The Sargasso Sea is the only sea on Earth defined without a coastline, instead bounded by ocean currents in the North Atlantic.
Its approximate boundaries are formed by the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east and the North Atlantic Equatorial Current to the south. These currents create a large rotating system called the North Atlantic subtropical gyre.
The sea takes its name from Sargassum, a floating brown seaweed that can form extensive mats at the surface. Portuguese sailors noticed the weed during Atlantic voyages and called it sargaço. Unlike seaweeds rooted to the seafloor, Sargassum in the Sargasso Sea floats and provides shelter and feeding habitat for many organisms.
The Sargasso Sea is not a sharply outlined body of water like a conventional coastal sea. Its position and boundaries shift with ocean conditions. It is also associated with the migration of European and American eels, which travel to the region to reproduce.