An ocean is a vast body of salt water covering much of Earth, and a sea is a smaller body of salt water partly enclosed by land.
Ocean
An ocean is one of Earth’s five major bodies of salt water: Pacific, Atlantic, Indian, Southern and Arctic. Together they cover about 71 percent of Earth’s surface and hold 97 percent of its water.
Sea
A sea is a smaller body of salt water, usually partly enclosed by land and connected to an ocean. Examples: Mediterranean Sea, Caribbean Sea, South China Sea.
Exceptions
The Sargasso Sea has no land boundaries, and the Caspian Sea is a lake.
Depth
The deepest point is the Challenger Deep in the Pacific, about 10,935 m (35,876 ft).
Salinity
About 3.5 percent on average.
Use
In everyday speech, sea and ocean are used interchangeably.
Definitions
An ocean is a very large body of salt water covering much of Earth. There are five named oceans: the Pacific, Atlantic, Indian, Southern and Arctic. Together they cover about 71% of Earth’s surface and hold roughly 97% of its water. A sea is a smaller part of an ocean, usually partly enclosed by land, such as the Mediterranean, Caribbean, North Sea and South China Sea.
Size
The Pacific Ocean covers about 165 million square kilometers, more than all the land on Earth combined. The Atlantic is about 106 million, the Indian about 70 million, the Southern about 20 million and the Arctic about 14 million square kilometers. The largest sea, the Philippine Sea, is about 5 million square kilometers, and the Mediterranean is about 2.5 million.
Depth
The average ocean depth is about 3,700 meters. The deepest known point is the Challenger Deep in the Mariana Trench in the Pacific, at about 10,900 meters. Seas are generally shallower, as they are along continental shelves. The Mediterranean averages about 1,500 meters deep.
Exceptions in naming
Some names do not follow the rule. The Sargasso Sea has no land borders, and is defined by ocean currents. The Caspian Sea and the Dead Sea are large lakes, enclosed by land and not connected to any ocean. The Sea of Galilee is a freshwater lake. The Southern Ocean was recognized by National Geographic in 2021 as the fifth ocean, defined by the Antarctic Circumpolar Current.
Differences in conditions
Seas are more affected by land, with variable salinity, temperature and river inflow. The Baltic Sea is brackish, with salinity of 0.7% to 1.0%, compared with ocean average of about 3.5% (35 grams of salt per liter). The Red Sea is one of the saltiest at about 4%. Seas can warm and cool faster than open ocean.
Everyday use
People often use the terms interchangeably. “Sea” can mean the ocean in general in phrases like “sea level” and “at sea”. Scientists use “ocean” for the large basin and “sea” for marginal or enclosed portions.
Importance
Oceans produce about half of the oxygen we breathe through phytoplankton, absorb about 25% of human carbon dioxide emissions and about 90% of excess heat from warming, and supply food to about 3 billion people.
Naming exceptions and scientific facts
Oceans hold about 97 percent of Earth’s water and cover 71 percent of the surface. There are five oceans: Pacific, Atlantic, Indian, Southern and Arctic. Seas are smaller and partly enclosed by land, such as the Mediterranean, Caribbean and South China seas. The Sargasso Sea is unusual: it has no land boundaries and is defined by ocean currents. The Caspian Sea is called a sea but is a large saltwater lake. The average ocean depth is about 3,688 m (12,100 ft), and the deepest point is the Challenger Deep in the Mariana Trench at about 10,935 m (35,876 ft). Only about 25 percent of the seafloor has been mapped at high resolution, according to the Seabed 2030 project.
Based on data from NOAA and the Seabed 2030 project
