Thanks for the clue, I'll watch it later. In some respects it's remarkable that any workings in shale are accessible, more so when they seem to have regularly used blasting as a means of getting usable material.
As much as anything it was the look of the faces of the two men when they reflected on the working conditions they had to contend with, who moves 15-20 tons (real imperial weights!), by hand, in a shift now.