Video narration
You're drifting through a crowded cloud of dust, ice, and rock, before Earth exists. About 4.5 billion years ago, gravity draws the cloud inward. A young Sun forms at its center while the leftovers spread into a spinning disk. As the disk spins, gravity gathers debris into larger clumps. Like a snowball gathering bits, rocky bodies collide and join, again and again. Each merger builds something larger, until Earth takes shape. But the collisions that built Earth left it burning. Its surface is not solid; it is molten rock, battered by impacts, with lava and eruptions reshaping the world. As heat escapes, the surface darkens into a rocky crust. Steam rises around the planet. Now icy asteroids drift past. They're one possible clue to Earth's water, not a proven source. Earth cooled into a rocky world. Where did its water begin?