The purpose of this paper is to establish a basis for discussion regarding underground space planning and its implementation in urban areas. More specifically, the intention is to document and analyse the overall space available instead of the two-dimensional surface traditional to urban planning. Several Québec examples are used to illustrate the relevance of the issue. This eventually leads to a proposal for a general methodology that will help urban and regional planners to take into account this new available urban space. These fundamental questions should help us manage urban space more adequately.