Only about 30 percent of the Earth's surface is dry land, oceans make up the remaining 70 percent. Earth's oceans are home to hundreds of thousands of marine life forms, but because most of the ocean depths remain unexplored, hundreds of thousands or even millions of unclassified life forms may exist.
There are four officially recognized oceans: the Pacific, Atlantic, Indian and Arctic oceans. Some scientists suggest that the Southern Sea or Southern Ocean is another ocean itself, while others counter that it is merely made up of parts of Earth's other oceans.
The Pacific Ocean is the largest ocean on Earth. The Pacific alone covers 30 percent of the Earth's surface, as much as all of Earth's continents combined. The word Pacific derives from the Latin word for peace, as the first European to traverse it, Ferdinand Magellan, found it much less turbulent than the Straits that bear his name. However, the Pacific is home to typhoons, earthquakes and volcanoes.
While Earth has four oceans, this has not always been the case. During the end of the Permian Period, over 250 million years ago, there was only a single ocean on the entire planet. This huge ocean was called Panthalassa, and it covered an even greater amount of the Earth's surface than all modern oceans combined.