Thanks for all the replies. Maybe I should have described the thing a bit better. It's metal, oval-ish, and the top sticks up through the pavement of workings. The thing leads down, in sections, through the backfill, to somewhere! Ventilation, or some kind of chute have been suggested. It does look like something for an air-course, or a chute for shovelling stuff down, and if they were working upwards from a level down below, I can understand the need for the sections. But if underneath where they were working was backfill, why would they need an air-course underneath them? Or if it was a chute for filling stuff down, where would it go to? Am I just showing my ignorance?