Which sea has no coastline?

The Sargasso Sea in the North Atlantic Ocean is unique as the only sea defined not by land borders but by ocean currents. It sits within a rotating system of currents called the North Atlantic Gyre, which encircle it on all sides. The sea is named after the free-floating sargassum seaweed that carpets its surface, providing a habitat and nursery for creatures such as young sea turtles, eels and countless fish. Its water is notably clear, deep blue and calm. European and American eels famously travel thousands of kilometres to spawn in the Sargasso Sea, a migration that puzzled scientists for centuries.