A tunnel is usually an underground passageway, completely enclosed except for openings for entrance and exit. One of the more memorable books related to tunnels I read recently was about the Battle of the Crater, a civil war battle where the North planned everything almost to perfection until the final execution of the plan. Failure to adequately take advantage of the opening created by exploding the tunnel led to eight more months of trench warfare.
This cache is just off the parking lot on a short trail. Once you get close to ground zero, the tunnel reference will make sense.
This cache qualified for the 2013 Battle Ground Geocoin Challenge but all Geocoins have been handed out! Look for the 2014 and future challenges for more geocoins. The codeword can still be used for the 2013 Bonus Cache, though.: