As the boys advanced they found the gangway considerably cluttered with "gob," or refuse, and the air was none of the best."I wish we could set the air shaft working," suggested Sandy."Well, we can't!" Tommy answered with a scornful shrug of his shoulders. "We can't set the whole works going in order to give us a midnight view of the Labyrinth mine. What gets me is how are we going to find our way back? There seem to be a good many passages here.""I've got that fixed all right!" Sandy exclaimed.As the lad spoke he took a ball of strong string from his pocket and tied one end to the cage which lay at the bottom of the shaft."Now we can go anywhere we please," he chuckled, "and when we want to return, all we've got to do is to follow the string."