Venice is built on a former lagoon. That means a lot of silt and mud which is why it is sinking. However, according to The Times there is something holding it back, stopping it from being a disaster. An older layer of soil dating back to the Ice Age, beneath all the soft and smelly stuff. This is keeping the city above the waters. It has become a stone, or sorts, known as caranto, a calcium rich paleosol which covered most of northern Italy during the last Glacial Maximum. It is also composed of downwash from the Alps and its foothills. Calcification did not occur until the early Holocene, forming nodules a minerals that solidified as they dried out. When the lagoon flooded the caranto was buried beneath softer silts so that, in the 5th century AD, when Venice began as a settlement on high points in the former lagoon, but as years went by more and more of the lower levels were built on, the layer has provided a firm base for those buildings that have deep foundations (see Geoarchaeology 26 pages 514-543).