The ocean is the vast, continuous body of saline water that covers the majority of the Earth's surface. It contains 97% of the planet's water and is a critical component of the Hydrological cycle and climate system. According to the National Oceanic and Atmospheric Administration (NOAA), while it is a single, interconnected body of water, it is geographically divided into five principal oceans: the Pacific, Atlantic, Indian, Southern, and Arctic.

Physical Properties

The ocean's water is characterized by its salinity, temperature, and density, which vary by location and depth.

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  Salinity: The average salinity of the ocean is about 35 parts per thousand (ppt), meaning that every 1 kilogram of seawater has approximately 35 grams of dissolved salts, primarily sodium chloride. As described by National Geographic, salinity is higher in regions with high evaporation and lower near river mouths or where polar ice melts.

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  Temperature: Ocean surface temperatures range from about -2°C (28°F) near the poles to 30°C (86°F) in equatorial regions. Temperature decreases significantly with depth, creating distinct thermal layers. A layer of rapidly changing temperature known as the thermocline separates the warmer, mixed surface waters from the cold, deep waters that make up the bulk of the ocean's volume, as detailed by the U.S. National Oceanic and Atmospheric Administration.

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  Density: Seawater density is a function of both its temperature and salinity. Cold, salty water is denser than warm, fresh water. These density differences are a primary driving force behind large-scale ocean currents, including the global Thermohaline circulation system which transports heat around the planet.

Oceanic Zones

The ocean is vertically stratified into several zones based on light penetration and depth.

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  Photic Zone: The uppermost layer, extending to about 200 meters (656 feet), where enough sunlight penetrates to permit photosynthesis. This zone supports the vast majority of marine life.

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  Aphotic Zone: Below the photic zone, where sunlight is insufficient for photosynthesis. This zone is further divided by depth into the mesopelagic (twilight), bathyal (midnight), abyssal, and hadal zones. Life in these deep zones has adapted to extreme pressure, cold temperatures, and the absence of light, often relying on chemosynthesis or marine snow for energy, as explained by the Woods Hole Oceanographic Institution. The deepest known point is the Challenger Deep in the Mariana Trench, at approximately 10,984 meters (36,037 feet) below sea level.

Geology and Seafloor Topography

The ocean floor is not a flat, featureless plain but has a varied topography shaped by Plate tectonics. Major features include the continental shelves, which are shallow, submerged extensions of continents; abyssal plains, which are vast, flat areas of the deep ocean floor; and extensive underwater mountain ranges known as Mid-ocean ridges, where new oceanic crust is formed. Other features include seamounts (underwater volcanoes) and deep-ocean trenches, which are the deepest parts of the ocean, formed at subduction zones. The study of the ocean's physical and biological aspects is known as Oceanography.

Marine Life and Ecosystems

The ocean is home to an immense diversity of life, from microscopic phytoplankton and zooplankton that form the base of the marine food web to the largest animal on Earth, the blue whale. Key marine ecosystems include:

• –Coral reefs: Often called the "rainforests of the sea," these are highly diverse ecosystems built by colonies of tiny animals.
• –Kelp Forests: Underwater areas with a high density of kelp, providing a complex habitat for many marine species.
• –Hydrothermal Vents: Fissures on the seafloor that release geothermally heated water, supporting unique ecosystems based on chemosynthesis.
• –Open Ocean (Pelagic Zone): The vast area of the ocean away from coastal boundaries, inhabited by species adapted to a life of drifting or swimming.

Human Impact

Human activities have profoundly impacted the ocean. According to the International Union for Conservation of Nature (IUCN), plastic pollution is a pervasive problem, harming marine life and ecosystems. Overfishing has depleted many fish stocks worldwide, disrupting marine food webs. Furthermore, the ocean has absorbed over 90% of the excess heat from greenhouse gas emissions and about 30% of anthropogenic carbon dioxide, leading to rising sea temperatures and Ocean acidification. These changes threaten marine ecosystems, particularly sensitive ones like coral reefs, and contribute to global sea-level rise.