The hidden side of cities—towards three-dimensional land planning

Vertical growth of cities into the ground is still to be developed, in particular for transport, storage of goods and geothermal exploitation. This article presents a synthesis of the potential resources offered by the urban underground and some experiences derived from different cities around the world. Analysis of these experiences shows that these resources have often been developed without any real long-term planning. This synthesis proposes a more complete approach to underground resource management, insisting on the compatibilities or incompatibilities of a parallel exploitation of certain resources due to their interactions (e.g. geothermics—groundwater). Since the impact of most underground work goes far beyond the space occupied, it must be conceived on the city scale. To attain this objective, the authors propose the notion of three-dimensional land planning for urban territory. This new concept goes much farther than the GIS applications used for the “cadastre” in the first metres of underground. A new way of conceiving underground management for the long-term development of the city is recommended.