In round figures the surface area of the planet Earth is 197 million sq miles of which 70.9% (about 140 million sq miles) is covered by water. Only 3% of this is fresh water, with the other 97% (about 135 million sq miles) making up the world's five oceans, plus a number of smaller seas such as the South China Sea, the Caribbean Sea and the Mediterranean Sea.
In order of size, the largest of the oceans is the Pacific Ocean, followed by the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. To give an idea of the range of size, the Pacific Ocean extends for 60 million square miles, while the Arctic Ocean is a mere 5.4 million sq miles.
The deepest point in the world's oceans is the Mariana Trench in the Pacific Ocean off the coast of Japan. There is some disagreement as to its precise depth, but it is somewhere close to 36,000 feet, or 6.8 miles. In fact the trench may be even deeper than this because only about 10% of it has been explored. At those depths the pressure of the water is around eight tons per square inch.
The Earth's oceans are unique in our Solar System. None of our neighbouring planets has any liquid water, although it is thought that Mars may have had some in the past.
The oceans are where life on Earth began and they continue to be the home to an incredibly diverse range of life. It has been estimated that there are around 230,000 different marine species, but some marine biologists think that there could well be ten times that number still waiting to be discovered.
Oceans have always played an important part in human life. They serve many functions. Most of the major cities of the world are close to the sea. Since the beginning of time oceans have provided the world with a transport network for goods and people. Countless numbers of migrants have travelled the seas to find a new and better life, while others have been transported very much against their will; the evil Slave Trade being a case in point. World trade has always depended on ocean transport and the world's navies grew up in order to protect these maritime trade routes.
The sea has traditionally been a source of nourishing food, but sadly the general view has always been that what was there was there for the taking. There has been little regard for sustainability and in the words of Greenpeace: ''we are now damaging our oceans on a scale unimaginable to most people.''
Large commercial fishing boats catch everything, but not everything is wanted or allowed due to imposed fishing quotas and vast amounts of dead fish or 'discard' are thrown back into the sea. Whole marine ecosystems can be wiped out as huge trawl nets are dragged across the ocean floor, destroying entire habitats forever.
Overfishing in this way does not enable the fish to sustain their population with the result that numbers fall until the point is reached when there are no longer any fish left to catch. According to the United Nations, over 70% of the world's fisheries are either ''fully exploited'', ''over exploited'' or ''significantly depleted''. Some species have been fished to extinction and some are teetering on the edge. Estimates say that due to overfishing, large predatory fish like cod and tuna are both down to the last 10% of their original number.
It is not just fish that suffer from the effects of these large commercial fishing operations. Other species, such as marine mammals and seabirds are killed after being trapped in fishing gear. This is known in the trade simply as ''bycatch''.
The hunting of whales for meat and oil dates back thousands of years, but in the early 20th century factory ships were introduced and the concept of whale harvesting began. By the late 1930's more than 50,000 whales were being killed each year and in spite of a reduction during World War II, by the 1950s it was clear that many breeds were facing extinction.
In 1986 the International Whaling Commission banned commercial whale hunting to give stocks a chance to recover. This has been largely successful and in spite of some countries, notably Japan, being anxious to resume whale hunting, there is now a considerable body of international opinion saying that whaling is immoral, unsustainable and should remain permanently banned.
Pollution of the oceans is a further factor causing considerable concern. At one time all rubbish from ships was simply thrown into the ocean and it was normal practice for coastal settlements to pump raw sewage directly into the sea. The same was the case with industrial effluent. Tankers regularly flushed out their tanks by pumping in seawater and then returning the now filthy and contaminated water straight back into the oceans.
In most countries in the developed world these practices have ended, but in many parts of the world the oceans are still regarded as convenient rubbish dumps and the situation is getting worse, particularly around the edges of the continents.
There is also a great deal of non-biodegradable plastic detritus in the oceans of our planet; oceanic currents mean a large amount of it eventually ends up in concentrated floating 'garbage patches'. The largest of these plastic waste concentrations is in the North Pacific, this area is named ''The Great Garbage Patch.''
Oceans can suffer from large scale and sudden pollution by 'man-made' accidents, which can have an almost immediate and devastating effect on entire ecosystems; examples of this are the BP oil spill in the Gulf of Mexico in 2010 or the 1989 Exxon Valdez oil tanker spill in Alaska.
The vast areas of the oceans also have an important role in modifying and controlling the world's temperature by absorbing incoming solar radiation and storing it as heat energy. Since the water in the oceans is constantly moving, currents distribute this energy around the globe, heating the land and air during the winter and cooling it during the summer.
There is much concern about the effects that climate change will have on the oceans. As the Earth warms up and the ice caps melt they will reduce in size. This will affect the amount of solar radiation that is absorbed, which in turn could seriously affect the temperatures of the oceans and disrupt current flow. A consequence of this could be that the warm Atlantic current that keeps the seas around North West Europe from freezing in winter might take a change in direction.
Additionally, if the ice caps melt, as is confidently predicted, sea levels could significantly rise. Since many of the world's cities lie in coastal areas, the consequences of this could prove disastrous.
With such a vast amount of water covering the surface of the Earth it has been very easy to take the view that with so much ocean, it doesn't really need much looking after. The answer to that argument is that it is because water makes up such a large percentage of the Earth's surface, its very size profoundly influences the whole world, so consequently its care is vital to us all.
The Oceans form a vast repository of resources, but as we have seen, they are very vulnerable to the actions of mankind. If these resources are not properly managed and if there is not greater worldwide concern about lack of conservation and the effects of pollution and climate change, the complex ecosystems of the oceans face a bleak future. Once these ecosystems have gone they can never be recovered.