The Museo dell'Opera della Metropolitana, also known as the Museo dell'Opera del Duomo di Siena, is located next to the cathedral, housed in what should have been the right aisle of the "Duomo Nuovo", according to a fourteenth-century expansion project never brought to completion. The gallery collects works from the cathedral (the "Metropolitana" in fact, being the seat of a metropolitan archbishop), among which most of the artistic production of Duccio di Buoninsegna stands out, with the absolute masterpiece of the Majesty, which once adorned the main altar of the cathedral, and the stained glass window dedicated to the Virgin of the Assumption and which was placed high up in the abyss. The sculptures of Giovanni Pisano (removed from the facade), Donatello, Jacopo della Quercia, Giovanni Duprè, etc. were also part of the cathedral's equipment, and are now exhibited on the ground floor of the museum. Among the pictorial works there are works by Pietro and Ambrogio Lorenzetti, Gregorio di Cecco, Sano di Pietro, Matteo di Giovanni, Domenico Beccafumi, Francesco di Valdambrino.