Sargassum is a floating brown seaweed that naturally occurs in the Atlantic Ocean. Since 2011, massive blooms have appeared regularly in the Caribbean and West Africa. These mats of algae drift with currents and are pushed ashore in huge quantities. At sea, they provide habitat and shelter for fish, turtles, and other marine life. On land, however, they decompose, releasing hydrogen sulfide gas with a rotten-egg smell. This gas can cause headaches, nausea, and respiratory irritation for nearby residents. The accumulation also blocks access to beaches, affecting tourism and fishing. Its sudden growth is linked to nutrient pollution, climate change, and altered currents. Management is costly, as collection and disposal require heavy logistics. Sargassum thus represents both an ecological challenge and a socio-economic crisis.