Oceans cover seventy percent of the Earth's surface is covered in water, and there's even more water trapped inside the Earth. Where did it all come from? And when? There have long been two possible answers to those questions: it could have been here since the very beginning, or it could have arrived later, carried by bombarding asteroids and comets. The prevailing thought has been that the latter is more likely because, when the planets were forming nearly four and a half billion years ago,