Sea

The sea is a vast expanse of saltwater, and the term can refer to two different things. One meaning is the single, connected body of saltwater that covers most of Earth, also called the ocean. The other meaning is a specific, named body of water, such as a marginal sea like the Mediterranean, which is a secondary section of the ocean, or a large, nearly enclosed body of water. Earth is the only known planet with liquid seas on its surface, though Mars has ice caps and other solar systems may have similar oceans. The sea holds about 1.335 billion cubic kilometers of water, making up roughly 97.2 percent of all known water on the planet and covering about 71 percent of its surface. Another 2.15 percent of Earth’s water is frozen, found in Arctic sea ice, the Antarctic ice cap and its surrounding seas, and various glaciers. The remaining 0.65 percent exists as underground water or in stages of the water cycle, including vapor, clouds, rain, lakes, and rivers.

Salinity varies widely across water bodies, with lower salt levels near the surface and at the mouths of large rivers, and higher levels in the deep ocean, though the relative proportions of dissolved salts remain fairly constant. The most common solid dissolved in seawater is sodium chloride. Other dissolved elements include salts of magnesium, calcium, potassium, and mercury, some in very tiny amounts. A huge range of organisms—bacteria, protists, algae, plants, fungi, and animals—live in marine habitats and ecosystems throughout the seas. These habitats stretch from the sunlit surface and shoreline down to the cold, dark, high-pressure abyssal zone, and from polar ice caps to warm tropical coral reefs. Many major groups of organisms evolved in the sea, and life may have originated there.

The ocean moderates Earth’s climate and plays key roles in the water, carbon, and nitrogen cycles. The sea surface interacts with the atmosphere, exchanging particles and temperature, and creating currents. Surface currents are driven by winds blowing over the water, producing wind waves and setting up slow, stable water circulations. Deep-sea currents, collectively called the global conveyor belt, carry cold water from near the poles to every ocean, greatly influencing climate. Tides, the roughly twice-daily rise and fall of sea levels, are caused by Earth’s rotation and the gravitational pull of the Moon and, to a lesser extent, the Sun. Tides can be very high in bays or estuaries. Submarine earthquakes from tectonic plate movements can trigger destructive tsunamis, as can volcanoes, large landslides, or large meteorite impacts.

The seas have been essential to humans throughout history and culture. People have studied and used the seas since ancient times and even prehistory. The modern scientific study of the sea is oceanography, while maritime space is governed by the law of the sea, and admiralty law regulates human activities at sea. The seas provide substantial food, mainly fish, but also shellfish, mammals, and seaweed, caught by fishermen or farmed underwater. Other human uses include trade, travel, mineral extraction, power generation, warfare, and leisure activities like swimming, sailing, and scuba diving. Many of these activities cause marine pollution.