I'm going to take a stab at answering this even though I personally believe in a RE.
I have read in variously locations on this forum that the Earth is being propelled upwards by dark energy. Taking this idea forward seeing the FE is moving in a straight line it makes sense to say that this dark energy is uniform in strength across the entire underside of the FE.
In order for this to be it also makes sense to say that the underside of the FE is both flat, in the literal sense, and made of the same material otherwise you would have to take into account the effect of friction and force etc on different materials which might affect the strength of the dark energy.
You could also suggest that the dark energy coming into contact with the underside of the FE causes it to heat up and that this is what causes the molten core.
So in answer to the question I would say that under the FE is a layer of molten rock on top of a mass of dark energy which, and this is a complete guess, may not technically exist in our universe in the way everything above the FE does. This could also explain why the earth would be flat as it is resting on the edge of space/time and so only affected by universal forces on one side.
hows that?