The ten miles had become fourteen when the professor, finding that everything was in good shape, proposed that the boys go to bed.  They, did not want to, though they were sleepy, and they feared to miss some strange sights.

But when the professor had promised to call them in case anything unusual developed, they consented to turn in, and Bill and Tom assumed their duties, which were light enough, now that the ship was merely falling into the immense shaft.

When Mark turned into his bunk he could not go to sleep at once.  It may have been the excitement over their new position, or because he had eaten too hearty a supper, but the fact was he remained awake for some time.

While thus tossing restlessly on his bed, wondering what ailed him, he thought he heard a noise in the main apartment out of which the storeroom opened.  He crawled softly from his bed, and looked from his stateroom door.

In the light of a shaded electric Mark saw the figure of some one glide across the floor and take refuge in the room, which Professor Henderson always was so particular about.

“I wonder what or who that was,” reasoned Mark.  “There is some mystery in this.  Can the professor have concealed some one on this ship whose presence he does not want to admit?  It certainly looks so.”

Not wanting to awaken the ship’s crew, and remembering what Mr. Henderson had said about any one entering the storeroom, Mark went back to bed, to fall into an uneasy slumber.

“Breakfast!” called Washington breaking in on a fine dream Jack was having about being captain of a company of automobile soldiers.  “Last call for breakfast!”

“Hello!  Is it morning?” asked Jack.

“Not so’s you could notice it,” Washington went on.  “It’s as dark as a stack of black cats and another one throwed in.  But breakfast is ready jest the same.”

The boys were soon at the table, and learned that nothing of importance had occurred during the night.  The Mermaid had been kept going slowly down, and about seven o’clock registered more than fifty miles below the earth’s surface.

Still there was no change in the outward surroundings.  It remained as black as the interior of Egypt when that country was at its darkest.  The powerful electrics could not pierce the gloom.  The ship was working well, and the travelers were very comfortable.

Down, down, down, went the Mermaid.  The temperature, which had risen to about ninety went back to sixty-nine, and there seemed to be no more danger from the inner fires.

They were now a hundred miles under the surface.  But still the professor kept the Mermaid sinking.  Every now and again he would take an observation, but only found the impenetrable darkness surrounded them.

“We must arrive somewhere, soon,” he muttered.

It was about six o’clock that night that the alarm bell set up a sudden ringing.  The professor who was making some calculations on a piece of paper jumped to his feet, and so did a number of the others.