Take a small gardening shovel and open the top of the next tunnel you find just enough to get a vacuum hose or whatever into it. Cobble up adaptors as necessary to get the other end on your mower exhaust and pump away. The CO will flow through the tunnels. So does propane from a grill tank, and the propane is more deadly because it's heavier than air. Do not be tempted to ignite it. You could wind up like someone I know on another forum/email list who blew his porch off his house trying to gas and blow out some gophers in his yard.

Based on your size description you more likely have shrews.