We use the words “ocean” and “sea” interchangeably all the time, but geologically speaking, they aren’t the same thing. While both are massive bodies of saltwater that are essential to marine life, there are distinct differences in their size, depth, and relationship to land.

If you’ve ever looked out at the water and wondered exactly what you were looking at, here is the breakdown of oceans vs. seas.

What is an Ocean?

Oceans are the largest bodies of water on Earth. They are vast, incredibly deep, and completely uncontained by land on a global scale. In fact, oceans cover about 71% of our blue planet’s surface.

There are five recognized oceans on Earth:

• The Pacific Ocean (the largest and deepest)
• The Atlantic Ocean
• The Indian Ocean
• The Southern (Antarctic) Ocean
• The Arctic Ocean

Because of their immense size and depth, oceans drive global weather patterns and regulate the Earth’s temperature.

What is a Sea?

Think of seas as the gentle, enclosed edges of the mighty deep. Seas are smaller than oceans and are usually located where the land and ocean meet.

Unlike oceans, which are open and boundless, seas are typically partially enclosed by land. For example, the Mediterranean Sea is almost entirely surrounded by land, connecting to the Atlantic Ocean only through a narrow strait. The Caribbean Sea and the Bering Sea are other great examples—they are distinct bodies of water, but they act as transitional zones between the coastlines and the open ocean.

The Key Differences at a Glance

Feature	Ocean	Sea
Size	Massive; the largest bodies of water on Earth.	Smaller; acts as a sub-section of an ocean.
Depth	Extremely deep (averaging around 12,000 feet).	Shallower, allowing sunlight to reach the ocean floor.
Borders	Open and boundless.	Partially or mostly enclosed by land.
Marine Life	Hosts deep-sea creatures and migratory species.	Rich in biodiversity due to shallow, sunlit waters.

Both oceans and seas are beautiful, mysterious, and absolutely essential homes for marine life. The next time you visit the coast, take a look at a map—you might just be swimming in a sea rather than an ocean!

Did You Know?

Most sea life hates the open ocean. While the ocean is massive, the vast majority of marine life actually lives in the shallower seas. Because sunlight can reach the bottom of a sea, it allows underwater plants to grow, creating the foundation for complex food webs.

There is one sea with no land boundaries. The Sargasso Sea, located entirely within the Atlantic Ocean, is the only sea on Earth that doesn’t touch land. Instead of coastlines, it is bounded entirely by four massive, swirling ocean currents.

We know more about space than our own oceans. Because the deep ocean is pitch black and under crushing pressure, humans have mapped more of the surface of Mars and the Moon than we have of our own ocean floor.

Oceans aren’t just an Earth thing. While our planet is famous for its surface oceans, scientists have discovered massive, hidden oceans sloshing around underneath the icy crusts of moons in our solar system, like Jupiter’s Europa and Saturn’s Enceladus.