The reminder had its effect. This linking up of the mysterious tunnel that had so nearly proved fatal to Andrew, with David's first adventure suggested the possible solution of a problem that had baffled them until now. In spite of Miranda's derisive comments on the schoolmaster's "fairy tale," there seemed to be only one thing to do--explore the tunnel. It might lead nowhere, and in that case the labor and the risk--if risk there was--would be of small account. If, however, it was the entrance to a subterranean dwelling, inhabited by people of whom the strange being described by Andrew was a specimen, the discovery was well worth making. "We will rescue David!" exclaimed Una, the eagerness of hope in her voice. "But, my young lady," protested Miranda; "he go away many mile from this tunnel." "That is true," assented Leighton. "All the same, David was lost there before," Raoul reminded him. "It is a clew we are bound to follow." The question remained, how carry out the proposed exploration? Equipped with miners' lamps, a number of which, of the best pattern, were still among the stores David and Raoul had brought to Colombia at the beginning of their venture, the worst difficulty--darkness--could easily be overcome. Firearms, a supply of provisions, and oil for the lamps, were other items obviously needed. But the essential thing was, as Doctor Miranda tersely put it, "brains"--a cool-headed leader who would bring them back to the entrance of the tunnel in case of danger. General Herran, with his military training and experience, was the man for this rôle. This hero of unfought battles was thereupon chosen captain of the expedition--not, however, without some modest disclaimers of ability on his part. "There will be five of us then," remarked Leighton. "General Herran, Doctor Miranda, Arthur, Parmelee and myself." "There will be six," amended Una. "Six?" "I will be one of the party." "Preposterous! You might as well make it seven, and include Mrs. Quayle." "I wouldn't think of going," declared that lady quivering with agitation. "It is not for the womens," argued Miranda, in his most conciliatory manner. "There may be troubles, and we want only the mens." Una turned on him fiercely. "I don't believe there is any danger," she cried; "but, anyway, I am going. I am certain David is there. I will go!" To all of which Miranda gave an untranslatable exclamation denoting sympathy, admiration for the pluck of this unexpected volunteer. Leighton, however, was less easily moved, and it was not until his niece assured him that she would return if the expedition promised to be a dangerous one, that he consented to her passionate plea. XII A DEAD WALL