earth can`t be hollow, otherwise it would have weak field of gravity, implying unstable scattered atmosphere, thus depriving us of oxygen.We know that we have volcanoes that erupt lava, . Where would the lava come from if the inner surface was hollow? Liquid, molten lava is a result of high temperatures, which in itself is a result of high pressure. any matter that has mass and is subjected to very high pressures will have high temperature, implying that core is of dense material, that is pressurized. The pressure is created in this case by gravity, but gravity is dependent on how dense an object is, implying that core should have denser materials such as iron. If you go down in coal quarries in RSA, the temperature increases dramatically once you go below 2 km mark, implying increased temperatures closer to the center of earth, which hints to high pressure of dense mass. i even believe that gas giants like SAturn or uranus have liquid cores that give off heat because of high pressure.