This image shows the development of a curved sloping tunnel through the outlines of a mountain (the gray mesh). First the outline shape of the tunnel (blue circular line) is created. Then the path it will travel through the mountain is laid down (orange line). The outline is then attached to the path and is lathed (the outline made to travel from one end of the path to the other), creating what you see in the next shot. I use terrain from UT2004 to assist in the shape of the path and the scale of the tunnel outline. The tiny red square you can see (lower middle of shot) is the size of the player - approximately 96 units tall.