Since we have these utterly untouchable wild things that run the surface world, perhaps we can have Dvergar that are essentially the underground version. Dvergar, like (presumably) dryads wouldn't have any visible effect on the game in general, but instead would merely be one of those creatures that you see at the edge of your vision who will then vanish into the stone.
If one wishes to provide some functional use, they could randomly restore stones to the surface under which they run, but I think it sufficient to have boulders reform on the surface and the blame be placed on these little critters rather than giving them any practical use.