A giant blob of brown seaweed may soon settle along swaths of beaches in Florida and across the Gulf of Mexico, scientists warn, fouling popular tourist destinations for months.

The seaweed a type of leafy, floating algae called sargassum usually spends most of the year bobbing in a 5,000-mile-wide mass across the Atlantic Ocean. The sargassum is generally beneficial while out at sea, providing food and breeding grounds for a variety of species, including fish, sea turtles and marine birds.

The true peril of sargassum comes when it washes ashore. The seaweed begins to rot after a few days on land, releasing hydrogen sulfide gas that smells like rotten eggs and leaving behind brown sludge that can foul beaches for weeks. Hydrogen sulfide can threaten human health, and the sheer amount of seaweed can be too much for local crews to deal with.

Parts of the Yucatn Peninsula in Mexico were shrouded in an estimated 200 tons of sargassum earlier in March, prompting warnings of excessive levels of seaweed near the popular Playa del Carmen. Officials warned some beaches were at risk of 3 feet of seaweed accumulation in a week, with no signs of the sargassum diminishing as summer approaches.