Ground, as a body incised by natural and human actions (European Landscape Convention), carries “stories”, going beyond quantitative values. As in a text, it holds the keys to understand what it covers or hides. In its thickness, it shelters “implicit projects”. Understanding its complexity requires a physical and perceptual commitment, challenging the body in space: dimensions gradually forgotten by Environmental Sciences. As a “threshold” between visible and invisible, Underground-Built-Heritage represents the reverse of the emerged world: hollow space, both generator and mirror of open space (cities, landscapes). The focus is on physical and mental relationships between these two worlds. Past and present relationships emerge, allowing hy...