I guess I have to ask, "what kind of tunnel is this?" If it is a cave, say carved by water erosion, then you need stalagmites and 'tites, with a whiter, limestone color. If this is man-made, like a mine, then you need either more bump, or a more chiseled shape to the walls. If this is a natural "cave" (like the neolithic finds in France) then you're pretty close. You need gravel and rocks on the floor, but otherwise, spot on. The lighting is excellent. Once you get it the way you like, you should animate a camera trip through!