As well as the tunnel, presumably a trial, in SE corner of the pit there was another tunnel at a lower level. It went from one of the lower levels (possibly the main level???) heading from the flat (mills??) are heading E for some distance before turning through 90 degrees to head N and presumably into the pit. It is blocked someway N of the turn. There are traces of track having been in the tunnel, in particular, steel "gauge retention bars" on the sharp curve.