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.

Key West does not "always" smell, but it is subject to a natural phenomenon known as "Sargassum Season" (typically from April to October) that can produce a distinct odor. Sargassum is a type of brown seaweed that floats in large mats across the Atlantic. When these mats wash ashore on Key West's beaches (like Smathers or Higgs Beach) and begin to decompose in the hot Florida sun, they release hydrogen sulfide gas, which smells remarkably like rotting eggs. This "Big Stink" can be quite pungent near the water's edge, but it usually doesn't penetrate into the heart of Old Town or Duval Street. During the winter months (November to March), the water is clearer, the seaweed is minimal, and the island generally smells only of salt air and blooming jasmine. For 2026, many resorts have increased their "beach raking" efforts to remove the seaweed daily, but on particularly heavy days, the smell remains an unavoidable part of the tropical ecosystem.