What is the only sea on Earth with no coastline?

The only sea on Earth with no coastline is the Sargasso Sea.

Rather than being enclosed by land, this unusual region of the North Atlantic is defined by four major 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. Together they form part of the clockwise North Atlantic Subtropical Gyre.

The Sargasso Sea takes its name from Sargassum, free-floating brown algae that gathers in surface mats. Portuguese sailors encountered the weed before Columbus, who wrote about it during his 1492 voyage. The calm conditions and thick-looking vegetation helped inspire old stories that ships could become trapped there, although the seaweed itself is generally not a serious navigation hazard.

The area is also an important ecosystem. Young sea turtles shelter among the algae, while European and American eels travel there to spawn. Its boundaries shift because they are based on currents rather than fixed shorelines.