This done the sailors return to the tug and Engineer Serko signs to me to go below.

A short iron ladder leads into a central cabin where various bales and packages are stored, and for which no doubt there was not room in the hold of the schooner.  I am pushed into a side cabin, the door is shut upon me, and here I am once more a prisoner in profound darkness.

I recognize the cabin the moment I enter it.  It is the place in which I spent so many long hours after our abduction from Healthful House, and in which I was confined until well out at sea off Pamlico Sound.

It is evident that Thomas Roch has been placed in a similar compartment.

A loud noise is heard, the banging of the lid as it closes, and the tug begins to sink as the water is admitted to the tanks.

This movement is succeeded by another—­a movement that impels the boat through the water.

Three minutes later it stops, and I feel that we are rising to the surface again.

Another noise made by the lid being raised.

The door of my cabin opens, and I rush out and clamber on to the platform.

I look around and find that the tug has penetrated to the interior of Back Cup mountain.

This is the mysterious retreat where Count d’Artigas lives with his companions—­out of the world, so to speak.

CHAPTER IX.

INSIDE BACK CUP.

The next morning I am able to make a first inspection of the vast cavern of Back Cup.  No one seeks to prevent me.

What a night I have passed!  What strange visions I have seen!  With what impatience I waited for morning!

I was conducted to a grotto about a hundred paces from the edge of the lake where the tug stopped.  The grotto, twelve feet by ten, was lighted by an incandescent lamp, and fitted with an entrance door that was closed upon me.

I am not surprised that electricity is employed in lighting the interior of the cavern, as it is also used in the submarine boat.  But where is it generated?  Where does it come from?  Is there a manufactory installed somewhere or other in this vast crypt, with machinery, dynamos and accumulators?

My cell is neatly furnished with a table on which provisions are spread, a bunk with bedding, a basket chair, a wash-hand-stand with toilet set, and a closet containing linen and various suits of clothes.  In a drawer of the table I find paper, ink and pens.

My dinner consists of fresh fish, preserved meat, bread of excellent quality, ale and whisky; but I am so excited that I scarcely touch it.  Yet I feel that I ought to fortify myself and recover my calmness of mind.  I must and will solve the mystery surrounding the handful of men who burrow in the bowels of this island.

So it is under the carapace of Back Cup that Count d’Artigas has established himself!  This cavity, the existence of which is not even suspected, is his home when he is not sailing in the Ebba along the coasts of the new world or the old.  This is the unknown retreat he has discovered, to which access is obtained by a submarine passage twelve or fifteen feet below the surface of the ocean.