It’s obvious, once you know it — or even stop to think about it — that the pipes and cables carrying water, gas, electricity and so on are under the paving stones of the streets.
(When they have to cross a canal, they cling to the underbelly of the nearest bridge in a marsupial kind of way.)
What happens with the water pipes is that they leave traces — not of the water itself, but of the condensation they cause because of the difference in temperature between the water in the pipe and its surroundings.
Example: It’s deeply hot now in Venice, the days are dazzling with heat and sun, though the air, thank God, isn’t very humid. At night, things cool down somewhat, and in the early morning, this appears on the fondamenta near our house:
In the winter, the opposite phenomenon occurs, as you see:
Nothing revolutionary here, I just find it diverting.