A Submerged Cavern                                     -      -      - 
(-------------------------------------------------)     - <---(M)---> E
                                                        -      -      -

Strong eddies and a surprisingly strong current make the normally restive
waters of this river froth and boil with activity.  This cavern is fully
submerged, with nary a drop of oxygen to be had, even at the ceiling. 
Bubbles roil out of a volcanic vent at the back of the cavern, but that
would really be more trouble than it is worth.  A few odd creatures, the
likes of which are not often seen within Sigil's walls, swim about
skittishly, disturbed by your presence.  
A massive frog-like thing swims about here.
A hulking frogocerous begins attacking you!
A hulking frogocerous has several minor scratches.