The Languedoc coastline is low and sandy, and contains within it a string of coastal lagoons. The largest of these is the Etang de Thau, with a surface area of more than 7,000 hectares. It is bounded by a 20-kilometre-long lido, or sand spit, which is anchored at its centre on the island town of Sète and at one end on the volcanic formation of Mont Saint-Loup (Agde). Channels – or graus – in the spit connect the brackish waters of the lagoon with the sea. This type of shore is a shifting milieu, which is today threatened by human activity and rising sea levels. Conversely, the lagoon's interior shore features an unchanging landscape of hills sculpted by an regular network of streams, which has determined the location of human settlements since the Neolithic.
On average, the Etang de Thau is about four metres deep. This is much deeper than most of the other lagoons, which serve as outlets for silt-laden coastal rivers. Its waters, which warm rapidly in springtime, are home to a rich ecosystem that is exploited to the present day. Seasonal fishing and gathering wild shellfish have given way to shellfish farming. Saltworks have been present since the medieval period. Until recently, the lack of compartmentalisation between the various lagoons made this "interior sea" a precious communication route