The vast bodies of water on Earth come in various sizes and depths, two of the most significant being oceans and seas.

While both are large bodies of saltwater, there are key differences in terms of size, depth, and location that distinguish the two.

Oceans: The Vast, Deep Blue

Oceans are massive bodies of saltwater that cover about 71% of the Earth’s surface, making them the largest water bodies on the planet. There are five oceans—Atlantic, Pacific, Indian, Arctic, and Southern—and they are characterized by vast, open waters that extend far beyond coastal areas.

Oceans are typically much deeper than seas. The Mariana Trench, located in the Pacific Ocean, is the deepest part of the world’s oceans, reaching depths of over 10,900 meters (35,000 feet). Oceans are deep and have expansive basins, making them the prime features of Earth’s hydrosphere

Seas: Smaller and Shallow

In contrast, seas are smaller bodies of saltwater that are typically partially enclosed by land. While seas can still be large, they are much smaller than oceans and are often found near coasts or in the vicinity of landmasses and islands. Seas are generally shallower than oceans because they are closer to the land.

The Ocean-to-Sea Transition: Size and Location Matter

There isn’t a specific point that defines when a sea becomes an ocean or vice versa. The distinction is based on size and location. For instance, the Caribbean Sea is considered a sea, even though it covers a significant area.

This classification is because it is largely surrounded by islands and continents. On the other hand, the Pacific Ocean, with its immense size, is defined as an ocean.

Depth: Oceans Are Generally Deeper

Oceans are typically much deeper than seas. The vast, open basins of oceans allow them to reach extreme depths, like the Mariana Trench, which holds the record for the deepest point on Earth.

In contrast, seas tend to be shallower because they are located closer to landmasses. While seas can still be deep in places, they generally don’t reach the same depths as the world’s oceans.

The Seven Seas Myth

The term Seven Seas is often used historically but doesn’t accurately reflect modern geography. In ancient times, the term referred to various bodies of water that were known to different civilizations. Today, we recognize five oceans on Earth: Atlantic, Pacific, Indian, Arctic, and Southern.

Summary: Oceans vs. Seas

In essence, oceans are vast, deep, open bodies of water that cover most of Earth’s surface, while seas are smaller, shallower bodies of saltwater usually located near land or partially enclosed by islands.

Oceans hold the deepest parts of Earth, while seas are more connected to the land, often with unique characteristics shaped by their proximity to shorelines. Despite their differences, both oceans and seas play essential roles in Earth's hydrosphere, supporting life and regulating the global climate.