The lake is called also ‘light of Chiusi’; it is in the province of Siena and of the town from which it takes its name, the eastern shore marks the border between Tuscany and Umbria, it corresponds to the ancient border between the Grand Duchy of Tuscany and the Pope State.
In the past the inhabitants of Chiusi called this lake “moonlight” as an ancient legend has it that on clear nights the beautiful Goddess of Heaven came here to contemplated herself in these waters. The width of the lake is about 300 hectares and 5 meters depth.
With the nearby lake of Montepulciano, which is connected by the canal called il Passo delle Querce, there is a residual area of the huge swamp that interested the whole valley of Chiana, including Siena and Arezzo. Even though it is not jet a Natural reserve, the lake and the surrounding lands, is an important natural site for both the fauna and flora, typical of humid environments. It is for resting and nesting for migratory aquatic birdlife. There are varieties of hydrophytes and eliofite, which are seldom seen because of drainage performed in the past. Very interesting as well, is the fish fauna represented by: pike, tench and carp which is allowed only regulated fishing.
The lake waters are of acceptable quality so much so that constitute the main source of drinking water supply of the city.

Poggio Antico is located just behind the Chiusi Lake, it is part of the Siena farmhouse circuit.