I may be delusional but a few years back was there a thread detailing the build of a tunnel? To prevent caving in the den was a large wooden box. I'm not sure of tunnel. He buried it and backfilled the hole. It had electrical and access to the den from a one man hatch on top. I'm trying to remember how he prevented rain from draining into the den. I'm not sure if I seen it here or another source.