Oceans

The ocean has an average depth of approximately 3.7 kilometres (or 2.3 miles). A calculation from satellite measurements in 2010 put the average depth at 3,682 metres (12,080 feet). However, at the time only about 10% of Earth's seafloor had been mapped to high resolution, so this figure is only an estimate.

Ocean depth is divided into zones: littoral, bathyal, abyssal and hadal. The deepest part of the ocean, the hadal zone, is anywhere deeper than six kilometres.

Challenger Deep, in the Mariana Trench, is the deepest point in the ocean known so far, at approximately 11 kilometres - deeper than Mount Everest is tall. The trench is located in the western Pacific Ocean.

The extreme depth of the Mariana Trench and other oceanic trenches is caused by subduction - where on the boundary of two converging tectonic plates, one descends down into Earth's mantle, creating a deep trough.

Salt in the sea, or ocean salinity, is mainly caused by rain washing mineral ions from the land into water.

Carbon dioxide in the air dissolves into rainwater, making it slightly acidic. When rain falls, it weathers rocks, releasing mineral salts that separate into ions. These ions are carried with runoff water and ultimately reach the ocean.

Sodium and chloride, the main constituents of the type of salt used in cooking, make up over 90% of all the ions found in seawater. Around 3.5% of the weight of seawater comes from dissolved salts.

Some mineral ions are used by marine animals and plants, removing them from the water. The leftover minerals have built up in concentration over millions of years. Underwater volcanoes and hydrothermal vents on the seabed can also release salts into the ocean.

Isolated bodies of water can become extra salty, or hypersaline, through evaporation. The Dead Sea is an example of this. Its high salt content increases the water's density, which is why people float in the Dead Sea more easily than in the ocean.

Coral reefs are made up of colonies of hundreds to thousands of tiny individual corals, called polyps. These marine invertebrate animals have hard exoskeletons made of calcium carbonate, and are sessile, meaning permanently fixed in one place. Polyps grow slowly, forming different shapes and sizes depending on their species.

Assisted by other animals with calcium carbonate skeletons and also coralline algae, corals form complex, three-dimensional reefs.

Coral reefs provide an important ecosystem for marine life, offering food and shelter among their crevices and branches for animals including fishes, molluscs, sea urchins and sponges.

Corals are found in all of Earth's oceans, from tropical to freezing temperatures, however they only build coral reefs in warm, shallow seas in the tropics. Among the biggest and best-known are the reef systems of the Great Barrier Reef of Australia, which is around 2,300 kilometres long. The most biologically diverse reefs in the world can be found in a region known as the Coral Triangle in Southeast Asia.

Watch a video about corals, the builders of the reef and explore more about coral reefs.

Coral reefs provide an important ecosystem for life underwater, protect coastal areas by reducing the power of waves hitting the coast, and provide a crucial source of income for millions of people.

Coral reefs teem with diverse life. Thousands of species can be found living on one reef. The Great Barrier Reef contains over 400 coral species, 1,500 fish species, 4,000 mollusc species and six of the world's seven sea turtle species. The Coral Triangle - a coral-rich marine region in Southeast Asia that encompasses the waters between Indonesia, Malaysia, the Philippines and Papua New Guinea - is the most biologically diverse marine ecosystem on Earth. 

Coral reefs have an estimated global value of £6 trillion each year, due in part to their contribution to fishing and tourism industries and the coastal protection they provide.

More than 500 million people worldwide depend on reefs for food, jobs and coastal defence. The ridges in coral reefs act as barriers and can reduce wave energy by up to 97%, providing crucial protection from threats such as tsunamis. They help protect areas such as mangrove forests and seagrass beds that act as nurseries for marine animals, as well as human coastal populations.

Extracts from animals and plants living on reefs have been used to develop treatments for asthma, arthritis, cancer and heart disease.

Explore the risks facing coral reefs.

Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body.

The oyster or mussel slowly secretes layers of aragonite and conchiolin, materials that also make up its shell. This creates a material called nacre, also known as mother-of-pearl, which encases the irritant and protects the mollusc from it.

When pearls are cultured commercially an irritant is manually inserted into a mollusc to promote the production of mother-of-pearl.

Nacre can form naturally around almost any irritant that gets inside the shell, creating some very unique and precious pearls.

Other bivalve molluscs and gastropods can produce pearls, but these aren't made of nacre.

Killer whales (also called orcas) are apex predators, meaning they are at the top of their food chain. They feed on fish and squid like other odontocetes (toothed whales) do, but will also target seals, sea birds and even whale species far bigger than themselves. Killer whales are also the only known predators of great white sharks.

Killer whales are the largest dolphin species. They are highly social and spend most of their lives swimming in large pods of family members. Hunting techniques are passed down through generations, so their diets depend on the region they inhabit and the pod's approach to hunting.

These highly intelligent cetaceans have been documented creating large waves to wash seals off ice floes, and even intentionally beaching themselves to catch prey on the shore.

Read more about what whales eat.

Blue whales eat krill - tiny, shrimp-like crustaceans that live throughout Earth's oceans. The huge whales can eat up to four tonnes of krill every day.

Blue whales lunge through large swarms of krill with their mouths open, taking in more food in one mouthful than any other animal on Earth. Krill make up the vast majority of a blue whale's diet.

The blue whale is a filter-feeder. Its throat has an expandable, pleated structure to engulf a volume of water and prey that is greater than the animal's own body weight. The water it takes in at the same time as its food is pushed out of the mouth by its enormous tongue, through strainer-like baleen plates which hang down from the upper jaw.

Watch how a blue whales lunges for its food.

Much of the plastic that does not end up in landfill or go through other waste management pathways (such as recycling or incineration) is thought to end up in the ocean.

Between 4.8 and 12.7 million tonnes of plastic enter the ocean each year, according to figures published in the journal Science in 2015.

Plastic can enter the ocean as large, identifiable items or as microplastics - pieces under five millimetres in length. Both pose a threat to marine life. Large pieces degrade over time to become microplastics, but never fully disappear.

Plastic has accumulated in huge quantities throughout the ocean - even in deep-sea areas previously thought to be untouched by humans.

A 2014 study involving Museum researcher Dr Lucy Woodall found high levels of contamination in deep-sea sediments. It revealed that around four billion microscopic plastic fibres could be littering each square kilometre of deep-sea sediment around the world.

Find out how plastic is affecting marine life.

Ocean acidification is mainly caused by carbon dioxide gas in the atmosphere dissolving into the ocean. This leads to a lowering of the water's pH, making the ocean more acidic.

Many factors contribute to rising carbon dioxide levels. Currently, the burning of fossil fuels such as coal, oil and gas for human industry is one of the major causes.

Deforestation results in fewer trees to absorb the gas. Also, when plants are cut down and burnt or left to rot, the carbon that makes up their organic tissue is released as carbon dioxide.

Some parts of the ocean are naturally acidic, such as at hydrothermal vent sites - underwater 'hot springs'.

In the past, ocean acidification occurred naturally but over much longer periods of time. It is occurring faster now than in the last 20 million years.

Find out how ocean acidification is causing problems for marine life.

Ocean acidification can negatively affect marine life, causing organisms' shells and skeletons made from calcium carbonate to dissolve. The more acidic the ocean, the faster the shells dissolve. 

Animals that produce calcium carbonate structures have to spend extra energy either repairing their damaged shells or thickening them to survive. Using energy for this could impact the animals' abilities to grow and reproduce.

Animals able to survive and reproduce in more acidic waters are likely to become smaller, potentially affecting the food chain that relies on them.

Discover more about ocean acidification and its impacts.

The longest ever recorded dive by a whale was made by a Cuvier's beaked whale. It lasted 222 minutes and broke the record for diving mammals. Other whales can also hold their breath for a very long time. A sperm whale can spend around 90 minutes hunting underwater before it has to come back to the surface to breathe. In 1969, a male sperm whale was killed off the coast of South Africa after surfacing from a dive lasting 117 minutes.

Whales' lungs are particularly efficient at taking up oxygen when they breathe air in and out through their blowholes at the water's surface. Special adaptations help them hold their breath for a long time.

Discover the secrets of the deepest-diving whales.

Rather than keeping oxygen in their lungs like humans do, whales' bodies are specially adapted to store oxygen in their blood and muscles. They have extraordinarily high levels of the oxygen-storing proteins haemoglobin and myoglobin.

Whales also reduce their heart rate and stop the blood flow to certain parts of the body, temporarily shutting down organs such as their kidneys and liver while they hunt. This helps them use the oxygen they have in their bodies more slowly.

Furthermore, beaked whales (which can dive for a particularly long time) have a streamlined body shape. Their flippers fit in indentations in the body, enabling them to take on a torpedo-like shape. This helps them to swim, and often to glide, with minimal effort and extend their oxygen stores for as long as possible.

Read the secrets of the deepest-diving whales.

Whales are accomplished divers. The deepest whale dive recorded so far was made by a Cuvier's beaked whale. A 2014 study used satellite-linked tags to follow the dives of eight beaked whales off the southern California coast. The deepest recorded dive was 2,992 metres, breaking the record for diving mammals.

Experts have suggested that this dive was unusually deep for this species. A more normal depth would be 2,000 metres.

Sperm whales also regularly dive 1,000 to 2,000 metres deep.

Read more about deep-diving whales.