We know how much the earth mass is, because we can figure how much influence it has on the moon (the moon orbital characteristics depends on only two factor: how far it is from the earth, and the mass of the earth).For this, and knowing the size of the earth, we can compute a density, which is pretty high at 5.5. Knowing that the center of the earth is very hot (the lava coming out of volcanoes is a fairly good proof) one can easily deduct that:- the earth is essentially molten rocks, with just a very thin solid crust- the pressure and temperature increase very quickly going downand thus no hollowness could survive, as there is no material strong enough to support that kind of pressure, especially in molten form.The mass of the earth is essentially constant, save for the contribution of falling meteorites.