To mark World Oceans Day on Tuesday, National Geographic officially recognized the Southern Ocean as the planet's fifth ocean, CBS News reports. Sometimes called the Antarctic Ocean or the Austral Ocean, the Southern Ocean is the body of water that surrounds Antarctica. All five of Earth's oceans—the other four are the Atlantic, Pacific, Indian, and Arctic Oceans—are connected, and the Southern Ocean's borders touch all of those except the Arctic. At first, it wasn't clear whether the body of water was unique enough to be given its own official name, or whether it should just be considered a southern extension of those three oceans.
Many scientists have long recognized it as its own ocean, "but because there was never agreement internationally, we never officially recognized it," a NatGeo geographer explains. Now that's changing, and as for the above-mentioned uniqueness question, NatGeo explains that the Antarctic Circumpolar Current (ACC) is what makes it so. While the other oceans are defined by the continents that surround them, the Southern Ocean is defined by the ACC, which makes the waters inside it colder and slightly less salty than its northern neighbors. That, in turn, makes the newly defined ocean a habitat for thousands of species that live only there. (Read more Southern Ocean stories.)