'Cept it wasn't the Earth, was it? No more than the other planet was the Earth. These were two previous worlds that slammed into each other, creating the Earth/Moon system. Parts of the original worlds mixed and mingled to create a whole new set. But neither of the those worlds were the Earth. Yet.

Since the first time cameras orbited the Moon, scientists have puzzled over the very different "faces" of our smaller partner. The "back" of the Moon is as heavily cratered as the "front," but has none of the dark lava "seas" we know so well. How could this be?

A recent hypothesis suggests that the original cataclysm actually create two moons, each with very different geology. These collided, but very, very slowly, so that instead of blowing each other to pieces, they merged, with the "front" of one and the "back" of the other being left exposed.