After removing some of the smoke shelf to make clearance for an SS liner for a new insert in my fireplace, I was able to get a better view up inside the smoke chamber. With this better view and cleaning it out up there I noticed wood planking at the top of the smoke chamber! This fireplace is in my basement with another fireplace directly above it in the same chimney on the main floor of the house. This is directly below the flue and smoke shelf of the main floor fireplace. What the heck is this?