"Then you've done the trick!" said the old cowboy. "You must have opened some gate, and the water's running away. Better swim over here while you have the chance. When the water comes back that rock may be covered!" But another strange part of their mysterious adventures was that they did not have to swim back. For the water receded so rapidly that, in a little while, it was possible to wade from the rock to the stone edge of the pool where the other members of the party stood. And wade back to their friends Bud, Dick and Nort did. "Oh, boy! But we're glad to see you!" cried Old Billee, as he caught Nort by the hand. "You let out a mouthful that time!" declared Yellin' Kid, and his voice nearly split their ear drums, so magnified was it by the echoing, vaulted roof of the cavern. "But what all happened?" asked Snake Purdee. "Is there some old Mexican grain mill under here that has a water-wheel, sluices and gates?" "I give it up," answered Bud. "All I know is that I pulled that copper lever--and it's copper so it won't rust off, I reckon--and the water began to rush out as fast as it must have come in here." "It is mighty queer," agreed Old Billee. "Let's go take a look," and he started to walk across the intervening space between shore and the great rock--a space in which only a few puddles of water now remained. "Will it be safe?" asked Bud, who had begun to dress, an example followed by Dick. "Why not?" asked Old Billee. "The water can't rise any higher than it was when you fellows were on the rock. An', according to your tell, there's room enough for us all t' stand there." "Yes, it's big enough," agreed Bud. "But suppose we all get there, and the water begins to come back?" "We'll turn it loose again with th' lever," answered the old cow puncher. "But I reckon it can't fill up this pool again until that lever is shifted hack where it was before you yanked it." "Maybe not," admitted Bud. "Well, let's take a chance. If worst comes to worst we can swim back, and I'd like to solve this mystery. I feel that we're getting at it now!" "That's right," said Nort, who was feeling stronger every moment. "When I fell in, and was carried away," he said, "I had a wild notion that this might lead to the discovery of something. I managed to keep my head out of water as I was swept along, until I got a knock on the noodle, and that put me partly to sleep. That may have been a good thing, too, for they say a partly unconscious person doesn't breathe much, and that's why I didn't swallow any water to speak of. "I was dazed when I must have been swept, or floated, past that rock but I came to in time to save myself. Gosh! but I was glad to hear you yell though, Dick!" he said. "Well, let's get over there an' start pryin' out this secret," suggested Old Billee. "This is gettin' mighty interestin'!"