Sea

Lore & Background

The sea has two distinct meanings: the interconnected body of seawater spanning most of Earth, and named bodies of water such as marginal seas or nearly landlocked bodies. Salinity varies widely, being lower near the surface and river mouths and higher in ocean depths, but the relative proportions of dissolved salts remain stable across oceans. The most abundant dissolved solid is sodium chloride, with other elements including magnesium, calcium, potassium, and mercury present in minute concentrations. The sea hosts a wide variety of organisms—bacteria, protists, algae, plants, fungi, and animals—in habitats ranging from the sunlit surface to the cold, dark abyssal zone, and from polar ice caps to tropical coral reefs. Many major groups of organisms evolved in the sea, and life may have started there. The ocean moderates Earth's climate and plays important roles in the water, carbon, and nitrogen cycles. Surface currents are produced by atmospheric winds, while deep-sea currents, known as the global conveyor belt, carry cold water from near the poles to every ocean. Tides, caused by Earth's rotation and the gravitational effects of the Moon and Sun, rise and fall generally twice daily. Submarine earthquakes, volcanoes, landslides, or meteorite impacts can lead to destructive tsunamis.

Reader's Guide

The sea has been an integral element for humans throughout history and culture, with its study and use recorded since ancient times and evidenced in prehistory. Modern scientific study is called oceanography, and maritime space is governed by the law of the sea, with admiralty law regulating human interactions at sea. The seas provide substantial food supplies—mainly fish, shellfish, mammals, and seaweed—caught by fishermen or farmed underwater. Other human uses include trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution. The sea's significance extends to its role in Earth's physical processes: it moderates climate, drives the water cycle, and influences carbon and nitrogen cycles. Its study has expanded into a multidisciplinary field encompassing physical oceanography, marine biology, chemical oceanography, and marine geology, which has provided evidence of continental drift and clarified sedimentation, volcanism, and earthquakes. The United Nations Convention on the Law of the Sea states that all of the ocean is 'sea,' underscoring its unified legal and scientific importance.

Did You Know?

• The sea covers approximately 71 percent of Earth's surface and contains about 97.2 percent of its known water.
• The most abundant solid dissolved in seawater is sodium chloride, making up about 85 percent of the solids in solution.
• Deep-sea currents, known together as the global conveyor belt, carry cold water from near the poles to every ocean.
• The Sargasso Sea has no coastline and lies within a circular current, the North Atlantic Gyre.