As sargassum washes ashore every year in Key West, locals and visitors alike are primarily annoyed by the smell of the sargassum quantity on the beach. This giant blob of seaweed breaks up along the shore to create a mess on public beaches in Monroe County, but there's not too much danger involved.

The occasional "bad smell" in Key West is almost always caused by the natural decomposition of Sargassum seaweed. This brown macroalgae floats in massive "mats" in the Atlantic Ocean and frequently washes up on the shores of the Florida Keys, particularly during the warm summer months. When the seaweed reaches the beach, it begins to rot under the intense Florida sun, releasing hydrogen sulfide gas—which has a distinct "rotten egg" or sulfurous odor. This is a natural ecological process and is not a sign of pollution or sewage issues. In some specific areas near mangroves, you might also smell "sulfur" from anaerobic bacteria breaking down organic matter in the silt, which is often referred to as "marsh gas." While the city and various resorts work hard to rake the beaches and remove the seaweed, the sheer volume during a "bloom year" can be overwhelming. The smell is usually localized to the immediate shoreline; just a block or two inland, you are much more likely to smell the pleasant scents of saltwater, tropical flowers, and grilled seafood.