I think that was asked a long time back in the forums somewhere, and the advice was that a separate effect was needed to create caves or underground structures. A different enough effect to need a separate spell. from memory there was also a discussion of using different techniques in the cave forming spell too (re and mu, or Cr & Pe together). That said, the Tower's foundations are meant to go a few floors below ground (I can't recall the text atm) so it seems plausible that the Tower could be linked with an underground area in that manner.
Edit: Found a thread, but isn't the one I recalled. It has a suggestion for the effect.