A local artist (Bob Mitchell) has been busy putting an up slate mining images at various places around North Wales (I think most of the underground installations are at Corris), but at least one is near the entrance to Cwmorthin, although I’d be surprised if it’s still there.
The images have been put up sympathetically, in that they are made of paper and held in place with slate dust so will decompose over time.
He’s put quite a stunning video together as part of it:
Man Made Mountains from Fourth Dimension on Vimeo.