A Lava Tube                                            -      -      - 
(-------------------------------------------------)     - <---(M)---> -
                                                        -      S      -

This tunnel was once formed from the action of molten lava flowing
through here, wearing away more and more of the tunnel as it went.  On the
ceilings, small glass-like spikes have been formed, thin and delicate, the
slightest noise would dislodge one and send it tumbling down to the ground.