How can a whole city stand on wooden stakes in a lagoon?

Venice stands on a forest turned upside down. Its early builders drove closely packed tree trunks down through the soft mud and sand of the lagoon until they hit a much harder layer of compressed clay. On top of these piles they laid plates of Istrian limestone, and on the stone, the buildings. Later poles were about 18 metres long. One church alone, Santa Maria della Salute, stands on a platform of a million wooden piles.

Why hasn't the wood rotted away? Because it is buried in mud. Wood normally decays when oxygen and microbes can reach it, but the lagoon mud keeps both out. Some wooden foundations in Venice are more than 500 years old.

The city does sink, though, very slowly. In the 20th century, wells pumping water from under the lagoon for industry made Venice subside faster, until they were banned in the 1960s. Today it sinks by about 1 to 2 millimetres a year, while the sea keeps rising. Since 1897 the city has lost 23 centimetres relative to the sea, and more than half of that loss was caused by people.

That is why high tides now so often flood the area around St. Mark's Basilica, the lowest part of the city. In 2019 the water covered more than 80% of the city. Since 2020 a system of giant gates on the seabed, called MOSE, can rise to shut the lagoon off from the sea during dangerous tides.