"Wonder what they want?" murmured Bob, in an undertone to his chum. "Fact is, I've noticed them before, but never said anything about it. They seem to follow us, for some reason or other." Joe nodded. "I've thought the same thing," he said quietly. "But as they made no move against us, I almost forgot about it till now." The youths said no more until they reached their stateroom, which was directly across the hall from that of their elders. As soon as their belongings were together, Bob decided to mention the matter to his father and friends. "Ten to one they haven't noticed these men," he said to Joe, "and it might be best for us to put 'em wise." The youths found their companions preparing to leave for the deck and motioned for them to come in the room. Then Bob told of the actions of the two Arabs, pointing out that they probably had no good intentions. The men listened closely, eager to get all the details. When the boys were finished, Mr. Lewis looked grave. "They probably heard Dr. Kirshner mention the hidden treasure," he said soberly, at the same time glancing about as if he expected to see the Arabs at the door. "Hidden treasure? What treasure?" demanded Joe excitedly, while Bob looked up in surprise. "It may only be a yarn," replied Dr. Kirshner. "In fact I just heard about it this morning. An intelligent Arab with whom I made friends pointed out that he had definite information that there were great riches amassed in a cave in the heart of the Sahara Desert. It seems that they had been placed there by native tribesmen, or Tuaregs, who inhabited this region hundreds of years ago. Tuaregs even today are very fond of raiding caravans and small towns. Got it in their blood, I guess." He paused a moment for breath, and Bob seized upon the opportunity. "But why did the Arab tell you this?" he asked. "Didn't he know----" "I know what you're thinking," the scientist interrupted. "You're wondering why I got in on this. I did too at the start. But as soon as he had finished with the details, he told me that he was not equal to making the expedition alone and had no relatives or good friends to accompany him. He went on to say that few expeditions are sent out into those remote regions, and that he would gladly share the treasure with us if we would go with him. "Now as this treasure cave is supposedly in the region that we intended to explore, your dads and I could see no reason for not taking him up. His services as a guide will also be invaluable." "But--but how does he know where to search?" questioned Joe Lewis, his tone indicating that he was greatly puzzled. "He has a map," returned the archæologist. "Got it from an old tribesman who was about to die. Whether or not it is accurate, we have yet to see--if nothing prevents us," he added significantly. "Nothing will--if we can help it," said Bob, delighted at such an opportunity.