What is the only sea in the world with no coastline?

The Sargasso Sea is the only sea in the world defined without a coastline.

Instead of being bounded by land, the Sargasso Sea is surrounded by ocean currents in the North Atlantic. Its approximate boundaries are formed by the North Atlantic Current, Gulf Stream, North Atlantic Drift, and Canary Current. These currents create a broad rotating system known as the North Atlantic subtropical gyre.

The sea is named for Sargassum, a floating brown seaweed that forms large mats at the surface. Portuguese sailors encountered the seaweed during Atlantic voyages and called the region after it. Unlike most seaweed, Sargassum in the Sargasso Sea spends its entire life floating rather than attaching to the seabed.

The area is an important habitat for young sea turtles, eels, seabirds, and many open-ocean organisms. Its boundaries are not fixed lines, because currents shift seasonally and the concentration of Sargassum varies. That moving, current-defined character makes it unique among named seas.