The phrase “the Seven Seas” has been around for centuries, but that term really refers to different parts of the ocean and several other large bodies of water. There are actually more than seven seas in the world. But what makes a sea different from other bodies of water?

That is not an easy question to answer, because the definition of a sea leaves some room for interpretation. In general, a sea is defined as a portion of the ocean that is partly surrounded by land. Given that definition, there are about 50 seas around the world. But that number includes water bodies not always thought of as seas, such as the Gulf of Mexico and the Hudson Bay.

Moreover, in some cases, a sea is completely landlocked. The Caspian Sea is the most famous example, though this sea, which lies between Russia and Iran, is also referred to as the world’s largest lake. Other seas surrounded by land include the Aral Sea and the Dead Sea. They contain saltwater and have been called seas for many years, but many oceanographers and geographers are more inclined to call them lakes.

Still, that leaves dozens of water bodies that fit the traditional definition of a sea, even though they can be quite different from one another. A sea can be more than 2.6 million square kilometers (1 million square miles) in area, such as the Caribbean Sea. Or, it can be as tiny as the Sea of Marmara, which is less than 12,950 square kilometers (5,000 square miles) in area. This tiny Turkish sea connects the Aegean Sea and the Black Sea.

A sea can also be very warm for most of the year. The Red Sea, for instance, has an average temperature of around 30 degrees Celsius (86 degrees Fahrenheit). It is also the saltiest sea, containing 41 parts of salt per 1,000 parts of seawater. Seas can be quite cold, too. The Greenland Sea, for instance, has surface water that hovers near the freezing mark most of the year.

The variety of the sizes, temperatures, and locations of the Earth’s seas also means that the marine ecosystems within each sea can vary greatly from one to the other. The Baltic Sea in Scandinavia is the world’s youngest sea having formed between 10 thousand and 15 thousand years ago from glacial erosion. It contains a unique mixture of saltwater and freshwater, making it the largest brackish water body on the planet. As a result, the Baltic Sea contains a small, but rare, variety of freshwater and saltwater plants and animals that have been able to adapt to their brackish environment.

Not surprisingly, the diversity of the world’s seas also draws National Geographic explorers, such as oceanographer Katy Croff Bell. She was part of the crew aboard the exploration vessel Nautilus, a ship that shared its scientific discoveries in the Mediterranean Sea, the Black Sea, and elsewhere with students around the world in online lessons and chats. She says the seas—big and small, cold and warm—can teach scientists about the rest of the world. “We’re going to places that have never been explored to see what’s there,” Bell told MIT Technology Review in 2015. “There are things we can’t even conceive of out there, and it will take a long, long time to fully understand our own planet.”