How many hours they slept they could not tell, for there were no timepieces. They would rouse, turn over, and drop asleep again, for each one was determined to sleep away as much of the waiting time as possible. It was probably early morning when at last Glen arose, stretched himself and carefully lighted a lantern.
"It's going down, boys," he announced. "The opening isn't uncovered yet, but it's two or three feet lower than it was last night."
They were all wide awake now, and all leaned over the ledge to form their own opinion.
"The current seems to run differently," said Glen. "It acts as if the rock we threw in has stopped up the old outlet and it was running back of the heap we pulled down instead."
"Yes, sir. Strikes me just that way," said Chick-chick.
"I'm going to take the other lantern and explore a little," said Glen. "You fellows needn't come. I'll holler if I find anything."
He disappeared behind the ruined arch, swimming and wading, but he was back in a minute, all excitement.
"There's a regular passage out this way, fellows. Seems to go clear through the Mound. The water's rushing down in a torrent. Come and see."
They needed no invitation, for they were down before he finished speaking. Around the crumbled masonry he led them, and pointed to an opening like a natural tunnel which, seemed to lead far into the bowels of the earth.