I had some problems placing green/red wires on underground conveyor belts. I am unable to tell whether it's because I did it wrong, or if it's just plain impossible. Can someone clarify which one is the case?
It is impossible (in 0.14) to attach circuit (red/green) cables to underground belts. The only way you can monitor underground items is by monitoring regular belts immediately before and after the underground belts.
This may change in 0.15. If it does, I'll update this answer.