For the past century, the urban underground has been developed piece-meal out of necessity rather than as the object of a long-term planning effort. The underground contains not only potential usable space, but also raw materials, groundwater and geothermal energy sources with which urban areas interact. Questioning the role of underground resources in future urbanization throws into relief the norms and procedures that are poorly adapted to a volumetric planning of the city, yet offers a new realm of possibilities for urban land use intensification, green space preservation and the restructuring of the urban topology. This paper presents the theoretical and practical issues at hand and then builds upon a methodological framework developed by the Deep City project at the EPFL in Switzerland, taking San Antonio, Texas, as a case study. The preliminary results of the urban and hydrogeological analyses illustrate the challenge of integrating a reflection about the third dimension into an already complex urban context.