Yup, big mole machine with a hatch and spell turret on one side, drill and another spell turret on the other side.  (spell turret behind the drill, capable of rotating to cover the sides and approaches to the machine when the drill isn't engaged.)  The thing has very slight hover capability to right itself, and seems to have more machinery under the drill bit.  The hatch itself is actually two hatches,  which open from the center.  When one hatch opens, the other closes, making opening both at once to get in very very hard.  (they're both capable of closing simultaneously as well.)  Fortunately, Cathy seems to be able to pull both damned doors open, but not quite fast enough.  On the other hand, are you really going to take advice from your enemy on when the tunnel will collapse?