Underground Atmospheres. Renewing the debate

Cities are moving underground. Cultural centres, data centres, organic urban farms, logistic centres are multiplying inside the bowels of metropolises worldwide, as realised/imagined equipment after the development/reappropriation of underground spaces. Such abundance echoes with the sophistication of projects, meetings of international professionals1], as well as publications in special issues of architecture journals2], which also demonstrate the buzz animating debates on the matter among professionals and scientists. The concept of underground spaces has emerged over the last twenty years (Admiraal & Cornaro, 2018) as a unifying issue for the professions working with the Urban (e.g. architects, engineers, geometricians, geographers). For a long time considered as a “residue” of the city, neglected by actors, bereft of natural light and air, underground spaces have responded to the needs for storing and burying vital functions of the city (all sorts of networks) or, in some cases, for climate protection (e.g. Montreal). Therefore, for a long time, the underground remained the domain of engineers, military officers, accommodating security installations, parking areas, technical galleries (utilidors)and various urban infrastructures (Goel, Singh & Zhao, 2012). As for urban planners, they have long imagined a retrieval of this common ground often involving the imagination around futuristic cities3]and less so the trial and updating of knowledge and techniques inherited from vernacular architecture (e.g. water reservoirs, troglodyte dwellings).

The end of the 20thcentury and the processes of metropolisation and urban sprawl now make global cities face issues regarding the densification of their functions. The underground is expected to prove itself as a structuring space of metropolitan life. It emerges as a promising response to the unsolvable shortage of land in dense areas, but also to the issues of energy performance, management of resources and mobility (Malone, 1996). From a perception of the underground as a bindingspace, we are now witnessing, in France and abroad, a renewal of the vocabulary and of the policies4]willing to connect the undergrounds to the grid of urban vital functions. However, despite this effervescence, the updating of the regulatory and operational tools has not yet started. The high cost of underground construction, the complexity of operational montages (type macrolot), the risks induced by the recognition of a common land to be shared and the absence of exhaustive census call for a careful reading with multiple dimensions (e.g. environmental, social) of underground spaces, as well as a revision of the analysis and design vocabularies.

Questioning the underground in its ability to welcome, please, affect, comfort also means recognizing an old debate (structure, land, safety) with topical issues, such as the hybridisation of public spaces, the tourist attractiveness of cities, the role of the senses in the urban experience… This special issue aims to open a debate on an updated reading of the underground space and its role in the construction of urbanity (Levy, 1994). We use in particular concepts of ambiance and experience (Malpas, 1999), which have been present for over forty years in humanities and social sciences in France (Amphoux, 2003) and in Anglo-Saxon research (Buser, 2014). How can a multi-sensorial approach of underground spaces shape modalities of production, practices and design of those places?

The debate remains open and comes alive with the recent evolutions of the global health crisis that has deeply shaken social interactions in closed and open sites. Cities have always been built with a direct connection to their underground. The remnants of chambers of historical cities5](Lavagno & Schranz, 2007), the re-use of natural caverns, of the many vernacular architectures6], of engineering work, show that humans have always inhabited the underground, either out of necessity (Ming, 2017) or out of ambition. While this universe remains to this day little studied, it expresses rooted reservations – linked to the technique, but also to the cultures and lifestyles. In what conditions can we imagine an underground life like we experience on the surface? The vertical stratification and the promotion of underground architecture (Meiss & Radu, 2014) ensure a gain in land, but they also allow to create efficient shortcuts to connect activities in the city7]. How can we nowadays rethink the new forms of proximity, the durability of the constructions, the superimposition of functions, the three dimensionality of studies (Harris, 2015)? Beyond questions regarding constructability and functions, the underground city calls for deep changes connected to the structuration of social lifeitself8], traditionally organised around spaces on the ground.

This special issue aims to highlight shared topics and possible bridges between actors who usually operate on different scales. We encourage reflections questioning the stakes and methods of the construction of inhabited space in its verticality by reintroducing the question of the underground, not as a soffit but as a living and evolving interface. The issue invites researchers, practitioners of the operational world and designers from different fields, who will share their work on the questions of undergrounds and ambiances – built, experienced and lived by humans – by highlighting case studies, in-situ experiences and new methodological tools.