The ocean and its zones

The ocean covers about 71 percent of Earth's surface and holds about 97 percent of its water. Its average depth is around 3,700 metres, and because life exists at every depth, the ocean makes up the great majority of the living space on the planet.

Despite this, humans have mapped the surface of Mars in more detail than much of the ocean floor, and new species are described from the deep sea every year.

The top 200 metres or so, the epipelagic or sunlight zone, receives enough light for photosynthesis. Microscopic drifting algae called phytoplankton live here, and they produce roughly half of the oxygen made by photosynthesis on Earth.

Phytoplankton are the base of most ocean food webs. Tiny animals called zooplankton eat them, and in turn feed fish, seabirds and even the largest animals on Earth, such as blue whales, which eat vast amounts of small crustaceans called krill.

Between about 200 and 1,000 metres lies the mesopelagic or twilight zone, where faint light remains but not enough for photosynthesis. Every night many animals here swim hundreds of metres up to feed near the surface and sink back down before dawn. This daily vertical migration is thought to be the largest movement of animals on the planet.

Below 1,000 metres is the midnight zone, where no sunlight reaches. Water is near freezing and pressure is crushing. Many animals here make their own light through bioluminescence, using it to lure prey, find mates or confuse predators. The anglerfish, with its glowing lure, is a famous example.

The abyssal plains, from about 4,000 to 6,000 metres, cover vast areas of the ocean floor. Food arrives mostly as marine snow, a slow drift of dead plankton, faeces and other particles from above. Animals such as sea cucumbers and brittle stars move across the sediment eating it.

Deep trenches plunge further still. The Challenger Deep in the Mariana Trench is nearly 11,000 metres deep, where pressure is more than a thousand times that at the surface. Even there, scientists have found amphipods, small shrimp-like animals, and snailfish have been filmed at more than 8,000 metres.

In 1977 scientists exploring the Galápagos Rift found hydrothermal vents, openings where hot, mineral-rich water gushes from the seafloor. Around them lived dense communities of giant tube worms, clams and crabs.

These ecosystems are powered not by the sun but by chemosynthesis: bacteria use chemicals such as hydrogen sulphide from the vents to make food. The discovery showed that life can thrive on chemical energy alone, and it shaped ideas about where life might exist elsewhere in the solar system.

• The ocean has absorbed a large share of the carbon dioxide released by humans, which makes seawater more acidic and harder for shell-building animals.
• It has also absorbed most of the extra heat trapped by greenhouse gases, driving marine heatwaves and coral bleaching.
• Overfishing has depleted many fish populations, although well-managed fisheries can recover.
• Plastic pollution has been found from surface waters to the deepest trenches.
• Marine protected areas, where fishing and other activities are limited, help ecosystems recover.