How many oceans are there?

The count most authorities now use is five: the Pacific, Atlantic, Indian, Arctic, and Southern. The first four have long been standard. The Southern Ocean, the band of water encircling Antarctica, was recognized by the International Hydrographic Organization in 2000 and adopted by the US Board on Geographic Names, but many atlases and schools continued with four until National Geographic formally recognized it in 2021. Its case rests on oceanography rather than coastline: the Antarctic Circumpolar Current forms a distinct boundary in temperature and salinity that separates it ecologically from the waters to the north.

The more fundamental answer is one. All five are connected in a single continuous body of salt water covering about 71 percent of the planet, and the divisions are conventions of naming and navigation rather than physical walls. The distinction between an ocean and a sea is likewise conventional, with seas generally being smaller, partly enclosed by land, and connected to an ocean, though the Caspian and Dead Seas are lakes and the Sargasso Sea has no coastline at all.

Scale worth having. The Pacific alone is larger than all of Earth's land combined and holds roughly half of the ocean's water. The average ocean depth is around 3,700 meters, and the deepest point is nearly 11,000 meters in the Mariana Trench. Over 80 percent of the ocean floor is unmapped at high resolution, which is why the comparison to the surface of Mars, mapped in far greater detail, is accurate rather than rhetorical.