My tunnels are made up of huge PVC tubes that I foamed up and placed rock over...You can see from one side of the tank all the way to the other.

From the right:


From the left:


I also have tubes that stretch across the back of the reef....one has a k3 pointing into it to get flow behind the reef....kind of hard to get a pic of that one though.

Would love to see what everyone else has...