Surfacing the urban underground: Knowledge production, modes of envisioning, and politics of visibility

• UUS is a finite, multifunctional resource requiring integrated governance. • Governing UUS requires critical reflection on how knowledge is produced. • Making the subsurface visible requires careful consideration of knowledge production. • Intersecting visions shape future urban subsurface developments. • Urban subsurface imaginaries are diverse, evolving, and continuously reconstructed. The subsurface is increasingly considered the final frontier of urban development, with marked potential to improve urban spatial problems, resilience and sustainability. At the same time, the literature recognises Urban Underground Space (UUS) as a finite and multi-functional resource that requires holistic and strategic management. However, knowledge of the invisible urban subsurface is obscured by secrecy, fragmented across disciplines, and built on extractive knowledge practices. A lack of long-term planning of UUS perpetuates a first-come-first-served development approach that risks jeopardising future infrastructures and urban sustainability. To address this, there is an effort to gather ever more subsurface data, rendering the invisible visible to urban decision-makers. We analyse and reflect on the limits of current underground planning knowledge, responding to the socio-political gap in UUS literature. We focus on the ontological challenges facing sustainable planning of UUS and present the need to critique how practices of power and politics in UUS knowledge production envision subsurface futures. We present four modes of envisioning the urban subsurface, which negotiate its characteristics of invisibility and complexity to shape volumetric potential and possibility. We argue that governance of this invisible urban frontier requires deeper reflection on how the subsurface is rendered visible and in anticipation of which futures. We suggest that a more nuanced lens is required to account for the interplay between power, depth, and volume through the intersections of plural urban subsurface imaginaries.