I can only second lozz' post, particularly in the light of having a direct professional connection with the whole issue of long-term stability of old workings.
Those big Welsh caverns are scary places. If I understand the matter, there is only one entry to one of the main setts? I wouldn't go in there for all the tea in China, if so....
''the stopes soared beyond the range of our caplamps' - David Bick...... How times change .... oh, I don't know, I've still got a lamp like that.