I started a new site, and was digging a 'plug' to get through an aquifer, but once it had been completed and the support removed, nothing happened. I noticed that the wall I had build was connected to leaves in the tree above (not sure if they are supposed to hold weight), so I cut it down, still nothing. As near as I can tell, and I may just be missing something here, the plug is still floating in this save, when it shouldn't be.