The sea, often known as the world ocean or just the ocean, is a salty body of water that covers more than 70% of the Earth’s surface. Second-order parts of the sea, such as the Mediterranean Sea, and vast, totally landlocked saltwater lakes, such as the Caspian Sea, are also referred to as seas.

The sea helps to regulate the Earth’s temperature and plays an essential part in the water, carbon, and nitrogen cycles. Humans have been utilizing and studying the sea since ancient times, with evidence dating back to antiquity, whereas oceanography is the contemporary scientific study of it. Sodium chloride is the most common substance dissolved in saltwater. The water also includes magnesium, calcium, potassium, and mercury salts, as well as a variety of other elements, some in trace amounts. Salinity varies greatly, with lower levels at the surface and at the mouths of big rivers and higher levels at the ocean’s depths; nevertheless, the relative amounts of dissolved salts are very little among seas. Winds sweeping over the sea’s surface create waves, which break as they reach shallow water. Surface currents are also created by wind friction, resulting in sluggish but steady water circulation across the seas. The forms of the continents and the rotation of the Earth all influence the circulation’s direction (the Coriolis effect). Deep-sea currents, often known as the global conveyor belt, transport cold water from the poles to every ocean on the planet. The rotation of the Earth, as well as the gravitational influences of the orbiting Moon and, to a lesser degree, the Sun, create tides, which are the twice-daily rise and fall of sea levels. In bays and estuaries, tidal ranges can be extremely wide. Submarine earthquakes caused by tectonic plate movements beneath the seas, as well as volcanoes, massive landslides, and the impact of big meteorites, can all result in devastating tsunamis.

The sea is home to a diverse range of organisms, including bacteria, protists, algae, plants, fungi, and animals, as well as a diverse range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the frigid waters beneath polar ice caps to the colorful diversity of coral reefs in the tropical Pacific. Life may have begun in the water, where many of the major types of creatures developed.

Humans rely on the sea for sustenance, mostly fish, but also shellfish, animals, and seaweed, which are collected by fishermen or cultivated underwater. Trade, transport, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving are all examples of human uses of the water. Many of these activities pollute the ocean. As a result, water has always been an important part of human culture and history.

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