Pozzo San Patrizio, 

well built on the orders of the pope. During the Sack of Rome in 1527 the Pope took refuge at Orvieto, and fearing that in the event of siege by Charles' troops the city's water might prove insufficient, he had this spectacular well constructed by the architect-engineer Antonio da Sangallo the Younger (1527‑37) with double helical ramps for one-way traffic, so that mules laden with water-jars might pass down then up again unobstructed. Its inscription boasts QUOD NATURA MUNIMENTO INVIDERAT INDUSTRIA ADIECIT ("what nature stinted for provision, let application supply") Mules carried water up the spiral staircase around the well.

The city of Orvieto has long kept the secret of its labyrinth of caves and tunnels that lie beneath the surface. Dug deep into the Tufo, volcanic rock, these hidden and secret tunnels are only now open to view through guided tours. Their spectacular nature has also yielded many historical and archeological finds. The underground city boasts tunnels, galleries, wells, stairs, quarries, cellars, unexpected passageways, cisterns, superimposed rooms with numerous small square niches, detailing its creation over the centuries. Many of the homes of noble families were equipped with a means of escape from the elevated city during times of siege through secret escape tunnels carved from the soft rock. The tunnels would lead from the city palazzo to emerge at a safe exit point some distance away from city walls.

The territory of Orvieto was under papal control long before it was officially added to the Papal States (various dates are quoted); it remained a papal possession until 1860.