Three-quarters of the Earth is covered in water. It's called "The Blue Planet." But imagine Earth without its oceans...and instead, with all the water in the world gathered together in a boll. There's not that much, really. But this water is what makes Earth different from every other planet in the solar system. The oceans have been here for almost 4 billion years-carving the coastline, driving the climate, and controlling the destiny of life itself. But four-and-a-half billion years ago, when our planet was born, it was just a molten infern. There was no place for any liquid water at all. Yet the ingredients needed to create water were there-locked away deep inside the Earth, when the planet formed. As the first volcanoes erupted, one of the gases that bellowed out was steam. And as the planet cooled, it formed clouds. It rained for thousands of years. As the first rivers formed, it began to fill up the lower-lying areas on our young planet, possibly creating the first oceans. But it's reckoned that even this relentless downpour only delivered around half the water that makes up our oceans today. The rest came from outer space. This is a comet filmed in 2005. It's about 4 miles across, and like all comets, it's made of roke and water in the form of ice. To discover just how muth water comets contain, scientists deliberately crashed a apace probe straight into it. After it plowed into the surface, more than 250 thousand tons of water burst into apace. In Earth's formative years, it was hit by thousand of comets like this. By the time the bombardment finished, it's thought they deliverd up to half the water in our oceans.