Today, 70% of the Earth's surface is covered in water.
But in Earth's infancy, 4.5 billion years ago, there was no water here at all.
Earth was a molten inferno. It was too hot for liquid water to exist!
But inside the Earth's core, oxygen and hydrogen had bonded to create the gaseous form of water.
As thousands of volcanoes erupted, water vapour was released into the atmosphere.
Over the next billion years the planet began to cool.
Enough to condense gases into rain clouds.
It rained for thousands of years.
Filling low-lying areas and forming the planet's first oceans.
Yet scientists believed this only provided half of the water on Earth, the other half came from outer space.
This is a comet filmed in 2005.
NASA decided to see just how much water a comet contained and flew a satellite straight into it.
As it ploughed into its surface, more than 250 million litres of water burst into space.
In Earth's early years, it was hit by thousands of these comets.
The water in our oceans today, is the very same water that created our blue planet, 3.5 billion years ago.