Venice and its Lagoon has been a UNESCO World Heritage Site since 1987, and the listing covers far more than the city: about 70,176 hectares of water, marsh and islands in the Venetian Lagoon, an enclosed bay of the Adriatic Sea in north-eastern Italy. The lagoon stretches about 550 km² between the mouth of the River Sile in the north and the Brenta in the south, and it is the largest wetland in the Mediterranean basin. A chain of sandbar islands, chiefly the Lido and Pellestrina, separates it from the open sea, with three inlets at Lido, Malamocco and Chioggia letting the tide in and out. Those inlets are why the site is fragile: twice a day the Adriatic flows into the lagoon, and when tide, wind and low pressure line up, it floods Venice itself.
Acqua alta: the Adriatic tide that floods St Mark's Square
The Venetian year has a wet season, and it is not the rainy one. Between autumn and spring the city braces for acqua alta, "high water": tide peaks that push seawater up through drains and over the pavements of the lowest districts, starting with Piazza San Marco. Three forces combine. The astronomical tide, driven by the Moon and Sun, can be predicted years ahead. The shape of the Adriatic, a long, narrow basin, sets up a seiche, a slow sloshing of the whole sea with a main period of about 21 hours 30 minutes and an amplitude of around half a metre at its ends, which amplifies the tide at the northern end where Venice sits. The third force, the weather, is the unpredictable one: when the sirocco blows north up the Adriatic, or the bora pushes into the inlets, water is held inside the lagoon and the next high tide stacks on top of the last.
Tides are measured at the station beside the Basilica di Santa Maria della Salute against a reference level averaged over 1897. A tide of 80 to 109 cm counts as intense, 110 to 139 cm as very intense, and anything from 140 cm upwards as exceptional. The record remains the flood of 4 November 1966, when the water reached 194 cm. The second highest came on 13 November 2019, when the tide peaked at 187 cm, more than 80 percent of the city went under water and over 50 churches were damaged.
Part of the problem is that Venice has sunk while the sea has risen. The city's tide centre estimates that Venice lost 23 cm of elevation relative to the sea between 1897 and the present: 9 cm from rising sea level, 3 cm from natural subsidence and 13 cm from extra subsidence caused by human activity, chiefly the pumping of groundwater for the industries of Porto Marghera, offset by 2 cm of rebound once pumping stopped. Filling in salt marshes to build that industrial zone also removed the barene that used to soak up surges. Tides of 110 cm, which occur about four times a year now, are expected to arrive around 60 times a year within 50 years.
MOSE: 78 gates across the Lido, Malamocco and Chioggia inlets
The answer Italy chose was to close the lagoon's mouths on demand. MOSE, from Modulo Sperimentale Elettromeccanico and a nod to Moses parting the sea, is a set of 78 hollow metal box gates lying in concrete housings on the seabed of the three inlets: two rows at the Lido inlet, one at Malamocco and one at Chioggia, 1.6 km of barrier in all. Normally the gates sit full of water and out of sight. When a dangerous tide is forecast, compressed air drives the water out and the gates swing up on their hinges until they break the surface and form a wall against the Adriatic. Raising takes about 30 minutes, lowering about 15, and the inlets stay closed for four to five hours per event. The largest gates, at Malamocco, measure 29.5 by 20 by 4.5 metres. A lock at Malamocco lets large ships reach the port while the barrier is up, and three small locks serve fishing boats.
The planning began in 1987 and construction in 2003. A 2011 completion date slipped to 2018, then 2021, amid cost overruns and a corruption scandal. The first full test came on 10 July 2020, and on 3 October 2020 the gates were raised against a real high tide for the first time, keeping Piazza San Marco dry. Designers had expected around five closures a year; within two years MOSE had been raised 49 times. The system is built for tides of up to 3 m and a sea-level rise of 60 cm; conservation groups such as Italia Nostra and WWF have opposed the project from the start, questioning its cost and its effect on the lagoon's hydrological balance and ecosystem.
A 550 km² lagoon behind the Lido and Pellestrina
Strip away the city and the lagoon is mostly mud and water. Only about 8 percent is land. About 11 percent is open water in channels, many of them dredged, and around 80 percent consists of mudflats, tidal shallows and salt marshes that flood and drain with each tide. It formed six to seven thousand years ago, when the sea rose after the last ice age and flooded the upper Adriatic plain; rivers carried in sediment, and sand drifting along the coast from the Po built bars across the bay. It is the main survivor of a string of lagoons that in Roman times ran from Ravenna to Trieste.
Left alone, it would have silted into marsh. Venetian hydraulic works in the 15th and 16th centuries reversed that natural course, and later centuries hardened the edge facing the sea. The barrier islands are essentially dunes. The Lido is 11 km long, home to about 20,400 people and the Venice Film Festival; Pellestrina, also 11 km long, has been protected on its seaward side by large stone embankments since the 18th century. The Malamocco inlet between them is the gateway for large ships heading to the port, which is why MOSE's main navigation lock was built there rather than at the Lido, the widest of the three inlets. The island of Venice itself covers just 5.17 km², and in 2021 about 50,400 people lived in the historic centre, against 26,800 on the other lagoon islands and 177,600 on the mainland.
Torcello, Murano, Burano and a city on alder piles
The lagoon was settled as a refuge. In the 5th and 6th centuries people from Roman towns on the mainland, including Altinum, Aquileia and Padua, fled the Huns and later the Lombards into the marshes, where invading armies could not follow. Torcello, at the northern end, was one of the first islands to fill up, especially after Attila destroyed Altinum in 452; in 638 the Bishop of Altino moved his seat there, and the island had a cathedral before Venice had St Mark's. Today it is sparsely populated: from the 14th century the marshes around it spread, boats could no longer reach it, and malaria grew worse. Tradition dates Venice's own founding to the dedication of the church of San Giacomo on the Rialto islet on 25 March 421.
Building in a lagoon took a technique of its own. The settlers drove closely spaced piles of alder, a timber that resists rot underwater, through the soft mud and sand until they reached a hard layer of compacted clay, then laid plates of Istrian limestone on top as a footing for brick walls. Sealed in waterlogged mud without oxygen, the wood has lasted for centuries. The city that grew on these foundations spreads across 118 small islands linked by more than 400 bridges.
The outer islands each found a trade. Murano, about 1.5 km north of Venice, became the glass island in 1291, when the Republic, fearing fire in a city of wooden buildings, ordered all glassmakers to move their furnaces there; it later made glass beads, mirrors and chandeliers for export across Europe. Burano, 7 km from Venice and linked by a bridge to Mazzorbo, is known for lace and for its brightly painted houses, and with about 2,800 residents on 0.21 km² it packs in more than 13,000 people per square kilometre, over twenty times the density of neighbouring Mazzorbo. Sant'Erasmo, at 3.26 km² the second-largest island after Venice itself, is known for its market gardens.
Venice and its Lagoon: UNESCO 1987 and the 2021 cruise-ship ban
UNESCO inscribed the site in 1987 as a single unit, recognising that the city cannot be separated from the water that protected and fed it. That is also why its status has stayed under review. By 2017 UNESCO was considering adding Venice to its list of World Heritage in Danger, citing mass tourism, with estimates of 22 to 30 million visitors a year and up to 60,000 on a single day, and the effect of cruise ships on the lagoon.
Large ships had sailed past St Mark's Square and down the Giudecca Canal for decades, eroding banks and stirring sediment. A ban on ships over 40,000 tonnes in 2013 was struck down by a court in 2015. After the cruise ship MSC Opera rammed a moored riverboat on the Giudecca Canal on 2 June 2019, injuring five people, pressure grew, and the Italian government diverted large cruise ships away from the historic centre from August 2021. In 2024 Venice began charging day-trippers an access fee on peak days, a measure its city council had first voted for in February 2019, and from June 2024 tour groups were capped at 25 people with loudspeakers banned.
For why the ground itself is dropping, our explainer on whether Venice is sinking goes further into subsidence, and if you want the landform in general, see what a lagoon is and how bars and inlets create one; the waterways inside the city are mapped out in our guide to the canals of Venice.