The right tunnel from the mosaic room spirals downward, narrowing to about five feet wide at spots, the floor is smooth, (requiring a Dex checks in areas), and becomes smoother the further in a person goes. About three hundred feet down the tunnel there is a break in the floor, with a rock fall to one side. Characters need to make a Dex check to keep from falling into the pit. There are not only rocks at the bottom, but spikes as well. Because of the rock fall there is only a 50% chance of hitting a spike and it is an easy climb out of the pit. There are two dead adventurers as the bottom of the pit and have a small amount of treasure on them. (Make it level treasure for your characters for each of the dead adventurers.)
The tunnel continues on to a large cavern which is finished in places, but natural in others.