Ether hides "in the cavity of a rock," and creeps out at night to look around.
This might just be a cave. On the other hand, it might be an evocation of the same imagery that underlies this passage in Isaiah. The foundation stone in the holy of holies of the Jerusalem temple has a hole in it, that leads to a cave underneath.
Hiding in the cavity of a rock might therefore just mean a cave. Or it might mean that Ether hid in a temple, a sanctuary. Or maybe he hid in a cave, and Moroni means to evoke for us that his hiding place thereby became a sort of hallowed ground.