It was narrower than the one which had led them to the cave, and its sides were rocky and uneven, while the roof was low. It was quite evident that none of the cars could have been driven through this narrow space, and as the boys went on they found that the roof was lower and the walls even closer together.
Finally, the flashlight showed them that it was almost impossible to continue, as projecting rocks jutted out and there was just enough space to admit passage of one person. Beyond that, the tunnel seemed to close altogether.
"Guess this is a blind alley," said Frank. "We may as well turn back."
He handed the flashlight to Joe, who led the way on the return trip through the tunnel.
Suddenly there was an uproar immediately ahead, a clamorous, deafening crash. The boys jumped with astonishment. In the darkness of the subterranean cavern their nerves had been keyed up to a high pitch, and this tremendous clatter was so unexpected in the dead silence that had surrounded them that they were almost paralyzed with momentary fright.
There followed a rattling and bumping of rocks, and then silence once more.
"What was that?" exclaimed Joe, recovering from his scare.
"Sounded to me like a fall of rock." Frank's voice was shaky, for he had a suspicion of what had actually happened.
"It seemed mighty close."
"That's what I'm afraid of. It may have blocked up this tunnel."