Hunting for Hidden Gold by Franklin W. Dixon

Frank Hardy could not have fallen more than ten or twelve feet, but he had the sensation of having dropped from an enormous height. The unexpectedness of it took his breath away, and when he finally crashed into a heap of earth and gravel with a jolt that jarred every bone in his body he could only lie there in the darkness and wonder that he was still alive.

Then, to his relief, came a voice from close at hand.