The mystery of the disappearance of the ten men—for mystery it was—remained, and as no side opening or passage could be found within the tunnel, it came to be the generally accepted explanation that the laborers had come out unobserved, and, for reasons of their own, had run away.  
This habit on the part of the Peruvian workers was not unusual. In fact, the Titus brothers had to maintain a sort of permanent employment agency in Lima to replace the deserters. But they were used to this. The difference was that the Indians used to vanish from camp at night, and invariably after pay-day.
 
"And that's the only reason I have a slight doubt that they walked out of the tunnel," said Job Titus. "There was money due em."
 
"They never came out of the front entrance of the tunnel," said Tom. "Of that I'm positive."
 
But there was no way of proving his assertion.
 
The third blast, while not as successful as the second in the amount of rock loosened, was better than the first, and made a big advance in the tunnel progress. Tom was beginning to understand the nature of the mountain into which the big shaft was being driven and he learned how better to apply the force of his explosive.
 
That was the work which he had charge of—the placing of the giant powder so it would do the most effective work. Then, when the fumes from the blast had cleared away, in would surge the workmen to clear away the debris.
 
Under the direction of Mr. Swift, left at Shopton to oversee the manufacture of the explosive, new shipments came on promptly to Lima, and were brought out to the tunnel on the backs of mules, or in the case of small quantities, on the llamas. But the latter brutes will not carry a heavy load, lying down and refusing to get up if they are overburdened, whereas one has yet to find a mule's limit.
 
After his first success in getting the natives to take a more active interest in the gathering of the cinchona bark, Mr. Damon found it rather easy, for the story of Tom's electric rifle and how it had killed the mad dog spread among the tribes, and Mr. Damon had but to announce that the "lightning shooter," as Tom was called, was a friend of the drug concern to bring about the desired results. Mr. Damon, by paying a sort of bribe, disguised under the name "tax," secured the help of Peruvian officials so he had no trouble on that score.
 
Koku was in his element. He liked a wild life and Peru was much more like the country of giants where Tom had found him, than any place the big man had since visited. Koku had great strength and wanted to use it, and after a week or so of idleness he persuaded Tom to let him go in the tunnel to work.
 
The giant was made a sort of foreman under Tim, and the two became great friends. The only trouble with Koku was that he would do a thing himself instead of letting his men do it, as, of course, all proper foremen should do. If the giant saw two or three of the Indians trying to lift a big rock into the little dump cars, and failing because of its great weight, he would good-naturedly thrust them aside, pick up the big stone in his mighty arms, and deposit it in its place.
 
And once when an unusually big load had been put in a car, and the mule attached found it impossible to pull it out to the tunnel mouth, Koku unhitched the creature and, slipping the harness around his waist, walked out, dragging the load as easily as if pulling a child on a sled.
 
Professor Bumper kept on with his search for the lost city of Pelone. Back and forth he wandered among the wild Andes Mountains, now hopeful that he was on the right trail, and again in despair. Tom and Mr. Damon went with him once more for a week, and though they enjoyed the trip, for the professor was a delightful companion, there were no results. But the scientist would not give up.
 
Tom Swift was kept busy looking after the shipments of the explosive, and arranging for the blasts. He had letters from Ned Newton in which news of Shopton was given, and Mr. Swift wrote occasionally. But the mails in the wilderness of the Andes were few and far between.
 
Tom wrote a letter of explanation to Mr. Nestor, in addition to the wireless he had sent regarding the box labeled dynamite, but he got no answer. Nor were his letters to Mary answered.
 
"I wonder what's wrong?" Tom mused. "It can't be that they think I did that on purpose. And even if Mr. Nestor is angry at me for something that wasn't my fault, Mary ought to write."
 
But she did not, and Tom grew a bit despondent as the days went by and no word came.
 
"I suppose they might be offended because I left Rad to do up that package instead of attending to it myself," thought Tom. "Well, I did make a mistake there, but I didn't mean to. I never thought about Eradicate's not reading. I'll make him go to night school as soon as I get back. But maybe I'll never get another chance to send Mary anythi............