"This," said he, "was a shaft. There is another further along. How deep they are, I don't know."
"But, look here!" cried Dick. "How could they venture to sink shafts, when at any moment a flood might rush in and drown them all?"
"Ah! That's just the point," said our friend. "Come outside again and you'll understand."
Returning once more to the bed of the pool, we faced the hole in the wall, when our guide continued:
"Now, you see, the floor of the tunnel is about ten feet above the creek-bed, and before the cliff fell down, forming the dam, the water ran freely past its mouth. But some time after the miners had got out all the copper overhead and had begun sinking shafts, this cliff came down, blocked the channel, and caused the water to back up into the workings. As you remarked just now, it filled the tunnel five feet deep, and, as a matter of course, filled the shafts up to the top."
"I see," said Dick. "You think, then, that the cliff fell in comparatively recent times. I believe you are right, too. That would account for there being no trees of any great size upon the dam."
"Yes. And as a consequence the mine was abandoned; for it would have taken years to dig away this dam, and as long as it existed it would be impossible to go on with the work with the water coming down and filling up the tunnel once every three days, or thereabouts."
"Every three days!" we both exclaimed. "Is this a regular thing, then, this flood?"
"Why, yes. I'd forgotten you didn't know that. Yes, it's a pretty regular thing, and a very curious one, too. Pedro says that up in that old crater near the top of the mountain there is a great intermittent spring which every now and then rises up and spills out a great mass of water. The water comes racing down this gorge, and half an hour later leaps over the fall here, fills up the pool and the mine, and gradually drains off again under the dam."