Venice was founded by Roman farmers (veterans from the legions) on the run from the barbarian invaders. Most of them starved on the Islands in the Venetian Lagoon but were safe from the horse riders under Attila the Hun who were destroying the old Empire in the 5th century AD.
They denuded the islands of their large trees, driving them deep into the water logged clay soils where, in the absence of oxygen, they petrified. More than 1000 years later, these still form the foundations for Venice.
Venittia Majore is made of 118, all joined by bridges. Other islands in the Venetian Lagoon are not considered Venice if they have to be reached by boat. There is only one artificial island where they have built a (very expensive) Parking Station, for there are no cars in Venus (but there is Italy’s most expensive Ferrari dealership
The picture shows 3 adjoining islands, joined by bridges, with the canals lapping up to the buildings.
The foundations mean that building have to be a maximum of about 6 floors and build for flexibility. There is no concrete. The terrazzo flooring is set in lime so floors that were flat once are now all sorts of angles but with no cracks.
The few tall structures, like church bell towers all have leans, like the one on St. Mark’s Cathedral, which is a reconstruction of one that fell down.