I'm reading through Feynman's lectures, and just read his demonstration that: a) a sphere generates the same gravity force on a body outside of it as if all its mass was concentrated in its center; b) on the other hand, on a body placed inside the same sphere there is no gravity force at all. I think I understand the proof well, but how does b) agree with the structure of the Earth? I understand that as you go towards the center of the Earth, the density and the pressure increase greatly... but now I realize I don't quite understand why. If a rock inside the Earth at depth 100km and another one at depth 1000km experience the same force of gravity towards the center, where does the increased pressure come from? I suddenly feel like I'm missing something very basic here... Thanks in advance!