Sargassum

Since 2011, a fleet of seaweed patches double the size of the contiguous U.S. has cycled from West Africa to Florida, threatening beaches from Martinique to Miami. This year, it could grow bigger.

Sargassum, a stinky algae, is choking beaches in Mexico and the Caribbean and wreaking havoc on the ecosystem — and tourism. From fertilizer to food, locals are experimenting with new uses for it.