Ok. I suppose this could be posted in something scientific in nature but considering Philosophy is a science I guess its as good a place as any, So:

If indeed the earth is flat like flat Earthers believe then there can be no hollow earth. On the other hand if the earth is hollow like the hollow Earthers believe then a flat earth can not exist. This would make it so that one proves the other wrong and that a spherical earth is the reality, with its cores and mantle and crust.

This of course leaves other options open such as dimensional existences of all three earth configurations, the sphere being the one we exist on daily and the hollow and flat versions at times of transcendental exploration.

That said and some reasonable solidity is found in this philosophy it seems the problem could thus be settled that everyone is correct. However now we have another dimension tossed into the salad. That is...we are not real at all but just a program running in a holographic chamber being manipulated by some being or beings like so many characters in a a game.
Proof you may require. There isn't any, just the conjecture of thousands... millions of people who believe their version of out planet an existence is the correct version.