Every sea in the world is bordered either entirely or mostly by land. Except for one. In the North Atlantic Ocean, there lies a sea that breaks this rule—the only sea in the world to be entirely surrounded by, well, more sea.

Named after Sargassum, a species of seaweed that covers the area’s waters, the Sargasso Sea is the only sea without shores or coastlines. Instead of being bounded by land, it is defined by four ocean currents. It borders the North Atlantic Current on the north, the Gulf Stream on the west, the North Atlantic Equatorial Current on the south, and the Canary Current on the east. These currents form a clockwise-circulating gyre that surrounds the Sargasso Sea much like a terrestrial coastline would.

In addition to giving the Sargasso Sea this unique geographical claim to fame, the gyre that surrounds the sea also makes for excellent visibility, with warm, dark blue water transparent for up to 200 feet. The Sargasso Sea was even mentioned in Jules Vernes’ Twenty Thousand Leagues Under the Sea. More unfortunately, the stillness of the Sargasso’s calm waters allows the sea to trap up to 200,000 pieces of trash per square kilometer in some areas, forming what’s been dubbed the North Atlantic Garbage Patch.