The differences between the two hemispheres could not be clearer. While the facing side of the moon features vast, dark plains of cooled lava — which the ancients assumed were seas — the far side is mostly an expanse of tens of thousands of impact craters. It is the tug of the Earth, astronomers believe, that is responsible for the different topography. Earthly gravity pulls with greater force on the dense, iron-and-magnesium interior of the moon than on the lighter upper layers. This causes the core to shift slightly earthward, thinning out the crust on that half of the moon.