Dark and sinister nebulosity, Bert! The mosaic is looking fantastic. Such a large FoV. Watch your clipping in the shadows.
I enjoyed seeing the bright knots of hydrogen. On the outskirts of the coal sack is actually a very large, yet faint band of hydrogen which few images pick up. It really makes me wonder what could be behind all that dust and gas. You can clearly see it in Galaxy-map's Milky Way explorer [MWE] (
http://galaxymap.org/mwe/mwe.php). You really are pushing out many images of late. Productive imaging. Well done!