How the lagoon was born in Venice

In a nutshell: six thousand years of water, mud and human genius - how the lagoon formed, why it kept armies out, and how the city learned to read its own water. It is the foundation story behind the city's history and the islands you can still visit today.

The Venetian lagoon is nearly 6,000 years old. It is a vast stretch of brackish water caught between the Italian mainland and the Adriatic Sea: the rivers of the terraferma deliver fresh water and sediment down to it, while the sea pushes saltwater back in with every tide. Roughly 212 square miles (about 550 square kilometres) lie sheltered from the Adriatic's open-sea swell behind the long sandy barrier of the Lido, and the whole expanse is a maze of sandbanks and mudflats. The depth is almost nowhere dramatic - over large stretches the water is so low that you could wade across on foot - and that is precisely the feature that shaped the city's destiny.

The balance of land and water is surprising. Of the entire lagoon surface, only about 5% is permanently emerged land - the islands where the city actually stands. Another 20% is made up of barene, the salt marshes: zones of ground sitting right at water level, carpeted with vegetation that tolerates the salt. Everything else is open water, tidal flats and shifting sand. This is a landscape in constant motion, reshaped twice a day by the tide and slowly but inexorably by the rivers that feed it.

A moat of mud: why Venice never needed walls

For centuries the shallowness of the lagoon was the city's best defence. The water was too low for the deep-draft galleys of would-be invaders and the sandbanks too treacherous for shallow ones: an attacking fleet simply could not find a safe channel, and an army marching across the flats would sink into the mud. Venice could therefore protect itself from attack by both land and sea without a single fortification - alone among the great medieval cities of Europe, it had no wall at all. The very name of the commercial heart of the city remembers that humble beginning: Rialto is a corruption of riva alto, the "high bank" rising above the marsh.

Refugees in the marsh: the first Venetians

People had lived along the northern Adriatic coast long before the city existed: the Veneti, who gave the region its name, had dominated this shore from around 800 BCE onward and had counted as Roman citizens since 49 BCE. The lagoon itself was settled only when the mainland became dangerous. As the Western Empire unravelled in the 400s and 500s CE, Huns, Visigoths and Lombards took turns ravaging the towns of the Veneto, and the frightened population fled into the wetlands. The exodus accelerated once Altinum, on the mainland, was destroyed by Attila's Huns in 452 CE: the first refugees made their base on Torcello, in the north of the lagoon, which grew into a substantial city. By 466 the settlements had reached the neighbouring islands and had begun acting as a loose confederation; in 540 the whole lagoon was absorbed into the Byzantine Empire, and in 726 the lagoon dwellers elected their own first independent doge, Orso Ipato. His son Teodato moved the seat of government to Malamocco, which served as the lagoon's capital from 742 to 811.

The centre of gravity shifted for good in 809, when a Frankish attack was repelled at the shallow banks of the Rialto islets by Agnello Partecipazio. His success won him election as doge, and the islets around the Rialto high bank became the centre around which the new capital grew. Marshland was drained, countless wooden piles were hammered into the yielding mud to consolidate the ground, and Venice gradually lifted itself out of the swamp - a city built deliberately, on water, by people who had no other choice.

Barene and bricole: reading the water

The whole lagoon is crossed by channels of varying depth, and the navigable ones are marked with poles driven into the bottom: a single pole is a palina, a cluster of three or more is a bricola. Since 1439 a regulation has governed the bricole, and in essence nothing has changed since - modern poles simply carry a reflector or a small light for night navigation. These markers draw the line between safe water and the secca, the shallows where a boat runs aground. Navigation here has always been a question of reading the water: the gondola was built flat-bottomed precisely because such a hull was the one design able to handle the lagoon's shallow, tricky waters, and the gondolier stood at the stern, scanning the water ahead for sandbanks. Today the shifting banks are tamed by well-marked thoroughfares, and the public boat network of ACTV threads the lagoon along routes defined by those same pilings.

The Lidi and the port mouths

Between the basin and the open Adriatic lies the chain of the Lidi - long, narrow strips of sand, sometimes reinforced by man-made works, that act as a natural breakwater for the whole basin. Communication with the sea passes through just three openings, the bocche di porto: the port mouth of the Lido, that of Malamocco, and that of Chioggia. Everything that floats in or out of the lagoon - water, fish, ships, tides, and the flood itself - must pass through these three gates.

The tide and acqua alta

Twice a day the sea breathes through those mouths: the water floods into the lagoon over roughly six hours and drains back out over the next six. Mostly this rhythm is a mild, almost imperceptible rise and fall of a few tens of centimetres - but when the astronomical tide is high and the sirocco blows up from the south, warm and wet, it pushes extra water into the basin and Venice experiences acqua alta. The lowest parts of the city - first and foremost St Mark's Square - find themselves under water. The city is ready for it: the busier flooded squares are crossed by raised wooden walkways, and the tabacchi and souvenir shops sell rubber boots to keep feet dry. Since 2020 the modern answer has been MOSE, the system of movable barriers at the three port mouths that rises out of the seabed to hold the Adriatic at bay when the forecast demands it.

Caranto: the hidden bedrock

Anyone who digs in Venice discovers that the city floats on nothing but soft mud - and yet the palaces stand. The secret is a layer called caranto: a band of compacted clay, hard as rock, lying beneath the mud across much of the lagoon. The Venetian builders drove countless piles of oak and larch down through the soft silt until they bit into the caranto, then laid a platform of planks across the pile heads, and on top of that the foundations in Istrian stone and the brick walls. The submerged wood does not rot: sealed from oxygen in the waterlogged mud, it undergoes a slow process of mineralisation that makes the ground under the building ever more resistant. Every great palace lining the Grand Canal still rests on this invisible forest of piles.

The rivers that wanted to bury it

The same rivers that created the lagoon also threatened to destroy it. Carrying sediment down from the mainland, they would in time have filled the basin in completely, turning the lagoon into dry land. Left to nature, the lagoon's destiny was progressive silting - and the Venetians knew it. Through patient works of hydraulic engineering they diverted the rivers, moving their mouths outside the lagoon so that their sediment would be carried away into the Adriatic instead of settling among the islands. Torcello shows what would have happened without that vigilance: even after the rivers were diverted, the basin gradually filled with sediment around the island; by the year 1000 or so the tiny island had eleven churches, yet it lacked drinking water, its farmland was poor, and malaria was endemic - so it was slowly abandoned. The fight against the mud is as old as Venice itself - and it is on this unstable ground of tiny islands and barene, held together by human ingenuity, that the most original city in Europe was built.