Further reading on the subject of Venice’s rainwater cisterns has clarified a few points which I now want to correct (I will revise the earlier post accordingly).
There are conflicting accounts on the average depth of the well. One source says they never exceeded 16 feet (5 meters), others carry it downward to 21-40 feet (10-12 meters). I’m not qualified to referee this point.
One source says they would stop at the layer of caranto, another says they dug past it. Ditto.
As for the purpose of raising certain campi, one sharp-eyed reader asked me outright the question I had also wondered about (let that be a lesson to me to let sloth, even momentarily, get the upper hand). One source, which I referred to, says that it was to facilitate reaching the necessary depth. Another source makes more sense by stating that raising the surrounding area protected the well from the danger of being polluted by salt water in the case of an exceptional acqua alta. I mentioned accounts of wells being ruined by infiltration, so the campo, or part of a campo, with a well that was dug in an area known to be susceptible to high water (San Marco comes to mind) would have been raised.
Bonus information: Speaking of wells that have been ruined, of course the Venetians didn’t just sing a dirge, drape it with black and leave it there. They manned the pumps, as you see here:
