The lagoon is located inside a vast basin of 2.5 x 1,750 km, formed by hydro-wind and even snow-wind, which it almost completely fills. It is the result of the action of snow and the mistral at the end of the Riss glaciation. A permanent body of water, at variable level, therefore with an equally fluctuating surface area, it is supplied regularly by injections of industrial brine. It is part of the municipalities of Istres, Saint-Mitre-les-Remparts and Fos-sur-Mer, according to a demarcation kept in the municipal archives of Saint-Miter, dated 15 April 1551. It is bordered by the east and west by hills covered with scrubland and pine forests. To the south and north, there are palustrine formations and elements of riparian forests.