The 25-km network of medieval underground tunnels that carried water to the city's fountains for nearly 800 years. Guided tours descend a short section under the centre — typically the Bottino di Fontebranda or the Bottino Maestro — through low, cool, dripping passages of cut sandstone.
Tours run a few times a month, in Italian and occasionally English, and book up; check with La Diana association or the Comune di Siena. The bottini are still partly functional and the engineering, given that they were dug with hand tools, is astonishing.