And he was right. When the water around the rock lapped the erosion mark, which had been worn in the hard stone by centuries of the flow of the fluid, the flood ceased. The roaring, bubbling and seething, like that which takes place in a canal lock, came to an end, and the water of the pool became quiet. "There! What'd I tell you?" cried Old Billee. "I closed th' water gate, that Bud opened to let th' water out, an' she come back. Now all we have t' do, so we can walk back, is t' yank this lever again." "Does it only work two ways?" asked Yellin' Kid, his voice again softened, as the mystery of the place seemed to cast a shadow over him and the others. "Seems to," Bud answered, holding his lantern down close to where the copper handle entered the rock. There appeared to be a slot cut in the hard stone--a slot about three inches wide, and a foot long, in which the copper lever could be moved backward and forward, but not from side to side. "Let's try the other way, now," suggested Dick. Once again Old Billee pulled on the copper shaft, which, as they could see by the light of all their lanterns combined, seemed to have been rudely hammered out, for it bore the rough marks of a primitive forge. And no sooner had the lever been pulled to its limit in the slot than there sounded again the rushing, roaring tumult of noises, and, after a little, the water began receding once more. "We've discovered the secret!" cried Dick. "No, only part of it," said Bud. "We've got to find where the water goes, and if pulling this lever sends it into our reservoir. That's the main thing to discover." "But we're on the track of part of it," went on Dick. "I wonder who built this secret water gate, and the lever that operates it?" "It may be part of the work of the ancient Mexicans, the old Indians or the Aztecs, who inhabited this land ages ago," said Bud. "Copper will last almost forever, you know, even in water, as it doesn't rust. And you've read how the ancient Aztecs used to build great vaults under the mountain, and arrange to flood them to keep their gold away from the Spaniards." "Yes, I've read of that," admitted Dick. "Say, where can you get a book like that?" demanded Old Billee. "I've got one at the camp," Bud answered. "I'll let you take it. Of course my theory may be all wrong," he went on. "But I begin to believe we've stumbled on some ancient Aztec water system." "You don't mean to say those old Mexicans, for that's what the Aztecs were, are still hanging around in this cave, turning your water on and off, do you?" demanded Dick. "No, it's some one more modern who's making trouble for us," Bud declared. "But we're on the track of a big discovery, I believe. Look, the water is almost gone!" This was true. The pool was emptying itself as it had done before, and, in a short time they could walk back to where Nort awaited them. "What's the next thing to do?" asked Dick.