When the Earth formed, it was an infernal planet of hell-fire and brimstone, covered in a magma ocean over 124 miles deep. When it cooled and solidified, the water vapour in the atmosphere condensed and fell into vast canyons and pits to form oceans, rivers and lakes. These would have been toxic, acidic and boiling, but bodies of water none the less.
The one problem with this theory is the sheer amount of water vapour that would have had to have been present in the atmosphere to form the natural features which make our planet unique.
Scientists suggested that this arrived on Earth via comets and other such bodies. There have been many theories recently suggesting that many of our planet's resources were brought to us by meteorites, such as the heavy, precious metals and indeed the chemicals which may have played a role in the origin of life. Around 4.4 to 3.8 billion years ago, the inner solar system endured the brunt of a vast meteorite shower called the late heavy bombardment which pelted the Earth with billions of tonnes of rock.
These rocks contained many chemicals and elements. Scientists have found evidence to support the theory that a great part of the water in the oceans had an extraterrestrial origin. The Herschel Space Observatory recently analysed a comet, known as Hartley 2 which exists in the Kuiper Belt, and found that it contained water with the same chemical signature to that on Earth. Heavy water is a type of water in which one of the hydrogen atoms is replaced by an isotope called deuterium. Chemically, it is no different to normal water.
The ratio of heavy to regular water in the tail of the comet of Hartley 2 was exactly the same as that on Earth. By comparing the chemical ratios from the comet to those from the Oort cloud, an inert shell of gas, dust and ice that surrounds the solar system, at a distance 10,000 times further away than the Kuiper Belt is to the Earth, scientists now believe that as much as 10% of all water on Earth may have arrived during meteorite showers such as in the Late Heavy Bombardment.