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

Allen Glen Cumaya Gil

Cities are increasingly measured through surface land cover and aboveground form, even as urban activity expands below ground. This Perspective develops an integrated approach for making that expansion visible and governable. First, it defines subsurface urban expansion as durable growth or intensification of urban activity below the local ground surface through new or expanded occupation, use, or modification of underground volume. Second, it identifies an underground monitoring gap produced by connected failures of observation, data integration, and accountability. Third, it proposes a framework for volumetric urban monitoring that combines four analytical dimensions, two governance mechanisms, and a five-stage implementation cycle. Spatial occupation, environmental interaction, infrastructure interdependence, and governance accountability characterize underground change. Responsible visibility governs sensitive information, and volumetric accountability assigns enforceable lifecycle obligations. Earth observation supports screening, validated proxies, deformation detection, and surface-impact monitoring within a broader evidence system that includes institutional, technical, and in situ information. Applications in New York City, Lyon, and Singapore illustrate how integrated evidence could reveal cumulative effects, coordinate infrastructure, and clarify responsibility. Bringing the subsurface into urban measurement and governance can help cities identify hidden risks earlier and prevent unresolved obligations from becoming lasting public burdens.