What is the only sea with no coastline, defined instead by ocean currents?

The Sargasso Sea is the only sea defined by ocean currents rather than a coastline.

The Sargasso Sea lies in the North Atlantic Ocean and is bounded by four major 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. Because these boundaries are moving water, the sea has no land coastline.

Its name comes from Sargassum, a type of floating brown algae that forms large mats at the surface. Portuguese sailors encountered the algae during Atlantic voyages and gave it a name related to the grapes they saw in the floating masses. Sargassum provides shelter and food for young fish, crabs, shrimp, and other organisms.

The sea’s warm, relatively calm waters became a major subject of maritime lore, including stories about ships trapped by seaweed. In reality, currents and weather—not giant algal barriers—explain the region’s physical character. The Sargasso Sea is also important for the life cycle of European and American eels, which migrate there to spawn.