How do you make a bridge with a tunnel going through it?
I have cut pvc pipe in half and used that to support the tunnel part. the cake I made was covered in fondant so I just covered the pvc when I covered the cake. I'm sure you could find other things that are easier to cut in half if you don't want to go through the hassle of cutting pvc pipe. Depending on how large your tunnel is, I have also used cleaned oatmeal containers cut in half, and the middle cardboard of unused toilet paper rolls cut in half for smaller ones.