Well you are talking about agartha. It is a theory wherein there is an advance civilization inside the Earth thats why it was hollow. There are theorist that are believing on it and they are saying that they have their own sun there that's  why it is hot inside. How can they live??maybe they evolve yo a body that can cope the temperature. Maybe it is just magma inside causing convection current and seafloor spreading subsequently.
Well personally I don't believe hollow Earth theory..It is just magma inside..