It is often the case that cities are densely populated, and this creates  problems relating to quality of life, infrastructure and the environment. This calls for an increased incorporation of the third dimension, especially in an upward direction, but also, and to a lesser extent, in a downward direction. Like all other objects on the earth’s surface, the dimension in an upward direction is regulated through spatial planning. By contrast, however, the underground is rarely included in such planning. It is mostly perceived as an alien, hidden milieu and its benefits are thus barely perceived.

The Deep City project (Parriaux et al., 2010) identifies four principal resources that lie beneath the foundations of towns and cities:   

Spatial resources for the construction of buildings: these include infrastructure for transport and parking, plus commercial space and recreation zones. 

Geothermal energy resources: the sharp rise in the use of geothermal probes conflicts with the protection of groundwater, and restricts the use of space deep below the surface. With the progression of climate change, the use of heat and refrigeration will gain in importance.

Drinking water: some cities are fortunate in having deep-lying and well protected groundwater reserves at their disposal. The advantage of using these is that there is no need to construct hundreds of kilometres of pipelines to provide the city with its supply of drinking water.

Utilisable materials resulting from excavation operations: what to do with earthen materials resulting from urban excavation activities is a matter of increasing concern. Here the tendency is towards recycling.