A seemingly ordinary grass, Spartina anglica, has transformed one of France's most iconic landscapes, the bay of Mont-Saint-Michel, over the past century. Introduced in the 1920s to stabilize shifting sediments, this grass has now colonized about a fifth of the bay. It traps sediments and accelerates siltation, so much so that it has connected the Tombelaine rock to the coast during high tide. To combat this, a dredging project with an estimated cost of over 190 million euros has been launched, aiming to contain the phenomenon and preserve the area's biodiversity. In the 1920s, French engineers sought to stabilize the bay's sediments and facilitate poldering, the process of reclaiming agricultural land from the intertidal zone. They chose Spartina anglica, a grass native to England known for its ability to thrive on mudflats. The idea was that this hardy plant would fix loose soils and help expand land from the sea. However, a century later, the outcome is quite different. The grass has taken over about a fifth of the bay, turning once-tidal areas into dense, compact meadows. What was once a beneficial plant has become a major contributor to siltation, gradually eroding the intertidal zone and reducing the open landscapes and bird habitats that make the site famous. The spread of Spartina anglica is not unique to Mont-Saint-Michel. It has also been found in Normandy, the Manche, the Gironde estuary, and the Arcachon basin, where it was introduced as early as 1924. In each location, the same pattern has repeated: a plant intended as an engineering tool has become invasive and difficult to control. The species is particularly adaptable, thriving in unstable and frequently submerged soils where other plants struggle. This has led to multiple eradication campaigns, none of which have successfully reversed the spread of the grass over the long term. The mechanism behind Spartina anglica's impact is both simple and highly effective. Its stems and leaves form a natural net that traps fine sediments carried by tides and currents. Over time, this process accelerates the accumulation of sediment, transforming the bay's landscape. In Mont-Saint-Michel, this has led to an unexpected scale of change, with the grass forming dense colonies that compete with native plants like the common sea beet, which typically thrives on the exposed mudflats known as slikke. A direct and symbolic consequence of this transformation is the connection of the Tombelaine rock to the coast during high tide, a change that has displaced birds from their feeding grounds. The dense vegetation now covers nearly 3,500 hectares, playing an essential ecological role by retaining sediments and providing food for the famous salt meadow sheep. However, the unchecked spread of Spartina anglica has disrupted the area's ecological balance, reducing the diversity of environments that once supported a wide range of wildlife. Shorebirds, which rely on open mudflats to feed on worms and crustaceans, are now losing their hunting grounds as the grass closes in. In response to this challenge, a large-scale dredging project has been initiated in the Mont-Saint-Michel basin, with an estimated budget of over 190 million euros. The goal is to halt the siltation process and preserve the area's unique maritime character, which has been designated a UNESCO World Heritage Site. Managing Spartina anglica, however, remains a complex task, as its spread is closely tied to the specific sediment dynamics of each estuary. What works in one location may not be effective in another, making sustainable eradication efforts particularly challenging.