"No, we couldn't if the old underground river course was _full_," agreed Bud, "but it _isn't_. There's only a comparatively small amount of water flowing through the old course, which is wide enough for two of us to ride or walk abreast, and twice as high as you need. I've ridden through more than once. It's like a long, natural tunnel under the mountain, with water flowing in the center depression, so to speak." "Must be rather spooky inside there," suggested Nort. "It is a little; and it's nearly an all-day's ride. But it's the only way to find the trouble. Professor Wright said that some day the water might work through, and go off on a new course, and in that case I'd be dished until I could stop up the break." "Well, we'll help all we can," offered Nort. "Sure thing!" echoed his brother. "We'd better take it a bit easy now," spoke Bud, as the ascent of the mountain became more steep. "We don't want to wind the ponies, and we may have a hard day ahead of us to-morrow." "It _is_ quite a climb," admitted Nort. "Are we going to ride all night?" "No, we'll turn in about midnight," said Bud. "But this will give us a start so we can get to the Pocut River end of the flume by morning. We can stop any time you fellows want to." "Oh, we aren't tired!" Dick hastened to say, a sentiment with which his brother agreed. "This is as much fun as riding herd, and driving off the cattle rustlers." "Glad you like it," commented Bud. "And the rustlers might as well drive off our stock, if we don't soon get this water to running again. Old Billee said I'd have bad luck when that black rabbit crossed my path, and it sure is coming!" "What black rabbit was that?" asked Nort, curiously. "One that gave me a tumble when I was riding to meet you," answered Bud. "I never saw one before, and I don't want to again. Not that I'm superstitious, but there sure is something queer about _this_! I don't like it for a cent!" The boy ranchers and the Zuni Indian rode on, mounting higher and higher along the mountain trail, heading for the summit. And when they reached it, and Bud, by a glance at his watch, announced that it was midnight, he followed with the suggestion that they camp there for the remainder of the night. "We can make the rest of the trip in a couple of hours, for it's down hill," he said. "Camp suits me," murmured Nort, and soon, after a bite to eat, they rolled themselves in their blankets, having tied the ponies to scrub bushes, and went to sleep. The riding of the boys, coupled with the pure air they had breathed, brought them slumber almost at once, and even Buck Tooth, alert as he usually was, neither saw nor heard anything of the sinister visitor who came softly upon the sleeping ones during the night hours. For there did come a visitor in the night, as evidenced by a scrawled warning, on a dirty piece of paper, fastened to a stubby tree by a long, sharp thorn.