Water is pretty incredible. Oceans cover 70% of the planet and our bodies are something like 60% water. Where did all this water come from?

One interesting theory is that water came to Earth from space. Specifically, from comets.

Comets in their simplest forms are nothing more than dust and ice, or giant dirty snowballs.

The idea is that about four billion years ago, hundreds of millions of comets bombarded the infant Earth. Comet ice either melted or vaporized when it hit and condensed in the earth's atmosphere.

Scientists have discovered huge ice balls, some as large as six-hundred miles across, in the Kuiper belt, an outer area of the solar system beyond the orbit of Neptune. It's possible that, billions of years ago, some of these objects were knocked out of orbit and spiraled in toward the sun. On the way, they collided with Earth, releasing trillions of tons of water.