Originally posted by Draconifors:

I've also wondered about those particular structures.

I don't know why I've never dug around in the centre of it (I would guess if something is hidden there it would be in the middle) when I've been meaning to try for a long time now.

I dug around there i think maybe when it was 4 or 5 weeks into the game when it first came out (Took me a while to beat the first boss with a friend) but we never really uncovered anything) just a crater that looks like a meteor that just smashed into centre of it) XD