Sea

The sea is a large body of salt water. It covers most of our Earth.

Some seas are small and near land. Others are very big. The salt in the water makes it taste salty.

Many things live in the sea. Tiny plants and big animals live there. Some live in warm water. Others live in cold, dark water.

Plants in the sea make the air we breathe. They use sunlight to do this.

The sea is a very busy place.

The sea is a large body of salt water. It covers about 71 percent of Earth's surface. Most of this water is in the big oceans. Some seas are smaller and sit near land. They are often partly closed in by land.

Seawater is salty because it has many dissolved salts. Sodium chloride is the most common salt. This is the same salt used in food. The water also has magnesium and calcium.

The salt levels can change. For example, rain can make the water less salty. High heat can make it more salty through evaporation.

Many living things call the sea home. These include plants, animals, and tiny bacteria. Some live in warm coral reefs. Others live in the dark, cold abyssal zone.

In the sunlit parts, plants use light to make oxygen. This oxygen helps marine animals breathe. The sea also helps control Earth's weather. Deep currents move water around the whole world. This is called a global conveyor belt. It carries cold water to many places.

The sea is a huge system of salt water. It covers about 71 percent of our planet. Most of this water is in the five big oceans. These are the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. We also use the word "sea" for smaller bodies of water. These are often partly or fully closed in by land.

One famous example is the Mediterranean Sea. Some seas, like the Sargasso Sea, have no land borders at all.

Seawater has a very special way it works. It contains many dissolved salts like sodium chloride. This is the same salt you might use on food.

Other salts like magnesium and calcium are also present. The amount of salt changes in different places. In the Red Sea, high heat causes evaporation. This makes the water more salty. In the Baltic Sea, many rivers flow in. This makes the water less salty, which is called brackish.

Scientists study the sea in many different ways. Oceanography is the main study of the sea. It looks at waves, tides, and the ocean floor.

Some experts study marine biology. They look at the plants and animals in the water. Others study physical oceanography. This looks at how the water moves. There is even a field called chemical oceanography. It studies how elements like carbon behave in the water.

There are many amazing facts about the sea. The ocean holds about 1,335,000,000 cubic kilometers of water.

Deep seawater stays between −2 °C and 5 °C. Light can reach as deep as 1,000 meters. However, plants cannot grow deeper than 200 meters. This is because they need light for photosynthesis. Tides happen about twice every day. They are caused by the Moon and the Sun.

These forces pull on the water to make it rise and fall.

Humans have always lived near and used the sea. We get food like fish and seaweed from it. People also use the sea for travel and trade. Some people enjoy swimming or scuba diving for fun.

The sea also helps control Earth's climate. A system called the global conveyor belt moves water around. It carries cold water from the poles to other oceans. This helps keep our whole planet's temperature steady.

The sea is the interconnected system of all oceanic waters on Earth. This massive system includes the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. While we often use the word "ocean" for the largest parts, the term "sea" can also describe smaller bodies of water. Many seas are marginal seas, which are sections of the ocean partly enclosed by land.

Some are called Mediterranean seas because they are almost entirely surrounded by land. Others are inland seas that are wholly enclosed. An unusual exception is the Sargasso Sea, which has no coastline. It exists within a circular current called the North Atlantic Gyre.

Seawater is defined by its salinity, which is the amount of dissolved salt in the water. The open ocean typically has a salinity of 35 parts per thousand (‰). This means there are 35 grams of solids in every liter of water.

Sodium chloride, or common table salt, makes up about 85 percent of these solids. The water also contains ions of magnesium, calcium, potassium, and mercury. Salinity levels change based on local conditions. High evaporation rates can increase salt levels, as seen in the northern Red Sea. Conversely, heavy precipitation or river runoff can lower salinity. The Baltic Sea is considered brackish because many rivers flow into it.

Temperature and light also change as you move through the water. The amount of solar radiation hitting the surface determines the temperature. In tropical regions, surface layers can exceed 30 °C (86 °F). Near the poles, water can be as cold as −2 °C (28 °F).

Light also behaves differently at various depths. Red light is absorbed in the top few meters. Blue and violet light can penetrate as deep as 1,000 meters. However, photosynthesis cannot occur beyond 200 meters because there is insufficient light. This limit defines where most marine plants can grow.

The ocean is constantly in motion through several different mechanisms. Surface currents are driven by winds blowing over the water. These winds create waves and cause water to circulate through drag. Below the surface, deep-sea currents move in a massive system called the global conveyor belt. This system is driven by changes in temperature and density. Cold, dense water sinks near the poles and travels to every ocean. This movement helps moderate Earth's climate by distributing heat.

Additionally, tides cause sea levels to rise and fall roughly twice a day. Tides are caused by the gravitational effects of the Moon and the Sun.

Scientists use several specialized fields to study these complex systems. Oceanography is the broad study of the sea. Physical oceanography focuses on the sea's motion, forces, and waves. Marine biology, or biological oceanography, examines the organisms living in marine ecosystems. Chemical oceanography studies the behavior of elements like carbon and oxygen.

Marine geology, also called geological oceanography, explores the seabed and tectonic activity. This field provided evidence for continental drift and the structure of the Earth. Finally, marine and maritime geography charts the physical shape of the sea.

The scale of the sea is immense and vital to the planet. Earth's seas contain approximately 1,335,000,000 cubic kilometers of water. This covers about 71 percent of the Earth's surface.

The sea plays a massive role in the carbon cycle. The oceans absorb about 30–40% of the carbon dioxide added to the atmosphere. This absorption can lead to ocean acidification, which lowers the pH of the water. For much of history, the average pH was about 8.2, but it has recently dropped below 8.1. This chemical change is a significant part of modern climate study.

Humans have maintained a deep connection to the sea throughout history. We rely on the sea for food, including fish, shellfish, and seaweed. Beyond food, the sea is used for trade, travel, and mineral extraction. People also use it for leisure, such as swimming and scuba diving.

However, these human activities can lead to marine pollution. The management of these spaces is governed by the law of the sea and admiralty law. Understanding the sea is essential for protecting both human interests and marine life.