They are inferring mountains and trenches based on the observed effect of their gravity on the water above. But can't there be mass concentrations that aren't m

They are inferring mountains and trenches based on the observed effect of their gravity on the water above. But can't there be mass concentrations that aren't mountains, like the lunar maria? http://en.wikipedia.org/wiki/Mass_concentration_(astronomy) http://en.wikipedia.org/wiki/Mass_concentration_(astronomy)