The Lackawanna Coal Mine Tour takes you down 90 metres below the surface in an old mine car, through a sloping shaft into a working face of the Clark Vein. Retired miners give the tour, which is the main reason to go: the geological detail is interesting, but the personal recollections — child labour, mine disasters, the wildcat strikes — are what stay with you. The whole tour takes about an hour and the temperature underground is around 13°C year-round, so bring a jacket even in summer.
The mine is in McDade Park on the south-western edge of Scranton, next to the Anthracite Heritage Museum (a separate ticket; useful for context). Tickets cost around 12 dollars; the mine is only open April through November. Reservations are sensible on weekends.