Explain the city of Sienna's water system

Below the medieval city of Siena there is a system of tunnels of some 25 km, formed by the conduits that transported the running water inside the houses of the city since the 12th century, this system of tunnels is called "Bottini". Bottini (true underground aqueducts, galleries excavated in the sandstone, partially bricked up, that collected the water from the springs and infiltrations of the rainwater to take them to the city.