26. All well and good
For most of Venice's history, the wells were the only source of fresh water for the dwellings and businesses nearby, and although no longer functional, most of the wellheads are still there, with their lids sealed shut.
You would think that a well dug into an island in a lagoon would quickly fill with nothing but undrinkable sea water, but these wells are not designed to penetrate down to a source of water that is naturally there, they are just the access points for underground water 'tanks'.
Under each wellhead is a cistern, a storage container for freshwater, consisting of a large clay lined pit, filled with sand. Rainwater is channeled into the outer edges of the cistern through grilles in the pavement. The sand provides some basic filtration of the water, which seeps through the sand and fills the centre column of the well, from where it was usually retrieved by lowering a container down to water level on a rope.
Because of the importance of fresh water to any Venetian community, there were strict laws protecting the purity of the source, prohibiting animals from contaminating it, and preventing people from dipping unwashed containers in it to collect their water, even from fetching water with unclean hands.
Goodness knows what the citizens of old Venice who relied on the integrity of this water supply would have done if they had caught any of today's vandals who have marked so many of these old wellheads with spray painted graffiti. They would not have been amused.