A portion of the ocean that's enclosed or partly enclosed by land is called a sea or a gulf—think of the Mediterranean Sea (nearly 2.5 million km²) or the Gulf of Mexico (~1.6 million km²).
What exactly is a partly enclosed sea?
A partly enclosed sea is a saltwater body that's mostly surrounded by land but still linked to the open ocean, letting water flow in and out like the Weddell Sea in Antarctica.
Some "seas" aren't seas at all—they're giant lakes with no outlet, like the Caspian Sea (roughly 371,000 km²) bordered by five countries. These form when ocean water gets trapped in basins as sea levels rise or when tectonic shifts cut off parts of ancient seas. They're vital ecologically but way more sensitive to pollution and salinity changes than open oceans.
So what's a gulf, then?
A gulf is a chunk of ocean partly cut off by land, usually bigger than a bay—the Gulf of Mexico covers about 1.6 million km², for instance.
Gulfs happen through erosion and tectonic shifts. They're often perfect spots for harbors and teem with marine life. Bays are smaller and cozier, like San Francisco Bay (about 1,240 km²), while gulfs push farther inland, like the Persian Gulf (~251,000 km²).
What usually borders oceans?
Oceans typically meet continents, islands, or underwater ridges, though their edges often include partially enclosed seas like the Bering Sea or Mediterranean Sea.
Seas act as nature's middle ground between land and open water, helping regulate climate and hosting incredible biodiversity. The Sargasso Sea, for example, is defined by currents rather than land borders. These zones are lifelines for global trade and critical pathways for migrating marine species.
What do we call water completely surrounded by land?
A water body totally cut off by land is called a lake, like Lake Superior (82,100 km²), the biggest freshwater lake by surface area.
Lakes can be natural or man-made, fresh or salty. They form from glaciers, tectonic shifts, or volcanoes. Salty lakes, such as the Dead Sea (605 km²), occur when evaporation outpaces water inflow, leaving behind super-concentrated minerals.
Which "sea" isn't really a sea on Earth?
The Caspian Sea is often called a sea that doesn't count because it's landlocked—scientists actually classify it as Earth's largest lake at about 371,000 km².
Other examples include the Dead Sea (605 km²) and the Aral Sea (mostly dried up since the 1960s). They're called seas but are technically lakes. These distinctions matter a lot for legal stuff like water-sharing deals between countries.
Is a gulf smaller than a lake?
Not usually—a gulf tends to be larger than most lakes, though the rules get fuzzy depending on where you are; the Gulf of Mexico (1.6 million km²) dwarfs Lake Victoria (68,800 km²).
A bay or gulf is defined by land partially enclosing it, while a lake sits completely inland. There are exceptions, like the narrow Gulf of Aqaba (174 km long), but most gulfs outsize freshwater lakes by a huge margin.
Which ocean is the saltiest?
The Atlantic Ocean holds the title for saltiest major ocean, averaging about 3.5% salinity, or 35 grams of salt per kilogram of seawater.
Salinity changes with evaporation, freshwater input, and currents. The Red Sea, an arm of the Indian Ocean, is even saltier in spots (up to 4.1%) thanks to intense evaporation and limited freshwater inflow. These areas often host marine life uniquely adapted to extreme conditions.
Are the Red Sea and Dead Sea the same thing?
Hardly—they're completely different; the Red Sea (438,000 km²) is a salty arm of the Indian Ocean, while the Dead Sea (605 km²) is a landlocked, super-salty lake.
The Dead Sea sits 430 meters below sea level with no outlet, packing salinity 10 times higher than the ocean (~34% vs. 3.5%). The Red Sea connects to the ocean, supports coral reefs, and hosts shipping lanes. They're over 300 kilometers apart and millions of years different in age—the Red Sea is about 25 million years old, while the Dead Sea formed around 3 million years ago.
What's the coldest sea on the planet?
The Arctic Ocean is the coldest sea, with average temperatures below 0°C (32°F) and areas frozen year-round.
Surrounded by landmasses and connected to the Atlantic and Pacific via narrow straits, the Arctic's icy, dense water drives global ocean currents. The Weddell Sea near Antarctica also gets brutally cold (often below -1.8°C or 28.8°F), but the Arctic takes the crown for overall coldness thanks to its isolation from warmer waters.
How many seas are there?
There are about 50 named seas worldwide, including the Mediterranean Sea, Caribbean Sea, South China Sea, and Red Sea.
Seas get their names from location, size, or historical importance. Some, like the Sargasso Sea, are defined by currents instead of land borders. Others, like Hudson Bay (1.23 million km²), are technically shallow inland seas. The exact number shifts depending on how geographers classify enclosed bodies.
Which seas are the biggest?
The three largest seas by area are the Australasian Mediterranean Sea (9.08 million km²), Philippine Sea (5.7 million km²), and Coral Sea (4.79 million km²).
These massive seas bristle with islands and vibrant marine ecosystems. The Australasian Mediterranean Sea, hugging Indonesia and Australia, includes the Arafura and Timor Seas. The Philippine Sea, in the western Pacific, features deep trenches and diverse wildlife, including endangered hawksbill sea turtles.
Can you name an example of an ocean?
The Pacific Ocean is the largest ocean on Earth, covering about 63 million square miles (165.2 million km²) and holding over half of Earth's free water.
The Pacific stretches from the Arctic down to the Southern Ocean, bordered by Asia and Australia to the west and the Americas to the east. It cradles the Mariana Trench, the deepest point on Earth (about 11,034 meters deep). Unfortunately, it also contains the Great Pacific Garbage Patch, a massive pollution hotspot caused by plastic waste.
What are the main types of water bodies?
The 10 primary water body types include oceans, seas, lakes, rivers, streams, glaciers, wetlands, estuaries, ponds, and groundwater aquifers.
Oceans and seas cover 71% of Earth's surface, while lakes and rivers store freshwater vital for drinking and farming. Glaciers in places like Antarctica and Greenland lock up about 69% of the world's freshwater. Wetlands and estuaries clean water and serve as fish nurseries, and groundwater aquifers quench the thirst of billions.
What's completely surrounded by land?
A lake is completely surrounded by land, like Russia's Lake Baikal, the deepest freshwater lake at 1,642 meters.
Lakes form from tectonic activity, glaciers, or volcanoes. They range from massive freshwater lakes to tiny, super-salty pools like the Dead Sea. Many lakes are biodiversity hotspots, recreation spots, and economic drivers through fishing and tourism.
What's smaller than a bay?
A run or creek is usually smaller than a bay; a creek might be just a few meters wide, while Chesapeake Bay spans over 11,600 km².
A run is a small, gentle stream often feeding into a river. Creeks and brooks are also tinier than bays, which are defined by their coastal indentations. Some bays are enormous, like the Bay of Bengal (2.2 million km²), and often provide safe harbors for ports and marine creatures.
Edited and fact-checked by the MeridianFacts editorial team.
