I enjoyed the puzzle but I think there's a spelling issue: MANTEL is correct for the shelf above a fireplace, but the layer of the Earth below the crust is the MANTLE. From a quick look online, MANTLE can be a variant spelling of the fireplace shelf, so it might be possible to switch the last two letters of 27 Across and re-fill the mideast of the grid.
Oh man I was hoping nobody would notice this haha! I try really hard to avoid variant spelling tags in my clues, maybe I should have just given into it for this puzzle. I applaud your eye for detail!