Ocean

These vast bodies of water surrounding the continents are critical to humankind. But overfishing and global warming threaten to leave this vital habitat barren.

The ocean is a continuous body of saltwater that covers more than 70 percent of the Earth's surface. Ocean currents govern the world's weather and churn a kaleidoscope of life. Humans depend on these teeming waters for comfort and survival, but global warming and overfishing threaten to leave the ocean agitated and empty.

Geographers divide the ocean into four major sections: the Pacific, Atlantic, Indian, and Arctic. Smaller ocean regions are called seas, gulfs, and bays, such as the Mediterranean Sea, Gulf of Mexico, and the Bay of Bengal. Stand-alone bodies of saltwater like the Caspian Sea and the Great Salt Lake are distinct from the world's oceans.

The oceans hold about 320 million cubic miles (1.35 billion cubic kilometers) of water, which is roughly 97 percent of Earth's water supply. The water is about 3.5 percent salt and contains traces of all chemical elements found on Earth. The oceans absorb the sun's heat, transferring it to the atmosphere and distributing it around the world via the ever-moving ocean currents. This drives global weather patterns and acts as a heater in the winter and an air conditioner in the summer.

Life began in the ocean, and the ocean remains home to the majority of Earth's plants and animals—from tiny single-celled organisms to the blue whale, the planet's largest living animal.

Most of the ocean's plant life consists of microscopic algae called phytoplankton that float at the surface and through photosynthesis produce about half of the oxygen that humans and all other terrestrial creatures breathe. Seaweed and kelp are big algae easily visible to the naked eye. Marine plants with roots, like sea grasses, can only survive as deep as the sun’s rays can support photosynthesis—about 650 feet (200 meters). Nearly half of the ocean is more than 9,800 feet (3,000 meters) deep.

The deepest reaches of the ocean are largely devoid of life, but biological hotspots appear around places like hydrothermal vents. These chimney-like structures spew gases and mineral-rich water from beneath the Earth's crust. Tube worms, clams, and mussels gather around the vents to feed on heat-loving bacteria. Bizarre fish with sensitive eyes, translucent fangs, and bioluminescent lures lurk about in nearby waters.

Other fish, octopuses, squid, eels, dolphins, and whales ply the open waters while crabs, lobsters, starfish, oysters, and snails crawl and scoot along the ocean bottom. Creatures like jellyfish lack their own way to get around and are mostly left to the whim of the wind and currents. Mammals like otters, walruses, and even polar bears also depend on the ocean for their survival and dip in and out as their biology requires.

Colonies of polyps form coral reefs as they die. The reefs are mostly found in shallow tropical waters and are home to a brilliant mosaic of polyps, plants, and fish. Coral reefs are also visible victims of human activity. Global warming, siltation, pollution, and other phenomena are stressing the corals to death, and over-zealous fishers net more food than the reefs can restore.

Human activities impact nearly all parts of the ocean. Lost and discarded nets continue to lethally snare fish, seabirds, and marine mammals as they drift. Ships spill oil and garbage and transport critters to alien habitats unprepared for their arrival. Mangrove forests are cleared for homes and industry. More than half of the U.S. population lives in coastal areas, spilling garbage and sewage into the ocean. Fertilizer runoff from farms turns vast swaths of the ocean into dead zones, including a New Jersey-size area in the Gulf of Mexico. The greenhouse gas carbon dioxide is turning ocean waters acidic, and an influx of freshwater from melting glaciers threatens to alter the weather-driving currents.

Conservationists urge international protections to protect and replenish dwindling ocean fish stocks and reductions in greenhouse gases to curb global.