The salterns of Guérande is a swamp of salt water about 1 700 hectares in size. The current saltmarshes began before the 9th century and lasted for several centuries. Around the year 1500, the marshes reached 80% of the current surface. The latest were built around 1800. In the middle of the 19th century, a gradual decline started for different reasons : competition from a salt mine, lower consumption of salt as a product of conservation and improvement of transport by land.