The rapid demographic growth of Siena as of the 12th century intensified the need for an efficient water supply for the population. The painstaking search for water, already testified by documents in 1176, was confronted with a system of tunnels, called "bottini", which with a network of more than 25 kilometres winds beneath the city. Built progressively between the Middle Ages and the Renaissance, this hydraulic system was capable of supplying water to the more than fifty fountains and five existing public wells, even in periods of draught.

Principally dictated by the need to guarantee water supply to one of the most water-impoverished areas of Tuscany, the work witnessed the commitment of famous Sienese engineers like Mariano di Iacopo, known as Taccola, and Francesco di Giorgio, both of whom left treatises testifying to a veritable local technological ability. These manuscript texts precisely record the techniques and instruments used to build the "bottini" system, as well as varyingly imaginative projects of hydraulic engineering: devices that could allow a man to float effortlessly in water or remain immersed at length, apparatuses to lift and draw water, projects for canalisations and river barrages.

The efficient functioning of the water system required, and continues to require, careful daily maintenance, originally entrusted to a "worker". In 1469 and 1492, this task was performed by Francesco di Giorgio himself, who was also entrusted with the project to increase the delivery of water to the fountain in Piazza del Campo (Fonte Gaia) by installing the Fonte Nuova tank there.

The visitor to Siena can admire the countless fountains, conceived as stupendous architectural structures, decorated by famous artists and, still today, supplied by the "bottini". He can also visit an underground Siena with traces of the city’s transformations, enlargements of piping, references to the districts where water arrived, and even the street numbers of houses.