Waterways have always been ideal places for the foundation and development of many cities. Siena, on the other hand, stands on a hilly landscape and has no rivers flowing nearby, so it was crucial to find a method of supplying the city's founts with water. The bottini, whose name derives from the barrel vault, are a network of underground tunnels 25 km long that cross Siena and until 1914 were the only source of water supply in the city. Their purpose is to collect rainwater, which accumulates clear in the so-called "gorello", and transport it to the whole city. The houses of wealthy families were supplied with water directly from the bottini, while the rest of the population drew on public sources. The network of tunnels was built between the thirteenth and fifteenth centuries and, like all excellent aqueducts, has an imperceptible slope, it is a few inches every hundred meters. The main stretches are the Bottino maestro of Fonte Gaia and the Bottino maestro of Fontebranda; some of these can be visited by appointment.