Suddenly he flung himself on his face, and pressed his ear close to the ground.
“Listen,” he said.
Yvette followed his example. There could be no mistake; the mysterious sound was coming from the ground beneath their feet! The earth was full of muffled thunder.
Dick took from his pocket a hammer and struck a sharp blow on the flat rock beneath their feet. It rang hollow! Unmistakably they were standing on the roof of a cavern.
Walking to the camp they roused the others and told them what they had seen and heard.
“We have got to catch that sailor if we wait here a month,” said Scott. “He must come out again some time. But how about food?”
“We have enough tinned stuff in the Mohawk for a week,” said Dick, “so we shall be all right for a few days. In the meantime we must watch the place closely.”
Next day passed without incident until evening was drawing on. Then Yvette, who was watching the top of the hill while the others rested, at six o’clock gave a low whistle. She was lying on the ground keeping observation between a couple of rocks which hid her completely. In a moment the others had crawled to her side.
“Look!” she said.
On the top of the hill, three hundred yards away, stood the sailor and the four mules, clearly silhouetted against the evening glow. He had appeared suddenly, Yvette told them, just on the spot where he had disappeared on the previous day.