Étang de Thau or Bassin de Thau is the largest of a string of lagoons (étangs) that stretch along the French coast from the Rhône River to the foothills of the Pyrenees and the border to Spain in the Languedoc-Roussillon. Although it has a high salinity, it is considered the second largest lake in France.DescriptionIt is about 21 km long and 8 km wide, with an area of 7,012 hectares. The mean depth of the lagoon is 4.5m, but in the central navigation channel it can be 10 metres deep. Near Bouzigues there is a 100 metre diameter depression of 30 metres. This 'Fosse de la Vise' is the source of a hot spring that feeds the spa in Balaruc.Its size and depth, which distinguish it from other lagoons of the region, is explained by the geomorphology of the region: it is the anticline formed from folding which produced the corresponding syncline of the Gardiole in the north east.Until relatively recently the lagoons from Marseillan to the Rhône were a continuous stretch of inland waterway. Early settlers described this as 'une petite mer intérieure et tranquille'. It provided access to, in particular, Marseillan - a fishing village that became a trade centre.Linked, now, by the Canal du Rhône à Sète to the river Rhône and by the Canal du Midi to Bordeaux via Toulouse, the lagoon also has access to the Mediterranean at Sète. There is also a small canal 'le canal des Allemands' or the 'pisse-saume' that links the western end to the sea at Marseillan Plage. This canal is only suitable for small craft since both road and railway bridges restrict height.