Can a planet exist with no core, just empty space? Perhaps a bubble of molter rock could solidify filled with gas. How thick would the crust have to be for the planet to remain intact? Water: If the planet contained some water would it pool at the bottom of the planet, or would it constantly slosh around? Gravity: Would the spinning of the planet act as a centrifuge forcing the contents of the planet to the sides? So people in a hollow Earth would walk on the crust internally, as we do externally? Their 'sky' would be a ceiling with people walking on it!