Sunday passed, and then they went at the cavern work in earnest. They cleaned out a place at the rear several yards in extent. Here they found a big slit, extending upward, but the rocks were exceedingly hard to remove.
"We ought to have some powder to blow them apart," said Mark.
"We must be careful," said Maybe Dixon. "We don't want a cave-in."
By the middle of the week they had twelve more nuggets and another small bag of dust.
"Ten thousand dollars' worth more," said Bob, when the find was placed in "the bank," as the hole had been designated. "We are getting wealthy, sure!"
Thursday found them all at the cavern, working away as industriously as ever. It was the middle of the forenoon and each had found a small nugget. In the center of the cavern floor was a heap of sand filled with golden grains, waiting to be washed. They were all happy and Bob was whistling merrily.
"Guess I'll go out and take a look around," said Si, and started for the opening.
Mark, Bob, and Maybe Dixon were at the rear of the cavern, working around a sharp, triangular rock. They imagined some more nuggets might be underneath.
"Hark!" cried Mark, suddenly. "What's that?"
All stopped work and listened. There was a strange rumble overhead. It kept growing louder and louder.