The project "Underground resources and sustainable development in urban areas" (called also "Deep City Project") is motivated by the critical congestion of many cities around the world leading to environmental and public health problems. Consequently, it is obvious that major changes are needed in order to reach new conditions, which are more compatible with sustainable development. The theses of the project are: • A more comprehensive use of the urban underground is one key-solution for getting more space and more healthy living conditions at the surface of the city. • However this increasing use of underground cannot be achieved without a long term 3D planning. • This planning must consider the four main resources of urban underground: space for construction, geomaterials, groundwater and geothermy. • The management of the urban underground must not be sectorial (e.g. transportation or building or water supply or geothermal extraction) but must integrate the potential of all these resources. The project is defining the rules for multi-uses of the resources according to the various geological formations present below the city and their properties. • As underground conditions are very variable from a city to another and even variable at the scale of a city, geological knowledge and 3D modelling is the first step of investment in order to define the long term multi-uses potential. • Shifting some activities toward the underground implicates a significant improvement of the human acceptability for this space. Research in sociology and architecture are done in this direction. Up to now, the project has studied experiences of some large cities like Paris and Mexico. It is presently defining the new methodological concepts, the principles of which are described in this paper. The fundaments of a real 3D - land planning is presented, which will be introduced in a new federal law.