The Venetian lagoon is a natural ecosystem in a precarious dynamic balance that must be maintained artificially. Since the appearance of the first anthropic settlements the lagoon has undergone hydro-geographical changes which have preserved its structure; in particular, during the venetian (well known as Serenissima) self-government period, interventions aimed at the hydraulic regulation of the lagoon were put into place: including levees construction upstream and the diversion of major rivers Brenta and Piave; the interface with the Adriatic sea ws also modified with the construction of coastal defences (Murazzi) and harbour entrance was reduced and gradually fortified , to mitigate the sea water ingression.
Currently there are three harbour entrances (in order from north to south): Lido-San Nicolò, Malamocco and Chioggia.
During the ‘90s, increasing ship traffic, both commercial and tourist, is one of the reasons (in addition to subsidence and eustatism) which helped to upset the delicate environmental balance of the lagoon, which is characterized by shallow waters (medium average depth of 1.2 m), unsuitable for the transit of cruise ships. For this reason, with the development of the Petrochemical Pole of Marghera, the Petroli Canal was excavated, which currently has a depth of 15m and a width of 90m and connects the harbour entrance of Malamocco with Marghera , which is until today the major terminal for commercial ships .
Cruise ships on the other hand, have access to the lagoon through the harbour entrance of Lido-San Nicolò, passing through the San Marco basin and continuing to the Giudecca Canal (depth of 12-17m and width of 300m) and finally docking in the touristic terminal of Marittima. It is precisely the path of the cruise ships that represents the core of the conflict between citizens and environmental associations on one side and institutions (the Port Authority) on the other. The transit of big ships causes significant environmental impacts, such as: erosion of the seabed of the canals; reduction of the surface of the adjacent sandbanks (called“barene”); the mutation of tidal currents; air pollution caused by discharges from the ships and water pollution due the release of wastewater and toxic substances .
Such impacts contrast with the protection principles of Venice and its lagoon, set out in the Special Law 16/04/1973 n. 171 , on: “Measures for the Safeguarding of Venice”, later supplemented by law 29/11/1984 n. 798 , on: “New Measures for the Safeguarding of Venice” which sets up the “Comitatone” (inter-ministerial committee integrated with representatives of local and regional authorities) that has tasks of planning, coordinating and controlling interventions in the Venetian lagoon.
Regarding the conflict, the first citizens’ mobilizations against the transit of big ships in the lagoon started around 2007 by the group Ambiente Venezia. In 2012, it is made up by the Comitato NO Grandi Navi – Laguna Bene Comune, that in addition to the organization of protests and the promotion of popular petitions and informational events, submitted several observations into the urban plan of Venice (Piano d’Assetto Territoriale – P.A.T.), managing to push through the Art.35 bis , through which City Council of Venice commits to exclude the incompatible ships with the environment of the lagoon and the historic centre of the city and further, to promote in-depth analysis of the environmental, social and economic impacts with the aim to define a maximum threshold of daily sustainability.
Besides this, the “Comitato NO Grandi Navi” presented several appeals to the European Commission, requesting the submission of alternative projects proposed to the Strategic Environmental Assessment (S.E.A.)  and the reduction of air pollution. The Comitato also demand a new Port Master Plan that, for the implementation of new projects, do not resort to the “Legge Obiettivo” , which would derogate from ordinary existing laws.
At the institutional level, in 2012 (following the great emotion aroused by the shipwreck of the Costa Concordia at Giglio Island) the D.L. “Clini-Passera” was enacted ; it would have prohibited the transit of the cruise ship into the Venetian lagoon, once an alternative solution would been found. In 2013, the T.A.R. (the regional administrative court) annulled the D.L. under the pressures of ship companies.
In recent years, several alternative plans have been presented though, in order to propose new routes for the big ships’ transit. In 2014 the Comitatone have opted for the project presented by Port Authority which provides for the excavation of a new canal called “Contorta Sant’Angelo”, which would link the Petroli Canal with the Marittima terminal.
Citizens and local institutions opposed to this project including the Comitato NO Grandi Navi, which prefers the solution of an off-shore port at entrance of Lido-San Nicolò as the only alternative project that respects the lagoon's environmental balance and the principles of gradualism, flexibility and reversibility. Instead, the City Council of Venice prefers the solution of a new tourist terminal at Marghera, even if it would be in conflict with the Special Law of 1973. Both subjects have filed an appeal against the plan of the new canal.