Also known as 'The Pope's Well' (Pozzo di S. Patrizio), constructed between 1527 and 1537 by architect-engineer Antonio da Sangallo the Younger of Florence as requested by Pope Clement VII during the sack of Rome. The pope was concerned that the city's water supply would not be sufficient during a siege. The name was inspired by medieval legends that St. Patrick's Purgatory in Ireland gave access down to Purgatory, indicating something very deep.