Use of Urban Underground Space (UUS) has been growing significantly in the world’s biggest and wealthiest cities. UUS has been long acknowledged to be important to the urban development agenda: sustainability, resilience, livability, and creating a better urban environment in particular. These issues are traditionally monitored using urban indicators, however UUS has not been properly included and considered in urban indicator lists (sets or systems) yet – the gap this paper is aiming to bridge. The paper reviews existing approaches to the composition of urban indicator lists, highlighting indicator types, challenges related to data collection, and agencies that are concerned with the issue. Further the paper has identified the importance of UUS inclusion in the lists that give integrated assessment and monitor urban sustainability, resilience, climate change adaptation and mitigation, as well as progress towards smart, livable, and compact cities. Existing global quantitative data on UUS have been examined in 8 cities; and three key indicators (descriptors) were suggested to monitor UUS use: Developed UUS volume (m3); UUS use density (m3/m2); and Developed UUS volume per person (m3/person). Current average UUS use densities in cities are identified as up to about 0.05 (m3/m2) (which can be interpreted as a virtual depth of UUS use of 5cm), and the developed UUS volume per person is up to about 10m3/person; while city central areas (central business districts) can have a virtual depth of developed UUS of several metres (m3/m2). Compatibility, comparability, uniformity, and sustained monitoring of urban indicators data (including UUS indicators) found to be posing significant challenges to the research across geographies, and industry/economic sectors

This paper examines the continued growth in the use of underground space beneath cities and the widespread lack of significant and broad planning efforts for its use. The paper is drawn from the collective experience and prior research of the authors together with information exchanges afforded by the activities of the International Tunnelling and Underground Space Association's Committee on Underground Space (ITACUS). The particular focus of this paper is on sustainability issues related to urban underground space use including contribution to an environmentally sustainable and aesthetically acceptable landscape, anticipated structural longevity and the maintenance of opportunity for urban development by future generations. However, owing to their initial cost and permanent alteration of the underground environment, underground structures put a special emphasis on long-range planning which considers life-cycle cost-benefits and the selection of projects that offer the highest contribution to urban sustainability rather than a short-term fix to an individual need.