You do realize that aliens aside, the Earth cannot be hollow for a variety of physics-related reasons, right? For starters, it would rip itself apart with its own spin if it were hollow - it's really that simple. Secondly, if the Earth was hollow, its seismic activity would echo, which we're not seeing on seismic monitors, and the crust would literally collapse into itself. The crust of the Earth is built of tectonic plates which are under constant friction againts each other - were the Earth hollow, these would bend inwards rather than outwards, hence we would have no volcanoes at all. When friction between two hard materials occurs, the plates will always give way to the weaker side, and you can't go any weaker than "empty". In reality, the plates gradually layer up ontop of each other, creating heat and pushing the crust as well as magma outwards.