If you look at the Earth from space it’s a beautiful blue planet and most of this blue colour comes from the oceans. There is actually more ocean than there is land on Earth, which makes ours a very watery world.
So where did all this water come from? Scientists don’t know for sure, but they suspect that some may have come from the inside of the planet, and some from the outside.
Before any of the planets or even the Sun was born, 4,500 million years ago, we all started off as a swirling cloud of gas and dust, which also contained water. Eventually all this cosmic stuff started to stick together in clumps. The bigger the clumps got, the more strongly they tugged on the clumps around them, and we ended up with a whole solar system of planets – and the Sun in the middle.
But there were also quite big clumps left over – like a sort of builders’ rubble – and these began slamming and crashing into the newborn planets like a giant pinball machine, making massive craters like the ones we see on the Moon, and heating up the Earth’s surface so much that any water probably boiled off.
What happened next is that, over the years, comets also began smashing into the Earth. Comets are big dirty snowballs made almost entirely of ice, so when they landed they handed over their ice to start making oceans.
More water might then have been puffed out by volcanoes from the interior of the Earth where it was trapped inside rocks. Put all this together over millions of years and hey presto – you have your oceans.
By the way, the reason the water has somewhere to slosh around in is an entirely different story. The oceans sit in gigantic basins like sunken baths, which are much lower than the surrounding continents. That’s because the continents, such as Europe, Asia and America, actually move around the surface of the Earth very slowly – about the rate your fingernails grow.
When two continents move apart they stretch out the space between them, which turns into a wide ocean basin like the Atlantic or Pacific, ready to receive all that water. When two continents move together they squeeze the space between them, until in some cases there is no ocean left.
The mighty Himalayas were formed when two pieces of continent moved closer and closer until they swallowed up the ocean between them and then crashed together to make Mount Everest and all the other high mountains around it.