Going back to Room #2 and taking the next left leads to room where the party can smell the strong stench of brimstone. Fires can be seen flickering in the tunnel and the party can feel heat.
Inside the room is a sight the party is not prepared for.
Inside the room are three Burning Bunnies.
These creatures are the Elemental Plane of Fire analogs to the common rabbit. They are here eating coal and sulfur. Thus the smell. Like regular bunnies, they are quite scared and mostly harmless.
Picking one up (if you can) causes 1d8 points of fire damage. When scared (which is all the time), they will explode. This is how they get back to their home plane. The explosion causes 2d8 points of fire damage to anyone within 5' of the rabbit. This behavior has given them the nickname Boom Bunnies.
In an interesting side effect of their diet, the Burning Bunnies leave behind 1d3 small diamonds worth 10 gp each as their "droppings."
They have no other treasure.