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

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.  
A large stalactite is dislodged from the ceiling, narrowly missing you!