Italians rightly call Venice an “upside-down forest” (foresta capovolta). The city literally rests on millions of wooden piles sunk beneath the water. Unlike the Taj Mahal, where the timber sits in a river's fresh water, Venice's foundations are bound up with the salty environment of a sea lagoon, its silt and its clay.

Venice's origins go back to the 5th century, traditionally dated to 421. As the Western Roman Empire collapsed, the local mainland population fled barbarian invasions – by the Huns and the Lombards – and sought refuge on the marshy, hard-to-reach islands of the Adriatic lagoon. At first these were simple huts of wood and reed. As the population grew, it became necessary to build heavy stone and masonry structures on an unstable, muddy base. The Venetians developed a unique method of soil compaction: sharpened wooden trunks were driven by hand, packed tightly side by side into the marshy bed – roughly 9 piles per square metre – forced through the soft mud until they reached the hard clay layer beneath. A horizontal deck of wooden planks was laid across the tops of the piles. Foundations of water-resistant Istrian limestone were then built up on the planks, and the building itself rose on top of that.

The famous Basilica di Santa Maria della Salute rests on more than a million wooden piles; the bell tower of St Mark's Square stands on around 100,000.

Enormous volumes of timber

To sustain its building programme, the Republic of Venice needed enormous quantities of timber, sourced and floated down rivers from both the surrounding regions and further afield across Europe. Black alder, prized for its exceptional cellular structure, was used for the longest (deepest) piles – underwater it does not rot but instead grows harder and more durable. Oak was used wherever the greatest structural strength and resistance to compression were required. Larch, thanks to its high resin content, offered excellent protection against water-borne decay and was used both for piles and for the load-bearing plank decking. Timber was brought in from mainland territories controlled by Venetian merchants. Vast tracts of forest were felled in the Dolomite Alps, in the Cadore valley for instance, and along the Piave river, which was used to float logs down to the lagoon. The second major source of timber was the Istrian peninsula and the Dinaric Alps, in what are now Croatia, Slovenia and Montenegro. Venice's appetite for timber was so vast that it permanently reshaped the landscape and forest structure of the Balkans and northern Italy.

The city's foundations today

Contrary to the myth that Venice's timber has turned completely to stone, recent scientific research shows that the wood has only partially mineralised. It is still wood, and it faces serious threats from the modern age. As long as the timber remains fully enclosed within the dense, oxygen-free mass of clay and silt, its decay is extremely slow, driven only by specific anaerobic bacteria. But modern climate anomalies bring extreme low tides, known as bassa marea, which expose the upper wooden layers of some buildings to the air. This activates fungi and insects, which can destroy the timber almost overnight. A further major problem is the wash from the engines of large cruise ships, motorboats and water taxis, which literally scours the silt and sand out from between building foundations. Once that protective layer of silt is gone, the wooden piles are left bare in flowing seawater.

How is Venice fighting back?

The city no longer relies solely on the skill of its ancient engineers, and is now running several high-technology projects.

The MOSE barrier system is the primary shield: a movable-dam project worth billions of euros, positioned at the gates of the lagoon. It is raised during major flooding, known as Acqua Alta, protecting the city from inundation while also helping to smooth out sharp swings in water level, so the foundations are never left high and dry.

During restoration of historic buildings, engineers waterproof and reinforce the foundations. Damaged wooden plank decks and the tops of piles are strengthened with injections of modern micro-cement or special resins, which seal off access to oxygen and water flow, effectively encapsulating the old timber structure inside a protective shell.

To stop the silt from being washed away, very strict speed limits have been introduced for all watercraft in Venice, and in many of the inner canals motorised traffic is gradually being restricted in favour of traditional rowing boats.

The “magic of wood” behind Venice's story

The stability of Venice's foundations depends directly on the interaction between physics and natural materials. The city's rescue efforts are unfolding at a time when local residents feel deep resentment and anxiety about the city's future. The magic of the wooden piles in Venice lies not in the wood itself, but in the environment they were driven into. The process by which silt and clay protect the city rests on three basic physical and chemical conditions.

The silt of the lagoon bed and the firm layer of clay beneath it act as an airtight seal. When an alder or oak pile is driven into this mass, the silt closes around it at once, entirely excluding air and any flow of water. Decay bacteria and wood-rotting fungi cannot survive without oxygen, so the wood keeps its original form for centuries.

Over the centuries, mineral-rich water carrying calcium and iron, along with fine clay particles, continually seeps into the wood's cellular structure. The timber gradually becomes saturated with these minerals, hardening into a stone-like material. It is no longer flexible wood but a solid, fossilised column. Because the piles are driven so densely, side by side, they compact the soft soil around them. The weight of a building is carried not only by the pile tips bearing against the hard clay, but also by the lateral friction between the wood and the surrounding silt. The soil becomes so solid that it can support masonry palazzi weighing thousands of tonnes.

What do Venetians think of the situation?

Venetians feel like the “last of the Mohicans” in a city that is rapidly turning from a living community into a soulless tourist theme park. Three main concerns shape residents' view of the current crisis. First, Venetians today worry less about the city sinking than about its “dying out”. The population of the historic centre has fallen below 50,000, while some 30 million tourists flood the city every year. Locals feel pushed out of their own homes as properties are converted en masse into Airbnb rentals. Grocery shops and everyday stores are being replaced by cheap souvenir stalls and expensive restaurants.

Second, Venetians remain sceptical of the MOSE barriers. Although the multibillion-euro MOSE project has successfully protected the city from the worst flooding since it began operating, locals view it with suspicion and a certain weariness. Construction dragged on for decades and was dogged by huge corruption scandals. Every time the barriers are raised, the lagoon's natural exchange of water with the sea is cut off. Local fishermen and environmental campaigners warn that this turns the lagoon into a “stagnant pond”, where the city's wastewater builds up and the marine ecosystem suffers. Knowing that sea levels will keep rising, Venetians understand that MOSE is only a temporary fix. In future the barriers will have to stay raised almost permanently, which would mean total isolation of the lagoon and the end of Venice as a maritime city.

Third, Venetians are angry about motorised traffic and the huge crowds of tourists who arrive in the morning and leave in the evening, contributing almost nothing to the city's economy while physically damaging its infrastructure. A particular source of irritation is the so-called Moto Ondoso, the wash generated by motorboats. Local gondoliers and owners of traditional boats regularly stage protests against high-speed motorboats and water taxis, whose traffic literally tears apart the canal banks and exposes the historic wooden foundations to erosion.

The municipal “entry fee” (a €5 levy on day-trippers) is regarded by locals as a farce. They point out that, rather than limiting the number of tourists, it merely gives legal sanction to Venice's status as a “museum with an admission ticket”, wounding the city's dignity and its future prospects.

Venetians take pride in their past and in the ingenuity of the engineers who built a city on wooden piles. But today they feel that the city is drowning not only in water, but in a sea of global tourism and political irresponsibility. Their greatest fear is that by the time engineers have fully worked out how to technically preserve the buildings' foundations, there will be no Venetians left to live in them...