A Lava Tube                                            -      -      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      -      -

This tunnel was once formed from the molten lava flowing through this
tunnel, slowly eating away, digging it deeper and wider.  On the ceilings,
small glass-like spikes have been formed, thin and delicate, the slightest
noise could dislodge one and send it hurtling down towards you.