Appraising the Environmental Advantages of Underground Storage Facilities in Athens, Greece Athanassios Mavrikos 1 , Dimitris Kaliampakos 1 1 School of Mining and Metallurgical Engineering, National Technical University of Athens, Greece. ABSTRACT The promotion of urban underground development confronts many obstacles. Among them, the absence of the environmental impact assessment from the evaluation process is one of the most important. Decisions solely based on the grounds of construction cost comparison and not including neither the operational cost nor the environmental impacts tend to penalize underground structures. The paper addresses the appraisal of the environmental benefits of underground structures. A comparative analysis of an underground and an aboveground storage facility in the urban area of Athens, Greece, is presented. For the appraisal of the environmental benefits, the Benefit Transfer Method is implemented. The results of the analysis prove that the environmental benefits of the underground storage facilities have a considerable economic value. Generally, the incorporation of the monetary benefits that stem from the environmental compatibility of underground works, in the evaluation and decision-making process can lead to sound decisions improving the environmental conditions and enhancing the quality of life in urban areas. 1. INTRODUCTION Urbanization has been one of the most distinguishable characteristic of the last decades, documented and studied both in developed and developing countries on a global scale. The unparalleled urban sprawl has resulted in megacities as well as accumulated urban problems. In a constantly growing and changing urban area the pressure is on urban planners to strive to alleviate the negative consequences of urbanization. In this context the prospect of utilizing the urban subsurface stood out as a feasible alternative offering multiple advantages with regards to the city’s structure. Underground metropolitan railways, road and railway tunnels, underground parking facilities, underground utility networks and underground sewage treatment plants are typical examples of urban underground development. However, the implementation of these prospects is still under consideration as traditional techniques employed in the decision-making process tend to penalize underground solutions. Usually comparisons between surface and underground alternatives are made on the grounds of construction cost, where the higher initial construction cost of underground structures render them as second-best options. The situation is sometimes reversed when decision-makers include operating and maintenance cost in the analysis or use a Life-Cycle Assessment approach (Sellberg, 1996). Nevertheless, a vital parameter that is missing from this process is the assessment of the environmental impact of each alternative. Granted that environmental awareness is rapidly raising worldwide, the need to include this field, when examining possible solutions, is becoming pressing. Until recently, researchers stressed the necessity of appraising the environmental advantages of underground development, especially in cases involving large urban areas, but at the same time they identified the lack in proper tools for expressing the advantages in monetary terms and relied on descriptive or qualitative approaches (Godard and Sterling, 1995). However, the continuous development in Environmental Economics and its relevant tools for appraising environmental goods 267 11th ACUUS Conference: “Underground Space: Expanding the Frontiers”, September 10-13 2007, Athens - Greece