I have a chicken tunnel. It runs from my coop that's half way up my hill to my side yard where they have a big run area. They even have an 8' section where they go under ground; under the gate to the side yard area. They all (9 chickens) do it well. We have had a raccoon that's attacked and killed one chicken through the tunnel. But he's been back and the chickens have wised up to him. They now know where to hide. Also a bob cat scratched one of my chickens pretty bad through the tunnel, but it got away and that chicken made a full recovery. It's not the most secure, but none of the predators have been able to break the tunnel. They have just reached into the tunnel.
It's been what we needed as the coop needed to be where it was, but the area for the run we wanted closer to our house.
Adding to the chunnel convo -
That is great that the chunnel has provided some measure of safety. Where the attacks in the daytime?
I have found shade cloth over the wire will slow down a predator that is trying to reach in.
A small flock in Sonoma also has an underground tunnel to get from their large coop to an enclosed play area which encompasses a tree. Watching them emerge into the play area was like a cartoon clip of circus chickens - they popped-up like they were tossed from a toaster. Mesmerizing and hysterical at the same time.
My flock has chunnels for hawk protection. It snakes around the orchard in open areas to provide quick cover. The tunnel is covered on the top and sides with stucco, poultry or aviary wire (depending what I have on hand), but stops short of the ground so the chickens can pop in there at any point. The long segments have become part of a "trail" system that the chickens have adopted.
The chunnels are 3 to 5 feet wide, utilizing a hoop system common to gardens: white pvc irrigation pipe makes an arc that slips over rebar which has been pounded into the ground; the wire is zip tied to the hoops.