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Abstract: In the late Middle Ages, the city of Siena (Italy) had a high population density and had to face the problem of supplying water within the city walls for housing, crafts, economic and commercial activities, as well as for the risk of fire. A network of underground drifts, namely “Bottini”, was then built, achieving a total length of about 25km in the late XIV Century. The Bottini have been capturing and conducting rain water from the countryside to the fountains in the city centre for centuries, and still provide an average 9.5Ls−1 of clean water, though it is not drinkable nowadays. Currently, water provided by the ancient aqueduct is only used to fill a set of monumental fountains, and is then wasted. In this paper, we have investigated the environmental performance of the water supply in Siena, comparing results from the analysis of the historical Bottini and the contemporary water supply system. In particular, an emergy evaluation was developed in order to account for the environmental resource use of the water management system, considering both the historical and the modern aqueducts, and providing information on their “sustainability”. Specific emergy, measured in units of equivalent solar energy (solar emergy Joules – seJ) per liter of water provided, as well as the Environmental Loading Ratio and the Emergy Investment Ratio, were used as indices of environmental performance. Results have shown that the Bottini have a lower environmental impact in terms of rate of renewability with respect to the contemporary system. Based on statistics on water use in urban centres (drinking, washing, gardening, etc.), we argued that keeping the Bottini alive is not only a good practice for the conservation of a precious cultural heritage, but could be a potential opportunity for improving urban ecology. [Copyright &y& Elsevier]