I placed my eye at the aperture, and almost recoiled with the violence of my surprise. The tunnel before me was brilliantly illuminated, and within three feet of the wall of rock behind which we crouched stood Dr. Syx, his dark profile looking almost satanic in the sharp contrast of light and shadow. He was talking to one of his foremen, and the two were the only visible occupants of the tunnel. Putting my ear to the little opening, I heard his words distinctly:

--"end of their rope. Well, they've spent a pretty lot of money for their experience, and I rather think we shall not be troubled again by artemisium-seekers for some time to come."

The doctor's voice ceased, and instantly I clapped my eye to the hole. He had changed his position so that his black eyes now looked straight at the aperture. My heart was in my mouth, for at first I believed from his expression that he had detected the gleam of my eyeball. But if so, he probably mistook it for a bit of mica in the rock, and paid no further attention. Then his lips moved, and I put my ear again to the hole. He seemed to be replying to a question that the foreman had asked.

"If they do," he said, "they will never guess the real secret."

Thereupon he turned on his heel, kicked a bit of rock off the track, and strode away towards the entrance. The foreman paused long enough to turn out the electric lamp, and then followed the doctor.

"Well," asked Hall, "what have you heard?"

I told him everything.

"It fully corroborates the evidence of my own eyes and ears," he remarked, "and we may count ourselves extremely lucky. It is not likely that Dr. Syx will be heard a second time proclaiming his deception with his own lips. It is plain that he was led to talk as he did to the foreman on account of the latter's having informed him of the sudden discharge of my men this morning. Their presence within ear-shot of our hiding-place during their conversation was, of course, pure accident, and so you can see how kind fortune has been to us. I expected to have to watch and listen and form deductions for a week, at least, before getting the information which five lucky minutes have placed in our hands."

While he was speaking my companion busied himself in carefully plugging up the hole in the rock. When it was closed to his satisfaction he turned on the light in our tunnel.

"Did you observe," he asked, "that there was a second tunnel?"

"What do you say?"

"When the light was on in there I saw the mouth of a smaller tunnel entering the main one behind the cars on the right. Did you notice it?"

"Oh yes," I replied. "I did observe some kind of a dark hole there, but I paid no attention to it because I was so absorbed in the doctor."

"Well," rejoined Hall, smiling, "it was worth considerably more than a glance. As a subject of thought I find it even more absorbing than Dr. Syx. Did you see the track in it?"

"No," I had to acknowledge, "I did not notice that. But," I continued, a little piqued by his manner, "being a branch of the main tunnel, I don't see anything remarkable in its having a track also."

"It was rather dim in that hole," said Hall, still smiling in a somewhat provoking way, "but the railroad track was there plain enough. And, whether you think it remarkable or not, I should like to lay you a wager that that track leads to a secret worth a dozen of the one we have just overheard."

"My good friend," I retorted, still smarting a little, "I shall not presume to match my stupidity against your perspicacity. I haven't cat's eyes in the dark."

Hall immediately broke out laughing, and, slapping me good-naturedly on the shoulder, exclaimed:

"Come, come now! If you go to kicking back at a fellow like that, I shall be sorry I ever undertook this adventure."

VII

A MYSTERY INDEED!

When President Boon had heard our story he promptly approved Hall's dismissal of the men. He expressed great surprise that Dr. Syx should have resorted to a deception which had been so disastrous to innocent people, and at first he talked of legal proceedings. But, after thinking the matter over, he concluded that Syx was too powerful to be attacked with success, especially when the only evidence against him was that he had claimed to find artemisium in his mine at a time when, as everybody knew, artemisium actually was found outside the mine. There was no apparent motive for the deception, and no proof of malicious intent. In short, Mr. Boon decided that the best thing for him and his stockholders to do was to keep silent about their losses and await events. And, at Hall's suggestion, he also determined to say nothing to anybody about the discovery we had made.

"It could do no good," said Hall, in making the suggestion, "and it might spoil a plan I have in mind."

"What plan?" asked the president.

"I prefer not to tell just yet," was the reply.

I observed that, in our interview with Mr. Boon, Hall made no reference to the side tunnel to which he had appeared to attach so much importance, and I concluded that he now regarded it as lacking significance. In this I was mistaken.

A few days afterwards I received an invitation from Hall to accompany him once more into the abandoned tunnel.

"I have found out what that sidetrack means," he said, "and it has plunged me into another mystery so dark and profound that I cannot see my way through it. I must beg you to say no word to any one concerning the things I am about to show you."

I gave the required promise, and we entered the tunnel, which n.o.body had visited since our former adventure. Having extinguished our lamp, my companion opened the peep-hole, and a thin ray of light streamed through from the tunnel on the opposite side of the wall. He applied his eye to the hole.

"Yes," he said, quickly stepping back and pushing me into his place, "they are still at it. Look, and tell me what you see."

"I see," I replied, after placing my eye at the aperture, "a gang of men unloading a car which has just come out of the side tunnel, and putting its contents upon another car standing on the track of the main tunnel."

"Yes, and what are they handling?"

"Why, ore, of course."

"And do you see nothing significant in that?"

"To be sure!" I exclaimed. "Why, that ore--"

"Hush! hush!" admonished Hall, putting his hand over my mouth; "don't talk so loud. Now go on, in a whisper."

"The ore," I resumed, "may have come back from the furnace-room, because the side tunnel turns off so as to run parallel with the other."

"It not only may have come back, it actually has come back," said Hall.

"How can you be sure?"

"Because I have been over the track, and know that it leads to a secret apartment directly under the furnace in which Dr. Syx pretends to melt the ore!"

For a minute after hearing this avowal I was speechless.

"Are you serious?" I asked at length.

"Perfectly serious. Run your finger along the rock here. Do you perceive a seam? Two days ago, after seeing what you have just witnessed in the Syx tunnel, I carefully cut out a section of the wall, making an aperture large enough to crawl through, and, when I knew the workmen were asleep, I crept in there and examined both tunnels from end to end. But in solving one mystery I have run myself into another infinitely more perplexing."

"How is that?"

"Why does Dr. Syx take such elaborate pains to deceive his visitors, and also the government officers? It is now plain that he conducts no mining operations whatever. This mine of his is a gigantic blind. Whenever inspectors or scientific curiosity seekers visit his mill his mute workmen a.s.sume the air of being very busy, the cars laden with his so-called 'ore' rumble out of the tunnel, and their contents are ostentatiously poured into the furnace, or appear to be poured into it, really dropping into a receptacle beneath, to be carried back into the mine again. And then the doctor leads his gulled visitors around to the other side of the furnace and shows them the molten metal coming out in streams. Now what does it all mean? That's what I'd like to find out. What's his game? For, mark you, if he doesn't get artemisium from this pretended ore, he gets it from some other source, and right on this spot, too. There is no doubt about that. The whole world is supplied by Syx's furnace, and Syx feeds his furnace with something that comes from his ten acres of Grand Teton rock. What is that something? How does he get it, and where does he hide it? These are the things I should like to find out."

"Well," I replied, "I fear I can't help you."

"But the difference between you and me," he retorted, "is that you can go to sleep over it, while I shall never get another good night's rest so long as this black mystery remains unsolved."

"What will you do?"

"I don't know exactly what. But I've got a dim idea which may take shape after a while."