Whoops, can't go this way! This is the end of a listening tunnel. Troops detailed to such duty would sit in this spot and listen for telltale scraping noises, revealing the dastardly digging of an attacker. Alert! In theory, countermines would be dug towards the burrowing enemy, and charges would be set to collapse the earth in on the bad guys. This all sounds terribly haphazard, so thankfully it never came into play.