This valley was originally called Buffalo Wallow, the source of the name being obvious. But once water was brought through the underground course, and piped to a reservoir, whence it could be distributed to drinking troughs for the cattle, and also used to irrigate the land, it enabled a fine crop of fodder to be grown. With the bringing of the water to Buffalo Wallow, or Flume Valley, as Bud called the place, it was possible to do what had never been done before--raise cattle there. Bud's father let him take this valley ranch as his own, and Nort and Dick were boy partners associated with their western cousin, Mr. Shannon putting up part of the needed capital to make the start for his sons. All would have gone well except for the mysterious stoppage of the flow of water, which stoppage, if continued, would mean disaster. How the water fight at Diamond X Second (as the valley ranch was sometimes called) ended, and how the strange mystery was solved, is the story in the second volume, and I absolutely refuse to go into more details about it here. It would not be playing the game square. At any rate the water was finally turned back into the underground tunnel, and then, in order to better guard this vital necessity, Mr. Merkel had the entrance to the tunnel boarded up--egress being possible only when heavy doors, at either end, were unlocked. I might say that while the tunnel was the old water-course of a vanished river, the shaft under the mountain appeared, in. ancient times, to have been used by the Aztecs, or some Mexican tribes, for hiding their store of gold away from the Spaniards. There were secret passages and rooms in the tunnel, to say nothing of hidden water gates. Who had constructed these, and what actual use had been made of them was, of course, lost in the dim and ancient past. But that it was the Aztecs, or some allied race, was the statement of learned men who examined the tunnel. After the water fight at Diamond X Second had terminated in favor of the boy ranchers, and great copper levers that operated the hidden water gates had been removed, the tunnel was boarded up, and was now seldom entered. But now, as Bud and his cousins rode back from the big round-up, and the western lad had, as he thought, seen some one sneaking about the forbidden gate, there was a feeling of apprehension in the hearts of himself and cousins. They had now reached the top level of the reservoir which held a storage supply of water. The reservoir was a great semi-circular bank of earth and atones, wide enough on top for two to ride abreast. "I don't see any one," remarked Nort, straining his eyes to pierce the gloom and shadows into which the face of the tunnel and the locked gate were thrown by the moonlight and clouds. "Nor I," added Dick. "Well, I saw some one!" insisted Bud. "It was a man, as sure as snakes, and he seemed to be trying to open the big gate."