The center tunnel winds for a good quarter of a mile, the width varying from one person to three people across. The first wide area is a cavern filled with stalactites and stalagmites, water dripping somewhere in the background. There is no natural light in this room and the east side of the room continues on down the tunnel. There is a lot of feces on the ground, though it's so dark it's hard to tell if it's dirt or feces, along with small bones. Bats fly in and out of this room, their small screeches filling the air if disturbed. The surprise in this room is a flock of giant bats.
You should be able to scale these to fit what ever level characters you have. Either pump up hit points, numbers, or both. For lower levels keep the numbers low. There is no treasure in this room, unless they find the dead adventurer in the far corner, who is covered in moss and feces. Most of his stuff is ruined, but there is a half decomposed map of part of the cavern. You can throw any treasure in here, if you wish.