"There isn't any human being but ourselves in sight, either on the wall or in the water.

Your light shows that." The light did not quite show that, and could not, since the huge prowler was now swimming gently under water, some seven or eight feet from the surface.
"We'll have to solve the question before we leave here," declared Tom.
"We can't have folks following us up in a ticklish place like this.
Besides, Harry, I'm willing to wager that your vision---whatever it was---has some real connection with the mystery that we're going out yonder to investigate.

So we'll solve the puzzle that's right here before we go forward to look at the bigger riddle that the dark now hides from us out yonder.

Use your eyes, lad, an I'll do the same with mine!" Neither Tom Reade nor Harry Hazelton are strangers to the readers of this series, nor of the series that have preceded the present one.
Tom Reade and Harry Hazelton, now engineers in charge of a big breakwater job on the Alabama gulf coast, were first introduced to our readers in the "_Grammar School Boys Series_." There we met them as members of that immortal band of American schoolboys known as Dick & Co.

Back in the old school days Dick Prescott had been the leader of Dick & Co., though, as all our readers know, Prescott was not the sole genius of Dick & Co.