It's a little hard to tell where the marble is in "3d" space. I think it might be a little better if it was even lighter while "underground". A lot of isometric

It's a little hard to tell where the marble is in "3d" space. I think it might be a little better if it was even lighter while "underground". A lot of isometric games use shadows to show exactly where objects are - that could help as well. Edit: now I figured out what the big problem is: you can't see where it's headed when it's underground. In Roller Coaster Tycoon, when your train went into a tunnel, there was a sort of transparent view of the underground track. I think having that here would make a lot of sense.