Five billion years ago, the universe was Earth-less. It remained that way till a vast number of asteroids smashed together and compacted into a giant rocky orb. But that raises a question: The Earth's surface is 70% water, so where'd the liquid come from? A long-standing theory is that a water-rich class of asteroids, called carbonaceous or C-type asteroids, could've pelted the Earth during its creation and brought along water. There's a caveat though, and the C-type asteroids may be only half the story....