The boy made his way as far into the interior as he could with his comparatively short air-hose, and then turned back to where he had left Jack and Frank. He had found it impossible, on account of the shifting to the prow of the hold cargo, to reach the cabin and the captain's offices without entering from the top deck.
As he turned around he stopped an instant, his attention attracted by a sound which seemed to come from beyond the bulkhead back of him. It sounded almost like the hiss of escaping steam. The lad knew that it must be a strong vibration which could thus make itself felt at that distance below the surface and through the heavy helmet he wore.
The more he considered the matter the clearer became the fact that it was actually uniform sound he heard. That is, sound brought to his ears by the water.
Some force might be moving the water, and the motion might be conveying to his ears, through the thin sides of the air-hose, the story of the action of the waves, if waves could be created at that depth.
As he listened to the steady beating he became convinced that some unknown power was at work in the wreck. What it was he could not even guess.
Then he heard sharper sounds which seemed to be created by steel striking steel. The jar brought the sound waves to his ears quite distinctly.
"Either I'm going daffy," the boy mused, "or there is some one at work on the wreck."
He left the hold and, without giving the others to understand that he had discovered anything of importance, began an examination of the sand along the line of the bottom. His air-hose was not long enough to admit of passing entirely around the vessel, so he motioned to the boys to accompany him and turned back to the submarine.
"Did you hear anything down there?" asked he as soon as the helmets had been removed.
"What are you talking about?" asked Frank, with a laugh. "Water would not convey sound to the ear."