Oceans

How deep is the ocean?

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.

What is the average depth of the ocean?

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.

How deep is the deepest part of the ocean?

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 Mariana Trench is 2,500 kilometres long, running north to south in a crescent-shape. It's located in the western Pacific Ocean near the Mariana Islands.

The trench's depth was first measured during the Challenger expedition in 1875. But it wasn't until the 1950s that scientists recorded its deepest depth.

Why are oceans so deep?

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

Why is the sea salty?

Seawater is known for being quite salty.

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.

Why is the Dead Sea so salty?

Some parts of the ocean are more saline than others, particularly where there are higher levels of evaporation, such as in the Red Sea.

Isolated bodies of water can also become extra salty, or hypersaline, through evaporation. The Dead Sea in the Middle East is an example of this.

The high salt content increases the water's density, which is why people float in the Dead Sea more easily than in the ocean. This effect is seen in other hypersaline bodies of water as well, such as in Senegal's Lake Retba.

In contrast, salinity in some areas may be decreasing due to climate change. Melting sea ice adds freshwater to the ocean. Changes in saltiness and temperature can disrupt the ocean currents that move important nutrients around the world.

Find out more about how climate change affects the ocean.

Do sharks have bones?

Sharks don’t have bones. Their skeletons are made of cartilage - the same soft, flexible stuff as your ears and the tip of your nose are made of. This is true for all sharks, from the formidable great white to the gentle giant the whale shark.

Do sharks have any bones at all? If so, how many?

Shark skeletons don’t include any bones at all. But their jaws, spinal column and the cartilage surrounding their brain are strengthened by calcium salts, which get deposited into their skeletal cartilage from the food they eat. Over time, this makes these body parts harder and stronger.

While humans have 206 to 213 bones in our body, most sharks have about 200 to 400 structures made of cartilage. The exact number varies depending on the species.

The only part of a shark's skeleton not made of cartilage is their teeth, which they're famous for.

Since cartilage is softer than bone and teeth, it doesn’t fossilise very well. But we’ve learned a lot about the history and evolution of sharks from their fossilised teeth.

Why do sharks not have bones?

‘We think early sharks developed a cartilaginous skeleton because it better suited their lifestyle,’ explains Emma Bernard, our Fossil Fish Curator. ‘Being light and more flexible than bone, cartilage means sharks can typically swim faster than bony fish.’

Cartilage is about half as dense as bone. Sharks don’t have the swim bladders that many bony fish have to stop them from sinking, so the low density of their cartilage skeleton helps them to remain buoyant.

Sharks have large livers filled with low-density oils that also help with this.

As well as being flexible, their skeletal structure helps give some species of shark a powerful bite. Their upper jaw is not fused to their cartilaginous skull, allowing them to open their mouths very wide. This makes their downward bite faster and harder. The great white shark has one of the most impressive bite forces in the world.

Did sharks’ ancestors ever have bones?

‘This is still being debated,’ Emma explains.

While bone fossilises, it’s still a very rare process and the fossil record only preserves a very small percentage of animals that have lived and died.

Emma adds, ‘It’s thought that sharks’ ancestors likely had a bony skeleton, but sharks and other cartilaginous fishes, such as rays and skates, reverted back to cartilage because it’s lighter and more flexible than bone. This offered advantages including escaping predators and being able to turn more quickly in the water in pursuit of prey.’

What is a coral reef?

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.

Why are coral reefs important?

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

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