In the process of urban growth, the underground is often only addressed once all surface alternatives have been exhausted. Experience shows that this can lead to unforeseen conflicts (e.g., subsidence, groundwater pollution) and to lost opportunities (e.g., combined geothermal systems and building foundations or recycling of excavation materials). One challenge is how the underground potentials are assessed by urban actors; data collection, analysis and visualization for the different resources are often conducted in separate disciplinary corners and administrative divisions. This paper presents a mapping method developed within the Deep City project at the Swiss Federal Institute of Technology in Lausanne (EPFL) and its application to San Antonio, Texas. San Antonio is interesting in its lack of major underground infrastructure and its few means and political support for short-term underground development. We will specifically look at the production of a series of interaction maps, an original mapping strategy that is complementary to the resource potential maps we have produced in prior work. After situating this research within larger theoretical and philosophical questions, we will show how mapping the combined potentiality of underground resources can serve as a compass for future interdisciplinary discussions that address the urban underground as a source of opportunity, rather than as an afterthought.