@Background Pony Number 17  
That’s ridiculous. Tartarus isn’t that far.

@Background Pony #0FB1  
So does that mean Twilight Sparkle will start using Maud’s home for a dungeon?

@Stotter  
I could get behind that. The cavern was left when a castle’s worth of gemstone vacated the premises.
 
Actually, another interesting revelation… this is the second Equestrian castle to have a confirmed cave of gemstone beneath it. Because as we learned back in season two, Canterlot has a network of crystal caverns beneath it. We learned that because the real Cadance was kept there as a prisoner.

@Stotter  
I like that idea.

So, interesting revelation: the castle is made of the same kind of gem material as the common gems underneath Ponyville. In other words, despite the extraordinary method of its creation, it was not just magically sprouted out of the box like a rabbit out of a hat, but instead amalgamated from surrounding materials.
 
Wait, what if Maud’s newly discovered cave was the result of all that mass being moved above ground, and the unique rocks were a side effect of the magical phenomena involved?