(Terribly sorry, but they aren't.  Fortunately, you only needed to deal 2 points of damage to dust the final skeleton)  The skeleton shattered from the impact of the arrow, leaving the group alone on a platform 25 feet deep into the ground.  A message came from inside the machine.  "Say your prayers, fools.  You've got about ten seconds before the tunnel collapses on top of you!"  Nergassa announced with cackling, gleeful laughter, while the roar of the machine's drill started to send cracks through the tunnel walls.