Aerial view of workers removing sargassum from the shore of Playa del Carmen Beach in the state of Quintana Roo, Mexico, on June 18, 2025. Large quantities of planktonic macroalgae known as sargassum are clogging Mexico's beaches and causing significant economic losses to tourism companies, both in terms of cleanup costs and canceled reservations. (Photo by Elizabeth Ruiz / AFP)

Record levels of sargassum seaweed are slowly moving west and north across the Atlantic Ocean, piling up on the shores of eastern Mexico, Puerto Rico, the Caribbean islands and Florida.

At the end of May, scientists at the University of South Florida Optical Oceanography Lab sounded the alarm: Sargassum seaweed was forming earlier and in more abundance than ever this season in the Atlantic -- worse than the record year of 2022.

Where is the seaweed now, and where will it go?

Averaged over the month of May, this map shows the concentration of Sargassum seaweed across the Atlantic ocean. Black areas did not have sufficient data to estimate. (USF COOL)

The concentration of Sargassum seaweed across the Atlantic ocean, averaged over the last week of June. Black areas did not have sufficient data to estimate. (USF COOL)

At the end of June, the largest concentration of the 33.5 million metric tons of sargassum was in the eastern Caribbean Sea and western Atlantic. This broke historical records for the basin in April, May and June, but the total amount in June was a decrease from the 37.5 million metric tons recorded the previous month.

In Miami, sargassum removal is an annual event, accounting for $9 million of the $11 million beach maintenance budget each year, and this year is no different. The Miami area and parts of Key West are listed as a high risk for sargassum by NOAA's CoastWatch website, but other areas of the United States coast are at a low risk.

This map shows the risk of Sargassum seaweed affecting coastal areas. Black areas or missing coastlines did not have sufficient data to estimate risk. (USF COOL)

Most of the Caribbean islands and parts of the Mexico coast are in a high risk. Piles of the stinky seaweed rolled up on beaches in Puerto Rico in early June, as well as eastern Mexico in early and mid-June. In the Dominican Republic, officials called for UNESCO to declare sargassum a regional environmental emergency on June 30.

More sargassum is likely to move into the Gulf during July, according to USF. Some of that seaweed could make it to Florida and the Gulf Coast later in the summer. Ultimately, currents and wind patterns will dictate which beaches are inundated with the brown stuff.

Averaged over one week periods, this map shows the concentration of Sargassum seaweed across the Atlantic ocean from early to late June. Black areas did not have sufficient data to estimate. (USF COOL)

What is sargassum?

Sargassum is a kind of seaweed, which is a type of algae. Sargassum never touches the seafloor until it dies. The algae tends to float in islandlike chunks and serves as breeding grounds for fish, turtles, birds and other animals.

Piles of sargassum stretch across the shore in Playa Lucía, Yabucoa, Puerto Rico, Monday, June 2, 2025. (AP Photo/Alejandro Granadillo)

Is seaweed dangerous?

The hydrogen sulfide gas it releases when it rots is not only stinky but can irritate your eyes, nose and throat, the Miami Herald says. The seaweed can also contain jellyfish or sea lice that can irritate your skin.

A young boy plays walks across seaweed that has accumulated on the beach, Tuesday, June 24, 2014, in Port Aransas, Texas. The Sargassum is piled and removed as it continues to wash onto the beach. (AP Photo/Eric Gay)