What is Sea? Everything You Need to Know

What is Sea? How many times have you asked this question without getting a proper answer? In this article, we will tell you everything you need to know about Seas. So let us start.

So, what is Sea? What is the definition?

In strict sense, a Sea is a body of salt water, that is partially or totally surrounded by land. So, a Sea can be landlocked from all of the sides or from most of the sides. A Sea may also be a body of salt water that is a part of one of the Oceans of the world.

A Sea is way smaller than an Ocean because it is either landlocked on all sides or most of the side or because it is a part of the Ocean. That doesn’t mean that Seas are small.

What are the different types of Seas?

The answer to the question – ‘what is Sea?’ – remains incomplete unless we define the categories or types of Seas. Broadly speaking, there are three different types of seas. They are:

  • Almost Enclosed Seas
  • Partly Enclosed Seas
  • Hypersaline Lakes

Let us take a look at each type individually.

Almost Enclosed Seas:

These are the types of Seas that reach very deep into the continents. However, they do stay connected with one or the other Ocean of the world through Straits, which are basically narrow waterways.

These types of Seas are almost enclosed or landlocked. It is because of this, the range of tides they get are limited. Some of these Seas may not have any tides at all.

Here is an interesting thing about Almost Enclosed Seas:

Because these Seas are connected to Oceans by Straits, the water exchange between these Seas and the Oceans are limited. This is why, the chemical composition of water in these Seas vary drastically from the chemical composition of the Oceans they are connected with. Similarly, their physical properties and their biological properties may also vary significantly from the Oceans they are connected with.


The best known examples are the Mediterranean Sea and the Baltic Sea. Red Sea is yet another popular ‘Almost Enclosed Sea’.

As we said, the ‘Almost Enclosed Seas’ are usually connected to Oceans through Straits that are narrow and hence, the water exchange between the two is very limited, causing differences in chemical, physical and biological properties. For example, Red Sea, which is connected to the Indian Ocean using Bab-el-Mandeb Strait is way saltier than the Indian Ocean.

Again, about 10% of the fish species that are found in the Red Sea are not found anywhere else in world – not even in Indian Ocean.

Partly Enclosed Seas:

Partly Enclosed Seas are those types of seas that are connected to the Oceans by a very wide opening. They behave almost in the same way like the Oceans they are connected to, especially in terms of the water circulation.

Again, a series of islands may also sit between a Partly Enclosed Sea and the Ocean to which it is connected. Such Seas are usually connected to the Oceans through a series of passages that pass right between the islands.


Antarctica’s Weddell Sea is a perfect example. It is connected to the Atlantic Ocean using a very wide opening. The characteristics of the Weddell Sea and the Atlantic Ocean are almost same.

Again, South China Sea is yet another example of Partly Enclosed Sea in which the Sea is connected to the Pacific Ocean using a series of passages running between numerous islands that sit in between. However, diversity of tides is not as much in South China Sea as it is in the Pacific Ocean because of the nature of the connection between the two.

Hypersaline Lakes:

Finally, there are types of Seas that are completely surrounded by land from every side. They are totally landlocked without any natural outlet. These are basically lakes – huge lakes.

Then, why are they considered as Seas? Nice question pal! The reason why they are called Seas is that they were once Oceans or Seas during prehistoric times. Over millions of years, the tectonic movements of our Earth rearranged the land masses and in the process, some of those prehistoric Seas and Oceans became landlocked from all sides, with their access to large water bodies getting completely locked out from every possible side.

Here is something interesting about Hypersaline Lakes:

They will not have a natural outlet but that doesn’t mean that no river will flow into those lakes. There may be rivers that drain directly into these Hypersaline Lakes.

The reason why they are called ‘Hypersaline Lakes’ that the salt concentration in their waters is extremely high.


Dead Sea is a perfect example of a Hypersaline Lake. Yes, Dead Sea is a lake because it is enclosed from all sides by land. It is completely landlocked. Dead Sea has no outlet. However, the Jordan River flows directly into the Dead Sea.

Similarly, Caspian Sea is also a Hypersaline Lake. The Volga River flows into the Caspian Sea but the Sea has not outlet.

Marginal Seas:

When you ask someone – ‘what is Sea?’ – you may come across the term ‘marginal Sea’. Hold your horses. This isn’t the fourth type of Sea. Actually, the first two variants, that is, “Almost Enclosed Seas” and “Partly Enclosed Seas” are together sometimes referred to as “Marginal Seas”. If you happen to be referring to Wikipedia regularly, you should be aware of this term. All ‘Almost Enclosed Seas’ and ‘Partly Enclosed Seas’ are Marginal Seas.

Different Names of Sea (More Like Synonyms)

Without giving the different names of Sea, the answer to the question – ‘what is Sea?’ – remains incomplete. Seas are also known by different names. They are listed below:

  • Bay (Such as Bay of Bengal).
  • Gulf (Such as Gulf of Mexico).

Then there are:

  • Sound – It is a wide Bay which is deeper than Strait or Bight. It is usually larger than a Bay (example, Broad Sound near to Queensland’s Clairview).
  • Fjord – This is basically a long Bay that has been created by a glacier (example, Adams Fjord in Antarctica).
  • Cove – It is a sheltered and a very small Bay.
  • Bight – It is a bay which is shallow compared to Sound.

What are Straits?

A Strait is a narrow waterway that will connect two wider water bodies. A Strait can connect to a Sea to a Sea or a Sea to an Ocean or Bay to an Ocean or just any combination that you can make from the list above.

While it is a common practice to list some Straits as Seas, they are not large enough to be categorized as Sea. So strictly speaking, Straits are not Seas even if some resources may list them under Marginal Seas.

How are Sea Boundaries Defined?

Sea boundaries are usually defined using three parameters:

  • Landforms that lock the sea from several sides.
  • Water currents. Yes, as vague as it may sound, it is true.
  • Imaginary lines like latitudes and longitudes.

Which are Some of the Biggest Seas in World?

There are over 50 Seas in this world. However, not all are big. Some are very small. There are 6 really massive Seas, which include:

  • Mediterranean Sea – Largest Sea with total area of 1,144,800 square miles.
  • Caribbean Sea – Second Largest Sea with total area of 1,049,500 square miles.
  • South China Sea – Third Largest Sea with total area of 895,400 square miles.
  • Bering Sea – Fourth Largest Sea with total area of 884,900 square miles.
  • Gulf of Mexico – Fifth Largest Sea with total area of 615,000 square miles.
  • Okhotsk Sea – Sixth Largest Sea with total area of 613,800 square miles.

Sea Depth

Seas are way shallower than Oceans. However, they are not very shallow either. Seas usually have a depth range of 12,000 feet to 22,000 feet. The deepest Sea in world is the Caribbean Sea with the maximum depth of 22,788 feet or 6,946 meters. The average depth of Caribbean Sea is 8,685 feet or 2,647 meters.

Waves and Beaches

Seas are either partially enclosed by land or totally enclosed by land. This is the reason why they almost always make beaches where the Sea water connects with continental land.

We can see large waves in Seas but not in Oceans. The reason for this is that when waves enter shallow waters, their speed decreases while their amplitude or size grows. This explains why waves in Seas are larger than those in Oceans.

This pretty much covers the basics of Seas in simplest possible way by avoiding jargon. In our next article, we will cover Oceans in a similar fashion. Stay tuned.

Sources: 1, 2, 3, 4, 5

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