"Well, I aim to ride over the mountain tonight," answered Bud, "and see if all is clear at the river intake end of the line. If it is, I'll know there must be a stoppage, or break, somewhere inside the old water tunnel."
"How you going to find that?" inquired Nort.
"Why, we'll get lanterns and ride through," replied Bud. "That's easy!"
"Ride through an underground river!" cried Dick. "You can't!"
"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."