"The object of this descent," said the stranger calmly, "is to see if we cannot solve the mystery of the abandonment of the mine." "Have you ever been down?" asked Billie. "Often." "There is no foul gas at the bottom?" "Not now, as you may see by the condition of Ambrosio, who has been clear to the bottom." "Then I am willing to be the first to descend; but first I must know more about you than I do." "What difference does that make? You will have three friends here with me. They are all armed and I can see they know how to use their weapons. I cannot possibly harm you. I will be the third to descend. I assure you that the descent and the ascent are comparatively easy for athletic young chaps, as the sides of the shaft are very uneven. By the aid of this rope you can come up almost as easily as you would climb a ladder. The adventure is well worth your while." "And you won't tell us who you are?" "I have already told you. I am an American soldier of fortune. My name, if that means anything to you, is Francis Strong, and I have assumed this character of a mountebank solely for the purpose of going about the country without being molested. What I hope to do, is of no interest to any one but myself." It was a straightforward statement and the boys saw no reason to doubt its truthfulness. "All right, then," exclaimed Billie. "Here goes!" He grasped the rope and lowered himself over the side. It was as Strong had said and in a very few minutes he found himself at the bottom. He could see nothing except the dim light at the mouth of the shaft. Giving the rope a vigorous shake, as had been agreed upon, he saw another figure begin to descend, and in a short time Pedro stood beside him. Strong was next to descend, then came Ambrosio, and after him Adrian and Donald in the order named--Donald having determined to be the last, that he might be sure that everything was safe above ground. "I should have given you this," was Strong's first remark upon alighting at Billie's side, and he drew from his pocket an electric torch. "But it slipped my mind." "We all have them in our trunk in the City of Mexico," replied Billie, "but I doubt if either of us has one with him." "This is sufficient, for I shall light some torches I have prepared as soon as we are ready for our work." When Donald had descended, Strong led the way through a lateral about thirty or forty feet, at the end of which another vertical shaft had been sunk. Around the mouth of this Strong had set a number of torches, which he now proceeded to light. By their glare it was possible to see part way down the hole. "The thing I hope to find," explained Strong, "is at the bottom of that hole, if it exists at all." "What is that?" asked Billie.