“But how is it we can breathe here, and things can grow?” asked Bill, who was beginning to lose his fright at the thought of being practically buried alive.

“I do not know what makes such things possible,” Mr. Henderson replied, “but that there is air here is a certainty.  I can hardly believe it is drawn from the surface of the earth, down the big hole, and I am inclined to think this place of the under-world has an atmosphere of its own, and one which produces different effects than does our own.”

“They certainly have larger flowers than we have,” said Mark.  “See how big they grow, and what strong colors they have.”

He pointed to the port, against which some of the blooms were nodding in the wind that had sprung up, for, in spite of the many differences, the under-world was in some respects like the upper one.

“Probably the difference in the atmosphere accounts for that,” the professor said.  “It enables things to grow larger.  And, by the way, Mark, that reminds me of something you said about seeing some horrible monster fleeing from the ship.  Did you dream that?”

“I did see something horrible, Professor,” he answered.  “I’m not positive what it was, but I’ll tell you as nearly as I can what it was like.”

Thereupon Mark detailed what he had seen.

“But how could anything, least of all some big monster, be concealed in the storeroom, and we not know anything about it?” asked Mr. Henderson.

“I thought you did know something of it,” replied Mark.

“Who, me?  My dear boy, you must be dreaming again.  Why should I want to conceal any being in the storeroom?  Come, there is something back of this.  Tell me all you know of it.  I can’t imagine why you think I was hiding something in the apartment.”

“I thought so because you were always so anxious not to have me go near it,” answered the boy.  “Don’t you remember when you saw me going toward it, several times, you warned me away?”

“So I did!” exclaimed Mr. Henderson, a light breaking over his face.  “But, Mark, it was not because I had hidden some human being or animal there.  I can’t tell you what it is yet, save that I can say it is merely a machine of mine that I have invented.  For reasons of my own I don’t want any one to see it yet.  Perhaps it may never be seen.  I thought, not long ago, that we might have to undertake a terrible risk in escaping from this place.  I directed you to go to the storeroom—­ but there, I can’t say any more, my friends.  Sufficient that I had nothing in the animal line concealed there.”

“But I am certain there was some beast or human being in there,” insisted Mark.  “I heard curious noises in there.  Besides, how do you account for the food disappearing and the door being open at times?”

“It might have been rats,” said Jack.

“I don’t believe there are rats in the ship,” put in the professor.  “More likely it was one of us who got up hungry and took the victuals.”