A 10 million-pound blob is riding ocean currents, heading for the tip of Florida.

The Great Atlantic Sargassum Belt—a goopy mass of leafy, floating seaweed stretching across 5,000 miles—is meandering its way past Florida and through the Caribbean.

“In the vast expanse of the ocean, it can be an oasis,” says Brian Barnes, a marine scientist at the University of South Florida. The patches of seaweed can be a home and source of food for passing fish and sea turtles. 

Historically, sargassum has been a natural part of the ocean ecosystem, but in the past decade that oasis has blossomed into a nuisance capable of causing serious damage—and a rotting, smelly one at that. Here's what we know about where it came from—and