Lake Chiusi sits in the heart of Valdichiana and its eastern shore marks the border between Tuscany and Umbria, corresponding to the ancient division between the Grand Duchy of Tuscany and the Papal States. In the past, the inhabitants of Chiusi referred to the lake as 'moonlight', for an ancient legend says that on clear nights the beautiful goddess of the heavens came to look at herself in the water.
The lake, with a surface area of 300 hectares and a maximum depth of five metres, is linked to the nearby lake of Montepulciano by the Passo alla Querce canal. It represents the residual area of a vast swamp that covers the whole valley floor of Valdichiana, both in its Siena and Arezzo areas. It is a nesting place for many migratory birds, such as the little egret, the purple heron, the seahawk and the black-winged stilt, making it a perfect location for birdwatchers.