Most modern structures are built to last 50 years or so, but ingenious ancient engineering has kept this watery city afloat for more than 1,600 years – using only wood.

As any local knows, Venice is an upside-down forest. The city, which turned 1604 years old on March 25, is built on the foundations of millions of short wooden piles, pounded in the ground with their tip facing downwards. These trees – larch, oak, alder, pine, spruce and elm of a length ranging between 3.5m (11.5ft) to less than 1m (3ft) – have been holding up stone palazzos and tall belltowers for centuries, in a true marvel of engineering leveraging the forces of physics and nature.

In most modern structures, reinforced concrete and steel do the work that this inverted forest has been doing for centuries. But despite their strength, few foundations today could last as long as Venice's. "Concrete or steel piles are designed [with a guarantee to last] 50 years today," says Alexander Puzrin, professor of geomechanics and geosystems engineering at the ETH university in Zurich, Switzerland. "Of course, they might last longer, but when we build houses and industrial structures, the standard is 50 years of life."

Building to last

Only once, early on in his career, Puzrin has been asked to provide a guarantee of 500 years for a construction a Baháʼí temple in Israel.

"I was kind of shocked because this was unusual," he recalls. "I was really scared, and they wanted me to sign. I called my boss in Tel Aviv, a very experienced, old engineer and I said, 'What are we going to do? They want 500 years.' He answered, '500 years? [pause]. Sign.' None of us is going to be there."

The Venetian piles technique is fascinating for its geometry, its centuries-old resilience, and for its sheer scale. No-one is exactly sure how many millions of piles there are under the city, but there are 14,000 tightly packed wooden poles in the foundations of the Rialto bridge alone, and 10,000 oak trees under the San Marco Basilica, which was built in 832AD.

"I was born and raised in Venice," says Caterina Francesca Izzo, environmental chemistry and cultural heritage professor at the University of Venice. "Growing up, like everyone else, I knew that underneath the Venetian buildings, there are the trees of Cadore [the mountain region next to Venice]. But I didn't know how these piles were placed, how they were counted and knocked down, nor the fact that the battipali (literally the 'pile hitters') had a very important profession. They even had their own songs. It is fascinating from a technical and technological point of view."

The battipali would hammer down the piles by hand, and they would sing an ancient song to keep the rhythm – a haunting and repetitive melody with lyrics that praise Venice, its republican glory, its Catholic faith, and declare death to the enemy of the time, the Turks. On a more lighthearted note, a Venetian expression still in use today, na testa da bater pai (literally 'a head that is good to pound down the piles') is a colourful way of saying that someone is dull or slow-witted.

The piles were stuck as deep as possible, until they couldn't be pounded down any further, starting at the outer edge of the structure and moving towards the centre of the foundations, usually driving nine piles per square metre in a spiral shape. The heads were then sawn to obtain a regular surface, which would lay below sea-level.

Transverse wooden structures – either zatteroni (boards) or madieri (beams) – were placed on top. In the case of the bell towers, these beams or boards were up to 50cm (20in) thick. For other buildings, the dimensions were 20cm (8in) or even less. Oak provided the most resilient wood, but it was also the most precious. (Later on, oak would only be used to build ships – it was too........

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