This paper questions “epigeic” understanding of the building of the city, on the basis of an ethnographic research research with foundation builders. It connects the conception of the urban ground as a blank page, on the surface of which human projects materialise, with a broader conceptual separation between the physical world of the ground and the aerial world of ideas. It shows how engineers manage to balance the soil’s underground flows with the buildings anchored on them, and how they learn how “to live with the ground”.