What is the only sea named in the five oceans that has no coastline?

The Sargasso Sea is the only sea named in the five oceans that has no coastline.

Instead of being enclosed by land, the Sargasso Sea is defined by ocean currents in the North Atlantic. The North Atlantic Gyre surrounds it, with the Gulf Stream forming its western boundary, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south. Its boundaries can shift because currents and ocean conditions vary.

The sea is named for Sargassum, a type of floating brown seaweed found in large mats at the surface. The seaweed provides habitat for young eels, crabs, shrimp, and fish. Unlike many coastal ecosystems, the Sargasso Sea is an open-ocean region whose identity comes from circulation rather than shorelines.

Christopher Columbus reported seeing floating seaweed in the area during his 1492 voyage, and sailors later developed legends about calm conditions and drifting vessels. The Sargasso Sea is not the same as the wider North Atlantic Ocean; it is a distinct region within that ocean.