I can't use that one, because the immovable rod my party has, they made themselves (I gave them a magic item formula as a reward). However... One of the ingredients in the formula I gave them was a stone from the highest pinnacle of a mountain (it had to have been there for at least 50 years prior, so they can't mass-produce immovable rods). So the logical choice for me is that the rod is motionless relative to Mt. Waterdeep. If they ever come to another mountain and make another, then that rod will be anchored to that mountain, which could have tectonic implications.

Come to think of it, a recent adventure did bring them to another mountain, but none of us, including me, remembered that detail at the time...