classically, acceleration towards the earth's surface is described by a=G*m/r^2 usually we think of using this formula to calculate the force of gravity as we rise away from the earth's surface. but consider what would happen if if you traveled through a tunnel to the center of the earth. Assuming constant density of the earth, m~volume~r^3 so a=G*r This means, as you travel inwards, the gravitational force is proportional to the distance from the center of the earth. At the center, there is no gravity. But the earth is spinning and generating centripetal acceleration. Therefore, as in a centrifuge, the denser objects will move away from the center. The core of the earth could thus be a giant gas bubble. Can this be experimentally examined?