Orvieto had a problem with water supply. This was due to its unique location on this isolated plateau of porous tuff. This water problem made Orvieto extremely vulnerable during long sieges against the city. Pope Clement VII ordered to make a very deep well (53m) through the plateau reaching the water. This well is known as the Pozzo di San Patrizio. It is remarkable that there is a tunnel around the well built in the shape of a spiral. This allowed the water to bring up by asses.