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10 facts about our amazing oceans

Oceans cover most of our planet, but we still have a lot to learn about this incredible ecosystem. Brush up on your knowledge of our watery world for World Oceans Day.

1. Earth's oceans are unique in the Universe — as far as we know

Earth is the only known planet or moon to have large bodies of liquid water on its surface. Our planet lies in the "Goldilocks" zone — not too hot, not too cold and with enough atmospheric pressure to prevent liquid surface water from evaporating into space.

Although we don't yet know of any other planets or moons with liquid water oceans, it's likely that they do exist and we just haven't found them. In our own solar system, there is growing evidence that the planet Mars may have liquid water not on the surface but underground. There is also strong evidence that liquid oceans may be hidden beneath the thick icy surfaces of three of Jupiter's moons (Europa, Callisto and Ganymede) and two of Saturn's moons (Titan and Enceladus).

2. For every species of marine life we know of, at least another three are yet to be discovered

Our oceans teem with life ranging from the blue whale — the biggest animal on Earth — to tiny microbes.

But nobody knows exactly how many different species live in this environment. There is no data for around 20 per cent of the ocean's volume.

The Census of Marine Life, a 10-year international project to identify life in our oceans, found nearly 250,000 species. But scientists believe a least a million species of marine life could be out there, and that's not counting the tens or even hundreds of millions of kinds of microbes that make up the majority of marine life.

What we do know is that ocean life survives in the most extreme environments. Scientists have found life that can survive in temperatures that melt lead, where seawater freezes into ice, or there's no light or oxygen.

In fact, the dark ocean zone between 1000 and 5000 metres known as the abyssal zone has a far greater range of marine life than we once thought.

The deep waters around Australia's Great Barrier Reef are home to a stunning array of life such as the deep sea jellyfish atolla (above), which was discovered by Australian scientists in 2006.

3. Water takes around 1000 years to travel all the way around the whole globe

The oceans not only have waves, tides and surface currents — they also have a constantly moving system of deep-ocean circulation driven by temperature and salinity.

Known as the global ocean conveyor belt or thermohaline current (thermo = temperature, haline = salinity), this deep ocean current gets one of its "starts" in the polar region near Norway.

As sea ice forms, the water left behind becomes saltier and denser and begins to sink, making room for warmer and less dense incoming surface water, which in turn eventually becomes cold and salty enough to sink.

The cold dense water flows along the ocean bottom all the way from the northern hemisphere to the Southern Ocean where it merges with more cold dense water from Antarctica and is swept into the Indian and Pacific Oceans.

Eventually it mixes with warmer water and rises to the surface before finding its way back to the Atlantic. It can take 1000 years to complete this cycle.

4. Half of all the oxygen we breathe is produced in the ocean

Some of this oxygen is produced by sea weeds and sea grasses, but the vast majority of the oxygen is produced by phytoplankton, microscopic single celled organisms that have the ability to photosynthesise. These tiny creatures live in the surface layer of the ocean (and in lakes and rivers) and form the very base of the aquatic food chain.

During photosynthesis, phytoplankton remove carbon dioxide from sea water and release oxygen. The carbon becomes part of their bodies.

5. The oceans may seem vast, but they're small compared to the overall size of our planet

About 71 per cent of the Earth's surface is covered by water, but the average depth of the oceans is only about 4.2 kilometres. The deepest point is in the Marianas Trench in the Pacific Ocean, around 11 kilometres below sea level.

To get some idea of how the average depth of the oceans compares to the scale of the Earth, imagine if the world's tallest building (the 829-metre Burj Dubai) was the distance from the centre of the Earth to the surface. The average depth of the oceans would be the equivalent of just 10 centimetres at the very top. Mount Everest would be another 21 centimetres above that.

All of the water on Earth (the oceans, which account for 96.5 per cent, plus groundwater, rivers, lakes, snow and ice) would fit into a sphere about 1,385 kilometres in diameter, according to the US Geological Survey. The Earth's diameter is roughly nine times larger (12,700 kilometres).

6. Rogue waves really do exist

For centuries maritime folklore has had tales of gigantic waves that would appear without warning in mid-ocean and sink ships, even in good weather. These rogue waves are not necessarily the biggest waves ever found at sea, but isolated waves way bigger than what any crew might expect in a given sea state.

By definition, rogue waves are ones whose height is more than twice that of the prevailing conditions.

The largest scientifically measured rogue waves were encountered in February 2000, by the British oceanographic research vessel, the RRS Discovery. West of Scotland it recorded individual waves of up to 29.1 metres, almost the height of a 10 storey building.

The causes of these waves are not completely understood, however they seem to happen more often when a strong ocean current runs counter to the direction of the waves.

7. Oceans hold around 50 times more carbon than the atmosphere

Cold water can dissolve much more CO2 than warm water, so the cold polar regions are net absorbers of CO2. But as the cold water finds its way to warmer tropical areas, the oceans release CO2 back into the atmosphere. The equatorial Pacific is thought to be the biggest single natural source of CO2 in the atmosphere. Most of this carbon is exchanged with the atmosphere on a timescale of several hundred years.

Prior to the industrial revolution, the uptake and release of CO2 on land and ocean was in a dynamic equilibrium. Since then, the oceans are thought to have absorbed about half of the carbon dioxide released from the burning of fossil fuels, with the rest remaining in the atmosphere.

Read more: The carbon cycle: blink and you won't miss it.

8. The great garbage patch is more like a plastic soup

The existence of theThe Great Pacific Garbage 'patch' in the North Pacific Ocean was confirmed in 1997. It lies at the centre of a large rotating ocean current — or gyre. Since then, a soup of plastic pollution has been found at the centre of all the world's major gyres — in the South Pacific, the North and South Atlantic and in the Indian Ocean.

Plastic trash takes a very long time to bio-degrade in the oceans. The actions of sunlight and wave motion tend to break plastic objects into smaller and smaller pieces until they eventually become smaller than a grain of sand. That's why there is no obvious floating island of plastic debris, but rather a fine soup of plastic particles floating in the water column.

These fine particles are now thought to act like sponges, concentrating pollutants such as PCBs, DDT and PAHs. When the particles are ingested by filter feeding sea creatures, they enter the food chain and ultimately into fish destined for human consumption.

They also harbour unique colonies of microbes, but it is too early to say what impact this emerging 'plastisphere' will have on marine ecological environments.

Plastics only came into widespread use after 1945, but can already be found in every part of the marine environment from the surface to the seafloor.

9. The Atlantic Ocean is getting bigger and the Pacific Ocean is getting smaller

Twenty million years ago there was no Atlantic Ocean. But then, thanks to plate tectonics, the South American and North American continents were separated by a rift valley that eventually turned into the Atlantic Ocean. The complementary shapes of the South American and African continents have long been noticed, but it wasn't until the 1960s that the theory of plate tectonics became accepted as the explanation.

These days, the Atlantic Ocean is growing at a rate of five centimetres per year, as new sea floor is created by volcanic activity along its mid-ocean ridge.

On the other hand, the much older Pacific Ocean is currently estimated to be shrinking by two to three centimetres each year. Again, this comes down to plate tectonics because the Pacific Ocean has subduction zones on three sides — where the Pacific plate submerges beneath other plates.

The Pacific's "Ring of Fire" broadly maps the areas where this subduction occurs and accounts for around 90 per cent of the world's earthquakes.

10. The most remote point in the oceans is called Point Nemo

The part of the ocean farthest from land lies in the South Pacific and is known as Point Nemo or 'The Pole of Inaccessibility'.

Point Nemo isn't an actual 'point' as (obviously) there's no land there — it's simply a spot in the ocean that happens to be 2,688 kilometres from the nearest land — Ducie Island, part of the Pitcairn Islands, to the north; Motu Nui, one of the Easter Islands, to the northeast; and Maher Island, part of Antarctica, to the south.

The name 'Nemo' comes from Jules Verne's Captain Nemo of Twenty Thousand Leagues under the Sea — not the film Finding Nemo. If you go to Google Earth and zoom in to 48°52.6′S 123°23.6′W you'll find it thoughtfully marked.

World Oceans Day is 8 June.

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