At the edge of Pont Saint Martin, the Marais de l'île forms one end of the Grand Lieu lake via the river Ognon.
This 50 hectare wetland, crossed by the river Ognon, is a classified site in the preserved Natura 2000 and Ramsar zones. Its distinctive and original feature is to be higher in its center, forming an ephemeral island during the floods. The site starts getting accessible from the borough in May. Thanks to a the chain boat you can walk around the heart of the marsh. The diversity of natural habitats (wet meadows, reeds, hedges and woods) is favorable for biodiversity and pleasant for hikers. Recent inventories highlighted a remarkable fauna : otter, Rosalie des Alpes beetle, Petite violette butterfly and many birds (guifette moustac, purple heron, Great white egret...). Historical documents attest that human works have shaped the landscape in order to generate resources (agriculture, hunting, fishing). These activities still contribute to the ecological value of the site : mowing and grazing maintain open spaces. Since 2017, the municipality has been working at preserving the Marais de l'île, and spreading awareness about it, in particular through the implementation of a management plan.