The north portal of #9. Note the solid concrete top to the portal. Likely an area that was rebuilt at some point to strengthen the portal against water that may have been backing up behind it.
Posted May 10, 2013, by Clark Vance (cvance [at] dogmail [dot] com)
The plate in photo 2 is a USGS survey mark. It indicates a point that is surveyed accurately enough that it can be used as a reference point for other surveys.
Thanks Clark, very interesting!