seeks to provide a new perspective on our cities, and consider how this might be used to engage more positively with them. So, tip your cities upside down to have a closer look, and let us rethink them from (below) the ground, up.