How do you know it is solid and doesn't resemble the form of an atom?
Because magnetism and gravity yo
How does the Earth can be hollow? It's a mindblown
+1. It can't be hollow just because we have oil and other awesome stuff
Attempting to play the devil's advocate here, those things don't necessarily imply the Earth, or more likely, some regions within the planet, couldn't be hollow. Oil would in theory form even if the Earth was hollow, with little more than the crust. Magnetism and gravity would require there to be something inside I suppose, though if the hollow region(s) were relatively small, they could perhaps go by unnoticed. In this case, for the "whole" Earth to be hollow, there would still need to be something equivalent to the core inside, and some as yet unknown material covering the inside of the crust to prevent the whole planet from collapsing (just don't ask me how the planet could have been formed that way).
On the other hand, seismic waves give us a very good idea of the interior composition of the planet; unless there is something fundamentally wrong with our understanding of them, they negate the idea that the Earth could be hollow, or even that large regions are, especially when combined with the other methods.
