A giant mass of seaweed is heading towards beaches in Mexico, Florida and the Caribbean, bringing with it toxic gases and a smell similar to rotting eggs. Visible from space, the Great Atlantic Sargassum Belt stretches from the coast of Africa all the way to the Gulf of Mexico. It is the biggest seaweed bloom on the planet, and for more than a decade researchers have watched as it has continued to grow in size. 2023 is predicted to be another record year. Madeleine Finlay speaks to sargassum expert Prof Brian Lapointe about why it’s getting bigger, what happens when it washes up on coastlines, and if anything can be done to deal with it. Help support our independent journalism at theguardian.com/sciencepod