The Oceans

Seen from space, Earth is a blue planet with some land on it. Water covers 71 percent of the surface, and almost all of that water, 97 percent, is in the ocean. We talk about the Atlantic, the Pacific and the rest as if they were separate, but they are one connected body of water, a single ocean so vast that if the land were flattened out it would cover the whole globe to a depth of nearly three kilometres.

Deep, dark and cold

The average ocean is about 3,700 metres deep. The deepest point, the Challenger Deep in the Pacific's Mariana Trench, lies 10,935 metres down; Everest would fit inside with two kilometres to spare. Only a handful of people have ever descended there.

Sunlight does not get far. By 200 metres down, too little remains for plants to grow; by 1,000 metres, it is utterly black. More than 90 percent of the ocean's volume is in this permanent darkness, at temperatures just above freezing and under pressures that would crush a car. Yet it is full of life, much of it making its own light. Deep-sea fish, squid and jellyfish flash and glow to find food, mates and each other, so that bioluminescence may be the most common form of communication on the planet.

Why the sea is salty

Rain falling on land dissolves tiny amounts of minerals from rock and carries them to the sea in rivers. Water evaporates from the ocean and falls as rain again, but the salts stay behind. Over hundreds of millions of years they have accumulated to about 35 grams in every litre of seawater, mostly ordinary sodium chloride. Undersea volcanoes and hot springs add more. The sea is not getting steadily saltier, though: salt is removed at about the rate it arrives, locked into seafloor sediments and evaporated into salt deposits.

The ocean in motion

The ocean never rests. Winds drag the surface into great circulating currents, the Gulf Stream carrying warm Caribbean water across the Atlantic and giving north-west Europe winters far milder than Canada's at the same latitude. Beneath the surface, a slower engine runs. In the North Atlantic and around Antarctica, cold salty water becomes dense enough to sink to the bottom, then creeps along the seafloor toward the equator, rising again centuries later in the Pacific and Indian Oceans. This global conveyor belt takes about a thousand years to complete one loop, and it carries heat, oxygen and nutrients around the planet.

Twice a day the whole ocean also rises and falls with the tides, pulled by the Moon's gravity, and in some bays the water climbs more than fifteen metres between low and high.

Life in the sea

Life began in the ocean, and most of its history has happened there. Microscopic floating plants, the phytoplankton, produce about half of all the oxygen in the atmosphere and feed everything above them in the food chain, from tiny shrimp to the blue whale, the largest animal that has ever lived. Coral reefs, built by small animals over thousands of years, cover less than one percent of the seafloor but shelter a quarter of all marine species.

In 1977 scientists exploring the seafloor near the Galápagos found something nobody expected: hot springs on the ocean floor surrounded by dense colonies of giant tube worms, clams and shrimp, two and a half kilometres down, in total darkness. These hydrothermal vents are fed not by sunlight but by chemicals in the hot water, which bacteria turn into food. Life, it turned out, does not need the Sun. Similar vents on icy moons like Europa and Enceladus are now prime targets in the search for life elsewhere.

The ocean and the climate

Water stores heat extraordinarily well, and the ocean has absorbed about 90 percent of the extra heat trapped by greenhouse gases over the past fifty years. That has spared the land some warming but is changing the sea: it is warming, expanding, and rising as glaciers melt into it. It has also absorbed about a quarter of the carbon dioxide humans have emitted, which makes the water more acidic and harder for corals and shellfish to build their skeletons.

What we still don't know

Despite its importance, the ocean is the least explored place on Earth. Only about a quarter of the seafloor has been mapped at high resolution; we have better maps of Mars and the Moon. Every deep-sea expedition finds species new to science, and estimates of how many marine species exist range from a few hundred thousand to millions. The deep ocean is the largest habitat on the planet, and we have barely glimpsed it.