How Venice was built and evolved over time

A city made from islands. Venice was built by adapting a tidal lagoon, not by placing a complete city on open water. It stands on 126 islands separated by open water and canals, with parts of the city linked by 472 bridges, in the shallow Venetian Lagoon between the mouths of the Brenta and the Sile.

Alder piles, driven until they hit clay. Those who took refuge on the sandy islands of Torcello, Iesolo and Malamocco learned to build by driving closely spaced piles of alder trunks — a wood noted for its water resistance — into the mud and sand until they reached a much harder layer of compressed clay. The piles are not floating in mud; they are standing on something.

And then stone plates on top of the wood. Building foundations rest on plates of Istrian limestone laid over the piles. Istrian stone is dense and close to impermeable, so it stops water rising out of the saturated timber into the brickwork above — the junction that decides whether a wall survives.

Materials chosen for weight and for water. Brick reduced load compared with solid stone, which matters when everything above is standing on timber. Durable stone was used where walls meet water and take the wear.

The largest work was the one that moved the rivers. Six hundred years ago Venetians diverted all the major rivers flowing into the lagoon, protecting the city from land-based attack and preventing sediment from filling the water around it. The consequence was an ever-deeper lagoon. It is the biggest piece of engineering Venice ever attempted, and it was done by taking things away rather than building them.

Why Venice does not stand perfectly still. The city moves through a combination of natural sediment compaction, historic groundwater extraction and the loading of the buildings themselves. Subsidence — the gradual lowering of the surface — is one reason the lowest ground floods at high tide.

Sixty-four centimetres is the whole problem. The lowest part of Venice is St Mark's Basilica, only 64 cm above sea level, and one of the most flood-prone places in the city. Between autumn and early spring the city is repeatedly threatened by flood tides pushing in from the Adriatic.

The Arsenal built ships the way later centuries built cars. The Venetian Arsenal is considered by several historians to be the first factory in history, and was the base of Venice's naval power — a production complex on a scale Europe would not see again for centuries.

Flood protection today. MOSE uses mobile barriers at the lagoon's three inlets to separate it temporarily from the Adriatic during a forecast high water. It is the current answer to a problem the river diversions of six centuries ago helped create.