The Crete Senesi refers to an area of the Italian region of Tuscany to the south of Siena. It consists of a range of hills and woods among villages and includes the comuni of Asciano, Buonconvento, Monteroni d'Arbia, Rapolano Terme and San Giovanni d'Asso, all within the province of Siena. Crete senesi are literally Siennese clays and the distinctive grey colouration of the soil gives the landscape an appearance often described as lunar. This characteristic clay, known as mattaione, represents the sediments of the Pliocene sea which covered the area between 2.5 and 4.5 million years ago. In the nearby is also the semi-arid area known as Accona Desert. Perhaps the most notable edifice of this area is the monastery of Monte Oliveto Maggiore. The region is known for its production of white truffles and hosts a festival and a museum dedicated to the rare tuber.