Pozzo di S. Patrizio or St. Patrick's Well

Being a historic well in Orvieto, Umbria, central Italy, it was built by architect-engineer Antonio da Sangallo the Younger of Florence, between 1527 and 1537, at the behest of Pope Clement VII who had taken refuge at Orvieto during the sack of Rome in 1527 by the Holy Roman Emperor Charles V, and feared that the city's water supply would be insufficient in the event of a siege. The well was completed in 1537 during the papacy of Pope Paul III. 

The name was inspired by medieval legends that St. Patrick's Purgatory in Ireland gave access down to Purgatory, indicating something very deep.

The architect-engineer Antonio da Sangallo the Younger surrounded the central well shaft with two spiral ramps in a double helix, accessed by two doors, which allowed mules to carry empty and full water vessels separately in downward and upward directions without obstruction. The cylindrical well is 53.15 metres (174.4 ft) deep with a base diameter of 13 metres (43 ft). There are 248 steps and 70 windows provide illumination.

A Latin inscription on the well states QUOD NATURA MUNIMENTO INVIDERAT INDUSTRIA ADIECIT ("what nature stinted for provision, industry has supplied"). (From Wikipedia Pozzo di S. Patrizio )

Il pozzo di San Patrizio storico è una struttura costruita da Antonio da Sangallo il Giovane a Orvieto, tra il 1527 e il 1537, per volere del papa Clemente VII, reduce dal Sacco di Roma e desideroso di tutelarsi in caso di assedio della città in cui si era ritirato. I lavori del pozzo – progettato per fornire acqua in caso di calamità o assedio – furono conclusi durante il papato di Paolo III Farnese (1534-1549).

L'accesso al pozzo, capolavoro di ingegneria, è garantito da due rampe elicoidali a senso unico, completamente autonome e servite da due diverse porte, che consentivano di trasportare con i muli l'acqua estratta, senza ostacolarsi e senza dover ricorrere all'unica via che saliva al paese dal fondovalle.