The Sargasso Sea is unique because it is the only sea on Earth that does not have any land borders. Instead, it is defined by four ocean currents: 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. This system of currents, known as the North Atlantic Gyre, traps a calm, blue area of water filled with a specific type of seaweed called Sargassum. Because it is surrounded by currents rather than land, its boundaries are constantly shifting. It is famously clear and was once feared by sailors because the lack of wind in its center could leave sailing ships stranded for weeks.