Sargassum is a catch-all term for over 300 species of brown algae. The algae is actually beneficial when it's adrift at sea, as it provides food and protection for fish, mammals, marine birds, crabs and other organisms. But when the seaweed hits the beaches, it accumulates in obstructive, rotten-smelling mounds.

The periodic foul odor in the Florida Keys is primarily caused by the decomposition of Sargassum, a type of brown macroalgae. While Sargassum is a natural part of the ocean's ecosystem, massive "blooms" known as the Great Atlantic Sargassum Belt have reached record-breaking levels in 2025 and 2026. When these thick mats of seaweed wash ashore and begin to rot in the intense Florida sun, they release hydrogen sulfide gas, which has a pungent "rotten egg" smell. This biological process is accelerated by warmer ocean temperatures and nutrient-rich runoff from major rivers like the Amazon. Beyond the smell, the rotting seaweed can be a health hazard for those with respiratory issues and an ecological nightmare that smothers coral reefs and nesting grounds for sea turtles. Local officials in the Keys often face a difficult choice: leave the seaweed to protect the beach's natural structure or spend millions on heavy machinery to remove the stinky piles to maintain the tourism-dependent economy.