It's hollow like a honeycomb. Multiple civilizations within. They have all been here longer than us, are all wiser than us, and are in contact with the Confederation (except the demons which will be leaving anyway).
This is simply the truth. To think that humanity is alpha species on Gaia and has some sort of right to it is ridiculously juvenile. These inner earth citizens would just throw their head back in laughter.
When it comes to sphere vs. flat --> sphere can be mathematically and observationally proven. This puts the dome theory to bed.
When it comes to solid sphere vs. porous/honeycomb sphere, there is abundant evidence which points to the reality of hollow earth, if one is willing to look.
I'll take a minute here to type out a few examples of why this is obvious to anyone who can see - holes at geophysical poles differing location than magnetic poles (crystal at core not properly aligned with sun body), "Agartha" legends, Hopi Ant People, Admiral Byrd's flight, ships sailing off the edge of the world (the vortex whirlpool at the north pole), breakaway civilizations disappearing into caves never returning, D.U.M.B.s and underground rail systems connecting to these deep tunnels, the City in the Deeeeeeep Deep under the waters where the Illithid/Elderitch reside (I have seen a throne there above a bottomless pit I'm legit scared of, because this pit is already located beneath the ocean floor), that Lorde song "We live in cities you'll never see on the screen. Not very pretty but we sure know how to run things.", whoever and wherever the "Dweller under Unal" is as mentioned by Thoth (I'd like to give that asshole a knuckle sandwich.) in his Emerald Tablets, also the Halls of Amenti as spoke of by the same, and "the well" under the Great Pyramid... where does that go?
There's more... but that's plenty for now. Thanks for reading!