For the past century, the urban underground has been developed piecemeal rather than as the object of a long-term planning effort. There is a growing awareness by urban planners and engineers that the underground contains not only potential usable space, but also raw materials, groundwater and geothermal energy sources with which urban areas interact. As a complex system, the city is not simply a projection off a two-dimensional plane, but a system of layers and interactions between urbanization and ecological processes. Questioning the role of underground resources in future urbanization throws into relief the norms and procedures that are poorly adapted to a volumetric planning of the city, yet offers a new realm of possibilities for urban land use intensification, green space preservation and the restructuring of the urban topology. This project draws from complexity theory in two principle ways. First, the urban form is investigated as emerging from a series of local topological rules. This exercise then crosses these logics with existing or potential uses of underground space. For example, locations in the city where geological conditions are favorable and property value is sufficiently high to justify the cost of land use intensification may or may not coincide with existing urban centers. Their development would raise questions about their impact on the logic of movement in the city and the existing relationships between places that would be strengthened or weakened. Second, complexity theory addresses not only urban form but also the processes and rules that structure that form. Legal instruments limit or expand the possibilities to divide land vertically, render the development of advantageous underground areas feasible and protect fragile or undesirable areas from exploitation. The potential of such legal instruments to support vertical (upward and downward) urbanization are of little use if actors aware of the underground potential or the information upon which their expertise is based is not present within the local planning system. The investigation will consider these legal instruments and the governance structures through the lens of complexity theory of cities as the rules of the game, how the game is (roughly) played and by whom. This poster summarizes the research questions and methods of the doctoral research upon which this investigation is based. The five minute presentation will also discuss the contribution that complexity theory makes as an important part of the project’s theoretical framework.