What’s behind the giant brown ribbon stretching thousands of miles across the Atlantic ocean?

The Great Atlantic Sargassum Belt has become one of the ocean’s most unusual environmental signals. A giant brown ribbon of floating algae is now stretching across the Atlantic, connecting waters near West Africa, the Caribbean, Florida, and the Gulf of Mexico.

Seen from space, the bloom looks like a massive natural stain. However, scientists say this is not simply an ocean surface change. It represents a deeper shift involving marine chemistry, currents, pollution, and climate-related pressures. In May 2025, satellite observations estimated the Great Atlantic Sargassum Belt at around 41 million tons, marking a record level for the Atlantic basin. The scale surprised researchers because sargassum was once mainly linked with the Sargasso Sea.