You can see the reclaimed washing machine lids at the far end of the tunnel as well...they make it easier to span the two cob walls and are both strong and fire proof, and won't restrict the heat conduction from flue to floor.
Now, we'll fill in the subfloor on both sides of the tunnel with gravel wheelbarrowed in. When that is done, we'll have wide, flat spaces to mix on, which will make the perimeter walls quick to put up. And the interior wall, which will hold up the south roof beams and 2nd floor joists, will mostly follow this tunnel but will have two wide arches! It's going to be fun!