Seeing Cities in Depth: Subsurface Urban Expansion and the Case for Volumetric Monitoring and Accountability

Allen Glen Cumaya Gil

Urban science can measure surface and aboveground change with increasing precision. Satellites, building-footprint datasets, and emerging three-dimensional products now track urban land, building height, and built volume. Yet these tools still struggle to capture urbanization below ground. This Perspective defines subsurface urban expansion as the extension of urbanization below the local ground surface through the creation, occupation, modification, and governance of underground volume for urban functions. Belowground systems support mobility, drainage, utilities, energy, storage, commerce, and service delivery. They can also create environmental impacts, carbon burdens, social exposures, spatial conflicts, and long-term liabilities that remain difficult to detect through surface-based monitoring. This mismatch is the underground monitoring gap: an observational, data-integration, and accountability problem. Existing underground-space research has shown that the subsurface is a strategic urban resource, while urban monitoring research has improved measurement of surface and aboveground change. The missing link is a framework that treats belowground urbanization as part of the measurable, governable, and publicly accountable city. Closing this gap requires volumetric urban monitoring supported by responsible visibility and directed toward volumetric accountability. Earth observation remains essential, but its value increases when connected with administrative, engineering, environmental, legal, and three-dimensional urban data. Sustainable urban monitoring must account for the full urban volume and make underground risks, responsibilities, and public-interest obligations visible enough to guide better decisions.