Vulgarly known as the "cisternone", it was designed by Francesco Paciotto, builder of the citadel of Torino (Turin), and largely completed between 1565 and 1567. After excavation work, conducted up to the groundwater level, two overlapping and independent helical ramps were built, modeled on the Pozzo della Rocca di Orvieto, covered with barrel vaulted ceilings, brick paved and lit by large open windows in an open-air cylinder. At the center of the well was the large circular tank for water collection. In 1607, with the construction of the columned loggia, above the portico on the ground floor, and the double-pitched roof, the above-ground building was finally completed, but destroyed in 1698 by the explosion of the city's biggest powder magazine. The helical ramps, restored after the siege of 1706, were unusable since the late thirties of the eighteenth century. Further damage during the course of the siege of 1799 finally determined the absolute abandonment of the work and its complete burial.
Recent archaeological excavations, carried out by the Museo Pietro Micca (Pietro Micca Museum) and the homonymous association between 1994 and 2006, have allowed the recovery of the lower terminal section of one of the two helical ramps.
