The Sargasso Sea is the only sea named in the English-language United Nations list of seas that has no coastline.
Unlike most seas, the Sargasso Sea is defined by ocean currents rather than land boundaries. It occupies a region of the North Atlantic surrounded by the clockwise-flowing North Atlantic subtropical gyre. Its approximate limits shift because the currents and water conditions change over time.
The sea is famous for floating mats of Sargassum, a brown seaweed that provides habitat for young turtles, fish, crabs, and other organisms. Christopher Columbus wrote about seeing the seaweed during his 1492 voyage, and early sailors sometimes feared that its calm waters could trap ships.
The Sargasso Sea should not be confused with a garbage patch. It is a natural ocean ecosystem, although plastic pollution and climate change threaten species that depend on its floating habitat. Its unusual current-defined geography makes it a distinctive feature of the Atlantic.