Batten down the hatches. Stay out of the water, and beware, because it’s coming. It isn’t a shark or a sea monster. No, this marine menace is…seaweed. More specifically, it’s a massive blob of the stuff twice the width of the continental United States. And that 5,000-mile-wide gunk raft is heading straight toward the Caribbean.

Since 2011, scientists have been monitoring colossal blooms of sargassum seaweed that have hit popular beach destinations in Florida, Mexico, and elsewhere in the Caribbean. This year’s bloom is one of the largest ever recorded, weighing an estimated 6.1 million tons, according to scientists at the University of South Florida Optical Oceanography Lab. Sargassum seaweed is a type of large brown algae, and there are more than 300 species in the Sargassum genus worldwide. The two types most often found in the Atlantic Ocean are narrowleaf gulfweed (S. natans) and broadleaf gulfweed (S. fluitans).

Seaweed clumps floating around Earth’s oceans is not always cause for alarm—they actually play an important role in healthy marine ecosystems. According to the Sargassum Information Hub, a joint project between the University of South Florida, the Intergovernmental Oceanographic Commission of UNESCO, and other institutions, these floating forests, at their normal distribution, provide a critical habitat for fish, marine mammals, seabirds, invertebrates, and reptiles. That includes threatened loggerhead sea turtles, which hide in the seaweed and eat crabs, mollusks, and various other critters that call sargassum home.

A juvenile loggerhead turtle hides in a patch of sargassum seaweed in the waters off Florida. The turtles can live 80 years or more and have a remarkable sense of direction. Females often return to lay eggs on the same beach where they hatched decades earlier.
A juvenile loggerhead turtle hides in a patch of sargassum seaweed in the waters off Florida. The turtles can live 80 years or more and have a remarkable sense of direction. Females often return to lay eggs on the same beach where they hatched decades earlier. Visual&Written SL/Alamy

Sargassum blooms, however—including the giant blob that’s approaching the Caribbean—are too much of a good thing. Scientists have identified several likely mechanisms behind the extreme overgrowth that turns a healthy habitat into a dangerous sargassum bloom, including warming oceans, shifting hurricane patterns, and increased runoff from deforestation and excess fertilizer use, which boosts levels of nitrogen and other chemicals that promote rapid seaweed spread. Eventually, the huge mat of sargassum sucks all the oxygen from the water, which forces out turtles and other animals and creates a dead zone.

For humans, the biggest problem with sargassum blooms occurs after it hits the beaches. Cleaning up the seaweed after it washes onto shore isn’t just difficult; it can also be dangerous. As sargassum rots, it releases toxic hydrogen sulfide, which can cause respiratory problems in people. The seaweed also contains arsenic, so ingesting it or using it in fertilizer can also pose real health risks.

As the Caribbean braces for this year’s sargassum invasion, Atlas Obscura compiled these photos that capture the beauty and peril of prior blooms. (We’d also like to formally pitch Syfy channel to make a horror movie about this monster algae.)

In coastal towns along Mexico’s Yucatán Peninsula, workers who clean up the masses of sargassum are nicknamed <em>sargazeros</em>. Already this year, the sargazeros have been hard at work in the tourist hotspot of Playa Del Carmen. There, even taxi drivers joined the effort after the leader of their union, Luis Herrera Quiam, called on them to assist with the cleanup.
In coastal towns along Mexico’s Yucatán Peninsula, workers who clean up the masses of sargassum are nicknamed sargazeros. Already this year, the sargazeros have been hard at work in the tourist hotspot of Playa Del Carmen. There, even taxi drivers joined the effort after the leader of their union, Luis Herrera Quiam, called on them to assist with the cleanup. Bret Reyes/Shutterstock

Park guard Roberto Varela walks over mounds of sargassum seaweed piled up on shore in Guanahacabibes Peninsula, Cuba. The westernmost point of Cuba, the finger of land includes the country's largest nature reserve. The surrounding waters are a crucial habitat for turtles, spiny lobsters, and red snapper.
Park guard Roberto Varela walks over mounds of sargassum seaweed piled up on shore in Guanahacabibes Peninsula, Cuba. The westernmost point of Cuba, the finger of land includes the country’s largest nature reserve. The surrounding waters are a crucial habitat for turtles, spiny lobsters, and red snapper. Alexandre Meneghini/REUTERS/Alamy

Workers remove heaps of sargassum near Tulum, Mexico, in August 2018. Scientists believe that the massive seaweed blooms of the last decade are likely the result of a combination of factors, including climate change, deforestation, and runoff from excess agricultural chemicals.
Workers remove heaps of sargassum near Tulum, Mexico, in August 2018. Scientists believe that the massive seaweed blooms of the last decade are likely the result of a combination of factors, including climate change, deforestation, and runoff from excess agricultural chemicals. ©Bénédicte Desrus/Sipa USA via AP Images

Normally crystal clear waters turned vivid orange when a seaweed bloom arrived off the coast of Saint Anne, on the French Caribbean island of Guadeloupe, leaving two boats usually rented out to tourists empty and unused.
Normally crystal clear waters turned vivid orange when a seaweed bloom arrived off the coast of Saint Anne, on the French Caribbean island of Guadeloupe, leaving two boats usually rented out to tourists empty and unused. ©GILLES MOREL/SIPA via AP Images

Two divers take photos of sargassum seaweed near Key Largo in Florida. The island is part of the Florida Keys and is well known for John Pennekamp Coral Reef State Park, where visitors can view tropical fish and a submerged Christ statue.
Two divers take photos of sargassum seaweed near Key Largo in Florida. The island is part of the Florida Keys and is well known for John Pennekamp Coral Reef State Park, where visitors can view tropical fish and a submerged Christ statue. Chris Gug/Alamy
In July 2011, a beach town in Martinique saw a huge swell of sargassum seaweed wash onto shore, covering the shoreline several feet deep with the foul-smelling algae. Scientists first began taking notice of the massive Caribbean blooms that year and have been tracking the blobs ever since.
In July 2011, a beach town in Martinique saw a huge swell of sargassum seaweed wash onto shore, covering the shoreline several feet deep with the foul-smelling algae. Scientists first began taking notice of the massive Caribbean blooms that year and have been tracking the blobs ever since. PATRICE COPPEE/AFP via Getty Images

Sargassum seaweed colors the water brown in Soliman Bay, north of Tulum, Mexico. In August 2022, workers painstakingly removed masses of seaweed from the beach by hand.
Sargassum seaweed colors the water brown in Soliman Bay, north of Tulum, Mexico. In August 2022, workers painstakingly removed masses of seaweed from the beach by hand. ©Eduardo Verdugo/AP Photo

In July 2022, Lakes Beach along the east coast of Barbados was covered in sargassum. The seaweed also smothered Long Pond, less than a mile to the north and one of the country's last ecologically significant coastal wetlands.
In July 2022, Lakes Beach along the east coast of Barbados was covered in sargassum. The seaweed also smothered Long Pond, less than a mile to the north and one of the country’s last ecologically significant coastal wetlands. ©Kofi Jones/AP Photo