Prison Architect puts you in charge of a prison where you need to make sure your prisoners carry out their full sentences. Naturally, they’re not going to like that idea and will sometimes try to escape through the use of tunnels. It’s your job to find and clear out these tunnels so prisoners won’t be able to escape through them like Shawshank Redemption.
Dog Handlers are your best bet against tunnels. Place a dog handler on a route surrounding your prison walls, and it will walk through that route until it sniffs out some tunnels.
Once a tunnel is found, it will show up on your screen with an icon hovering over the location. Go to the Demolish section of the build menu and choose the Remove Tunnels option and click on the tunnel exit. Once your workmen clear out the exit, the rest of the tunnel will reveal itself through all of the different cells that were using that underground network. Simply drag the Remove Tunnels over every last bit of tunnel and you’ll be rid of them all.